Geological and geophysical site characterization for ...

EXP03-2015

Geological and geophysical site characterization for marine renewable energy development and environmental assessment

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EXP03-2015 Geological and geophysical site characterization for marine renewable energy development and environmental assessment

A trade-mar k of the Canadian S tandards Association, operating as “CSAGroup”

TM

Published in May 2015 by CSA Group A not-for-profit private sector organization 178 Rexdale Boulevard, Toronto, Ontario, Canada M9W 1R3


To purchase standards and related publications, visit our Online Store at shop.csa.ca or call toll-free 1-800-463-6727 or 416-747-4044. ISBN 978-1-4883-0040-0 © 2015 CSA Group Portions of this work contained under Sections 1 to 8 are reproduced or derived from material authored by: P.R. Hill, J.V. Barrie, R. Kung, D.G. Lintern, S. Mullan, M.Z. Li, J. Shaw, C. Stacey, and B.J. Todd © Her Majesty the Queen in Right of Canada, as represented by the Minister of Natural Resources, 2014 All rights reserved. No part of this publication may be reproduced in any form whatsoever without the prior permission of the publisher.



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Contents Development Committee on EXP03 Preface

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1 Introduction 6 1.1 Objectives of the Guide 6 1.2 Scope of the Guide 6 1.3 Geotechnical requirements for offshore structures 7 1.4 Geotechnical requirements for submarine cable routing

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2 Data and information sources 8 2.1 Public domain information 8 2.2 Government databases 9 2.2.1 Natural resources Canada 9 2.2.2 Fisheries and Oceans Canada 10 2.3 Provincial Governments 10 2.4 Universities 11 2.4.1 University of New Brunswick Ocean Mapping Group 11 2.4.2 University of Victoria 11 2.4.3 Fundy Energy Research Network (FERN), Acadia University 12 2.4.4 NSCC: Applied Research, Nova Scotia Community College 12 2.4.5 Memorial University of Newfoundland (MUN) Ocean Engineering Research Centre (OERC) 2.5 Uses of GIS within an organization 13 2.6 GIS for renewable energy geologic/geophysical site assessment 14 2.7 Spatial data types managed by GIS 14 2.8 Data management roles 15 2.9 Storage and retrieval of GIS data 15 2.10 GIS data management system requirements 16 2.11 GIS data dissemination strategies 17

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3 Geophysical methods 18 3.1 Multibeam sonar 18 3.1.1 Bathymetry 19 3.1.2 Acoustic backscatter strength 21 3.2 Sidescan sonar 24 3.2.1 Field acquisition 25 3.2.2 Data processing 26 3.2.3 Visualization and interpretation 26 3.3 Sub-bottom acoustic profiling 26 3.3.1 Acoustic systems 27 3.3.2 Field acquisition 31 3.3.3 Data processing 32 4 Seabed sampling 34 4.1 Grab sampling 34 4.2 Coring 35

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4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.2.6 4.2.7 4.2.8 4.3 4.3.1 4.3.2 4.3.3 4.4 4.5

Gravity corers 35 Piston corers 36 Vibracorers 37 Box corers 38 Core storage 39 Core processing and automated core logging Core description 40 Geotechnical properties 41 Geochronology 41 14C (Radiocarbon) 41 210Pb 42 7Be 42 Microfossil analysis 43 Bottom photography and video 43

5 Geological mapping 44 5.1 Geological map flow 44 5.1.1 Data acquisition 44 5.1.2 Data processing 45 5.1.3 Data archiving 47 5.1.4 Creation of imagery for the maps 5.2 Seascape mapping 57 5.2.1 Definition of a seascape 57 5.2.2 Example of a seascape map 58 5.3 Seabed habitat mapping 60 5.3.1 Habitat mapping schemes 61

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6 Geohazard analysis 64 6.1 Principles of hazard and risk analysis 64 6.2 Earthquake hazard 65 6.2.1 Seismic source characterization 65 6.2.2 Ground motion characterization 66 6.2.3 Mapping active faults 67 6.2.4 Earthquake hazard analysis 68 6.3 Tsunami hazard 68 6.3.1 Earthquake sources 69 6.3.2 Landslide sources 69 6.3.3 Evidence of past tsunamis 70 6.3.4 Evaluating tsunami hazard 70 6.4 Gas venting hazard 71 6.4.1 Identifying subsurface gas 71 6.4.2 Identifying gas venting 72 6.4.3 Determining the activity of gas venting 74 6.5 Slope stability, submarine landslides and other mass movements 75 6.5.1 Identifying submarine slides and mass movement deposits 77 6.5.2 Determining the activity of submarine slides and mass movements 79 7 Current scour and sedimentation 80 7.1 Principles of sediment erosion, transport, and deposition May 2015

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7.1.1 7.1.2 7.1.3 7.1.4 7.1.5 7.1.6 7.2 7.3 7.3.1 7.3.2 7.3.3 7.4 7.4.1 7.4.2 7.4.3

Grain size classification and analysis 80 Thresholds of sediment erosion and deposition 82 Sediment transport equations 84 Sediment transport under tidal flows 85 Sediment transport under waves and oscillatory currents 87 Bedforms and flow regimes 88 Subaqueous dune classification 89 Methods for assessing bedform mobility 91 Repetitive survey techniques 92 Evaluation based on grain size and model predictions 92 Numerical models of sediment erosion, transport and deposition Instrumentation for assessing sediment mobility 93 Benthic landers 93 Cabled observatories 94 Instruments 95

8 Seabed characteristics of marine renewable energy sites 8.1 Characterization of tidal energy sites 99 8.2 Characteristics of wave energy sites 103 8.2.1 Gravel wave-dominated seabed 103 8.2.2 Sandy wave-dominated seabed 107 9 Acknowledgements 10 References

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Development Committee on EXP03 P. Hill

Natural Resources Canada, Sidney, British Columbia

A. Carlisle

OpenHydro Technology Ltd, Dartmouth, Nova Scotia

G. Decker

Nova Scotia Department of Energy, Halifax, Nova Scotia

G. Fader

AMGC, Halifax, Nova Scotia

S. Molloy

Glas Ocean Engineering and Dalhousie University, Halifax, Nova Scotia

G. Trowse

Fundy Tidal Inc., Shad Bay, Nova Scotia

T. Wright

Fundy Ocean Research Center for Energy, Halifax, Nova Scotia

V. Alleyne

CSA Group, Toronto, Ontario

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Project Manager

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Preface This is the first edition of CSA EXP03, Geological and geophysical site characterization for marine renewable energy development and environmental assessment. CSA EXP03 has been developed as a guideline and does not contain any mandatory language. It provides guidance and best practices for geological and geophysical site characterization for marine renewable energy development and environmental assessment. This Guide is being issued to assist the Canadian marine renewable energy community with geological and geophysical site characterization for marine renewable energy development and environmental assessment. This Guide aims to be as comprehensive in scope as possible but does not aim to be exhaustive in the level of details provided. References to more detailed work are provided for readers to pursue their own requirements. The most commonly used survey methodologies and equipment are presented but it should be noted that other legitimate tools exist and their exclusion should not be taken as a refutation of their potential value. CSA Group acknowledges that the development of this Guide was made possible, in part, by the financial support of Marine Renewables Canada through the ecoENERGY Innovation Initiative (ecoEII). The initial document was authored by the Geological Survey of Canada and has been reviewed by the Development Committee on CSA EXP03, formed with diverse stakeholders holding expertise in geological and geophysical site characterization for marine renewable energy development and environmental assessment. Notes: 1) Use of the singular does not exclude the plural (and vice versa) when the sense allows. 2) Although the intended primary application of this document is stated in its Introduction, it is important to note that it remains the responsibility of the users of the document to judge its suitability for their particular purpose. 3) To submit a proposal for change, please send the following information to [email protected] and include “Proposal for change” in the subject line: a) designation (number); b) relevant section, table, and/or figure number; c) wording of the proposed change; and d) rationale for the change.

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EXP03-2015 Geological and geophysical site characterization for marine renewable energy development and environmental assessment 1 Introduction 1.1 Objectives of the Guide Marine renewable energy (tide, wave and offshore wind) is available in large quantities for integration into the Canadian energy mix (Cornett, 2006), but the ocean is a harsh and unforgiving environment that presents major engineering challenges to industry. The development of standards and best practices for site and submarine transmission corridor characterization and environmental impact assessment are important requirements for moving marine renewable energy development forward. Seabed site characterization is or should be a critical part of the engineering design process required to install turbines as well as to understand environmental impacts. The Geological Survey of Canada (GSC) has conducted a preliminary review of site characterization requirements and methodology at a number of sites with high wave and tidal energy potential. Based on this experience and previous experience related to the offshore oil and gas industry, this document is a first attempt to summarize the range of geological and geophysical information that is required for characterizing the seabed at a marine renewable energy site. The aim of the document is to provide information on methodology and geologic interpretation that will be useful for both proponents evaluating a potential site for development and regulators attempting to identify potential environmental concerns related to the development. The present document will be subject to both peer review and review by industry experts and government regulators. However, given the relative youth of the marine renewable energy industry, it should be regarded as preliminary in nature. As the industry matures, it is anticipated that such a guide would become more finely tuned to the specific needs of both proponents and regulators.

1.2 Scope of the Guide The document aims to be as comprehensive in scope as possible but does not aim to be exhaustive in the level of details provided. References to more detailed work are provided for readers to pursue their own requirements. The most commonly used survey methodologies and equipment are presented but it should be noted that other legitimate tools exist and their exclusion should not be taken as a refutation of their potential value. Many instruments have been developed for specific purposes and may be applicable to specific problems encountered in site characterization. The character of the seabed depends on a large number of factors, but primarily (1) the geological history of the region, which determines the type of underlying bedrock and unconsolidated sedimentary deposits that occur at the seabed; and (2) the present day sedimentary processes that actively move, erode and deposit sediments. Correspondingly, two broad areas of seabed characterization are covered in this guide. First, geospatial mapping of the geological character of the seabed is presented. Modern May 2015

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geologic mapping in the marine environment uses a variety of geophysical remote sensing and sampling techniques to determine the surface morphology and nature of the upper part of the bedrock or sedimentary column immediately below the seabed. This information can be usefully summarized in map form as geological or seascape units with unique or distinguishing characteristics. By inference, these units will also be characterized by particular geotechnical properties. Second, the principles and techniques of determining the dynamics of sediment movement at the seafloor are discussed. Whereas these processes vary spatially and can to some extent be mapped, they also vary temporally and typically require time series data sets to understand the natural variability of the dynamics. As much as possible, the guide uses examples from high energy environments in areas of wave or tidal energy potential. In association with this report, there is a companion GIS-based map product that provides overviews of several of these study areas.

1.3 Geotechnical requirements for offshore structures Knowledge of the geological and geophysical characteristics of the seabed is critical to understanding the geotechnical conditions on which wave and tidal energy conversion systems are founded or anchored. CSA Group’s Code for the design, construction, and installation of offshore structures developed with and for the offshore oil and gas industry comprises several engineering standards for both gravity-based and anchored structures that would be generally applicable to offshore renewable energy structures (CSA Group, 2007; 2008; 2009). Design and placement of offshore structures require detailed regional and site specific information relevant to the seabed and sub-seabed properties to typical depths of tens of metres (CSA Group, 2008; 2009). Geological and geophysical interpretation is necessary for understanding the horizontal and vertical distribution and variability of soil properties. The CSA Group standard for Foundations (CSA Group, 2009) requires site investigations to be performed for all fixed offshore structures with the objective of characterizing “the site conditions and to define relevant bathymetric parameters, morphological features, geological context, and geotechnical design parameters”. These site investigations have to take into account both “near field and far field conditions”. CSA Group (2009) specifically includes determining the effects of bathymetry and geomorphology, surficial geology, bedrock geology, seismicity, slope instability and the sedimentary environment, including erosional processes on the design of structures. Det Norske Veritas (2011) has developed standards for offshore wind turbine structures that include specifications related to soil investigations and geotechnical data. These investigations are “divided into geological studies, geophysical surveys and geotechnical soil investigations”. Further details are provided in a guidance note: “A geological study, based on the geological history, can form a basis for selection of methods and extent of the geotechnical soil investigations. A geophysical survey, based on shallow seismic, can be combined with the results from a geotechnical soil investigation to establish information about soil stratification and seabed topography for an extended area such as the area covered by a wind farm. A geotechnical soil investigation consists of in-situ testing of soil and of soil sampling for laboratory testing.” While anchored structures depend less on the sub-surface conditions, geological maps provide information on seabed and shallow substrate properties (CSA Group 2007). The International Electrotechnical Commission (IEC) Technical Committee 114 (IEC/TC 114) has developed draft standards for the assessment of mooring systems (IEC/TC 114, 2011). Knowledge of sediment transport conditions is of particular importance to anchorages in the high energy sites required for marine energy conversion.

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1.4 Geotechnical requirements for submarine cable routing Submarine electricity transmission cables from energy conversion devices to shore are an inevitable component of renewable energy installations. Cable routing and issues related to the laying, burial or trenching of cables are therefore critical aspects of marine renewable energy projects. The International Cable Protection Committee (ICPC), an association of cable industry companies and governments, has developed formal recommendations in a number of technical areas related to these issues (ICPC, 2007; 2010). Geological characterization of the cable corridor, typically “500 m on either side of the engineered route” (ICPC 2010), is a recommended practice either through desktop study using existing data or through subsequent marine surveys (ICPC 2007). Included in the list of geological characteristics that are recommended for evaluation are the following: • tectonic setting and associated seafloor morphology and lithology; • geological history; • seismicity; • surface faulting; • turbidity currents; • sediment transport; • sand waves; • beach and near shore seabed stability; • other geohazards. Submarine pipeline routing involves many similar issues related to seabed conditions and therefore similar principles of information gathering can be applied. Det Norske Veritas (2010) standards require consideration of the following: • seabed characteristics (uneven seabed, unstable seabed, soil properties, hard spots, soft sediment and sediment transport); • subsidence; • seismic activity. Route surveys are required “along the total length of the planned pipeline route to provide sufficient data for design and installation related activities”. Survey results should be presented in map form indicating “location of the pipeline, related facilities together with seabed properties, anomalies and all relevant pipeline attributes”. Features that should be noted include obstructions and topographic features such as rock outcrops, large boulders, pockmarks, potentially unstable slopes, sand waves, valley or channelling and erosion in the form of scour patterns or material deposits.

2 Data and information sources 2.1 Public domain information Public geographic information of both land and marine areas is available through the virtual global map and geographical information programs, Google Earth and Google Maps. These free software programs make available high resolution satellite and air photo information for most of the planet and includes marine bathymetric imagery compiled at the global scale on a 1 km grid resolution. Although potentially useful as a reconnaissance tool, for most inshore areas where wave and tidal energy sites are likely to be located, this resolution is inadequate for seabed characterization requirements. Furthermore, the map information cannot be queried to provide water depth values. It is therefore not recommended for use as the basis for site characterization studies at this time. It is possible that, in the future, government agencies will publish public data sets of higher resolution in formats compatible with Google Earth and Google Maps. May 2015

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Google Earth has also developed an Ocean feature that posts additional oceanographic information at selected sites. For example, Canadian weather and wave buoys are located and provide real time information on local conditions, including wave height as well as links (via the U.S. National Buoy Data Center) to Environment Canada where archived data can be downloaded.

2.2 Government databases 2.2.1 Natural resources Canada The Earth Sciences Sector of Natural Resources Canada provides maps and information related to the Canadian landmass and offshore in the fields of geoscience, geodesy, mapping, surveying, and remote sensing. The following data products are available through the GeoConnections Discovery Portal at http://geodiscover.cgdi.ca:

Table 2.1 Data available at Natural Resources Canada Product

Contents

Contact

National Topographic Series (NTS)

General-purpose topographic coverage of Canada at the 1/50 000 and 1/250 000 scales. NTS maps depict terrain features (landforms and land cover), drainage (lakes, rivers and streams), official boundaries, the transportation infrastructure (rail and road networks) and many other manmade features (buildings, power lines, pipelines, dams, cut lines, etc.). Note: Depth of water bodies is not shown.

[email protected]

Canada3D

Digital elevation model (DEM) providing coverage of the entire Canadian landmass. Distributed in ESRI Arc/Info ASCII Grid files. Available at both horizontal grid spacing of 30 and 300 arc-seconds.

[email protected]

Earth Observation Imagery (GeoGratis)

GeoGratis distributes a wide variety of satellite imagery for Canada. The data is available free of charge, subject to copyright restrictions and disclaimer.

[email protected]

Canadian Marine Multibeam Bathymetric Data Web Map Service

Over 300 colour-shaded relief bathymetry images of high resolution marine multibeam bathymetric data, comprising survey data collected by NRCan and other partners.

[email protected]

Marine Geoscience Map Vectors

These data depict many marine subjects including bedrock geology, surficial geology, benthic habitat, sediment thickness and seabed morphology.

[email protected]

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Table 2.1 (Concluded) Product

Contents

Contact

Offshore and Coastal Canadian Geophysical Records Holdings

Record inventory for the archiving of geological and geophysical data (e.g., seismics, multibeam, bathymetry, gravity, magnetics and seabed videos). Includes logbooks, magnetic/reel-toreel/digital tapes, paper records, analog data, videos, microfilm, microfiche and photographs collected as part of research cruises by, or for, the Geological Survey of Canada.

[email protected]

Offshore and Coastal Canadian Samples Holdings

Sample inventory for the physical holdings archived at the Geological Survey of Canada Includes cores (both refrigerated and ambient), grabs, and other types of samples collected as part of research cruises by, or for, the Geological Survey of Canada.

[email protected]

2.2.2 Fisheries and Oceans Canada Original bathymetric data from both single beam and multibeam surveys, as well as field sheets can be obtained from the Canadian Hydrographic Service (CHS), a division of Fisheries and Oceans Canada. The portal for obtaining data and contacting CHS is http://www.charts.gc.ca/. Oceanographic data, including current and wave data can be obtained from the Integrated Science Data management division of Fisheries and Oceans Canada (http://www.meds-sdmm.dfo-mpo.gc.ca/isdmgdsi/index-eng.html). Data products include Tides and Water Levels, Current Moorings Data, Wave Buoy Data, and Wind and Wave Climatology Atlases.

2.3 Provincial Governments Provincial governments do not generally hold much marine data. However, provinces hold and provide access to land geographic and geoscience data, which may be relevant to site investigations.

Table 2.2 Data available from provincial agencies Province British Columbia

Québec

URL

Agency and Data Types GeoBC: Air and Climate, Air Photos, Fresh Water and Marine, Geology, Land Ownership, Permitting and other

http://geobc.gov.bc.ca/

Ministry of Energy and Mines: Geological Maps

http://www.em.gov.bc.ca/Mining/Geoscience/ PublicationsCatalogue/DigitalGeologyMaps/Pages/ default.aspx

Le Québec géographique: Air Photo, Satellite images, Topography

http://geoboutique.mrnf.gouv.qc.ca/edel/pages/ recherche/critereRechercheEdel.faces

Géologie Québec: Geological Maps, Geophysical Data

http://sigeom.mrnf.gouv.qc.ca/signet/classes/I1102_ indexAccueil

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Table 2.2 (Concluded) Province New Brunswick

Nova Scotia

Newfoundland

URL

Agency and Data Types GeoNB: Topography, Aerial photography

http://www.snb.ca/geonb2/index.html

NB Natural Resources: Geological Maps, Geophysical Data

http://www.gnb.ca/0078/minerals/Main_Menu_ Descrip-e.aspx#Databases

GeoNOVA: Geographic, Geoscience, Wind

http://www.gov.ns.ca/geonova/home/default.asp

NovaScan: Geoscience Maps and Publications

http://geonova.novascotia.ca/

MapsNL: Topographic Maps, Air Photos

http://www.mapsnl.ca/

Geological Survey: Geological Maps, Resource Atlases

http://www.nr.gov.nl.ca/mines&en/geosurvey/maps/

2.4 Universities Universities are centres of teaching, research and innovation and are therefore a resource available to those conducting site investigations and seabed characterization. The groups mentioned below bring particular expertise with regard to marine renewable energy development.

2.4.1 University of New Brunswick Ocean Mapping Group The Ocean Mapping Group, located within the Department of Geodesy and Geomatics Engineering at UNB, has been one of the pioneers in the utilization of multibeam sonar for seabed mapping. The research of the Ocean Mapping Group “is focused on developing new and innovative techniques and tools for the management, processing, visualization and interpretation of ocean mapping data.” The group has conducted numerous surveys in Canadian waters and conducts training courses on a regular basis. The group has also pioneered the use of multibeam mapping for the monitoring of large subaqueous bedforms (see Section 7.3.1). For more information on the Ocean Mapping Group, go to: http://www.omg.unb.ca/.

2.4.2 University of Victoria The University of Victoria has led the development of cabled underwater observatories both in Canada and worldwide through the two landmark projects VENUS (www.venus.uvic.ca) and NEPTUNE Canada (http://www.neptunecanada.ca), administered through Ocean Networks Canada (http://www.oceannetworks.ca). Cabled observatories are constructed by the laying of a combined electrical power and fibre optic cable on the seabed to join instrument clusters together. This configuration permits power to be supplied to instruments 24 hours per day and for high bandwidth data transmission to land in real time via the internet. In effect, instruments deployed on the cabled network can be monitored, programmed and controlled in real time from the comfort of the user’s office. A particular feature of both VENUS and NEPTUNE Canada is the sophisticated Data Management System (DMS) that allows data to be viewed in near real time and provides permanent archiving of very large data sets. Underwater observatory systems have great potential for seabed characterization particularly with respect to the dynamics of seabed sediment transport and monitoring of geohazards such as slope stability and seismicity. For example, as part of the FORCE site environmental monitoring and site

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resource assessment program (see below) ONC has established a system to feed data streams into its Oceans 2.0 system for public access through the ONC website. The University of Victoria’s Institute for Integrated Energy Systems hosts the West Coast Wave Initiative, a “multi-disciplinary group of academics and industry members committed to quantitatively determining the feasibility, impacts and possible structure of wave energy conversion on the west coast of Canada”. WCWI is active in developing wave energy resource assessment methods, numerical simulation tools for Wave Energy Converters (WEC) and numerical grid integration toolboxes for assessment of the feasibility of wave energy conversion in British Columbia.

2.4.3 Fundy Energy Research Network (FERN), Acadia University FERN is a non-profit organization, initiated by academic and government researchers, as a forum to coordinate and foster research collaborations, capacity building and information exchange to advance knowledge, understanding and technical solutions related to the environmental, engineering and socioeconomic factors associated with tidal energy development in the Bay of Fundy. The expertise and experience gathered and developed in FERN has and will continue to provide a valuable resource for seabed characterization efforts.

2.4.4 NSCC: Applied Research, Nova Scotia Community College Applied research at Nova Scotia Community College (NSCC) is an extension of the college’s mission to support the economic and social development of the province of Nova Scotia. Expansion of capacity and scope of applied research projects since 2000 has resulted in a multi-million dollar research operation that is equipped to respond to the needs of the population. The Applied Research Office facilitates and supports applied research endeavors initiated by the college and its partners and works to transfer this knowledge to these communities. The focus of applied research is to provide innovative and problem solving technologies that have practical relevance and commercial application. Applied research focus at NSCC cuts across a number of disciplines of relevance to marine renewable energy, including applied geomatics, oceans, energy and engineering technologies. For more information on Applied Research at NSCC, go to http://www.nscc.ca/about_nscc/applied_research/.

2.4.5 Memorial University of Newfoundland (MUN) Ocean Engineering Research Centre (OERC) The Memorial University of Newfoundland (MUN) Ocean Engineering Research Centre (OERC) is located within the Faculty of Engineering and Applied Science at Memorial University. OERC’s objectives are to advance ocean engineering research and technology development; promote interaction among researchers and ocean engineering technology stakeholders and the research community; foster an innovative, wealth-creating research climate, technology transfer, entrepreneurship and commercialization of research; and take an active role in shaping policies in the ocean engineering, offshore engineering and the marine technology sectors. The OERC facilities include a 58 m x 4.5 m towing tank with wave making capabilities, a structures lab with a range of testing equipment, an autonomous underwater vehicle (AUV) lab, a wind tunnel, a deep-water tank and more. The OERC has a long history working on complex projects and the Naval Architectural and Ocean Engineering department has world-class researchers specializing in ocean engineering research. MUN is also closely affiliated with C-CORE which is a Canadian research and development (R&D) corporation that creates value in the private and public sectors by undertaking applied research and development, generating knowledge, developing technology solutions and driving innovation. Established in 1975 as the Centre for Cold Ocean Resources Engineering to address challenges facing oil and gas development in offshore Newfoundland & Labrador and other ice-prone regions, C-CORE is now a multi-disciplinary ISO 9001May 2015

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registered R&D organization with world-leading capability in Remote Sensing, Ice Engineering and Geotechnical Engineering. Many complex projects require a multi-faceted approach; C-CORE combines this expertise for a complete, end-to-end solution. For more information, go to: https://www.c-core.ca/ and http://www.engr.mun.ca/oerc/index.php.

Data management and dissemination using GIS Modern surveying techniques generate large volumes of spatially specific data, requiring systems to manage, analyze and share the data. Over the last thirty years Geographic Information Systems (GISs) have proven to be a valuable and indispensable tool to manage, visualize, analyze and disseminate many types of spatially oriented data, and have revolutionized the way the earth sciences manage and work with spatial data. Section 2 is not a comprehensive guide to GIS technology, but rather focuses specifically on GIS data management and dissemination strategies, and even then outlines only the most basic of principles. A GIS is usually purchased as a licensed package of software applications which can be installed at different implementation scales ranging from individual computers to arrays of computer network servers. The applications can range from very complex and expensive modular systems like those provided by the companies ESRI, Intergraph or Autodesk, to small inexpensive stand-alone applications like Manifold System or MapInfo or Global Mapper. More expensive software generally yields more analytical capability, higher quality cartography, and the ability to work directly with relational database management systems (RDBMSs) on multiple operating systems. Less expensive GISs are made to be cost effective and within reach of the individual or small organization, the price being paid in reduced functionality or availability in only one operating system, typically Windows. More expensive GISs will typically run on Windows, UNIX, LINUX and in some cases, Macintosh OS as well. Some GISs are no cost and open source, such as GRASS or GMT. Systems like these, although having no initial economic cost, are generally cumbersome to use because they must be programmed to suit the users’ needs. Visualization interfaces must be built and are not available “out of the box”. Most organizations are better off using a commercial “off the shelf” GIS that includes technical support if programmers are at a premium or non-existent within an organization. A GIS can be implemented as a “stand alone” system or a network-based system. On a stand-alone system, the software license, the software applications, and data reside on a single computer. On a network, a GIS is typically configured as a client/server architecture, with the possibility of the software licenses, applications and data all residing on different network servers and being accessed by client computers throughout the organization. Network implementation of a GIS has the advantages of concurrent multiple user access to data and cost effectiveness in the management of large numbers of users and GIS licenses.

2.5 Uses of GIS within an organization A GIS is used for several data-centric, generic purposes: data management, data analysis, data visualization and data dissemination. Data management involves logically storing the data for ease of access, safety against loss, search and recall, and long-term archiving and documentation. Most GISs provide capabilities for documenting data through the use of metadata tags, which describe the data in question and are based on a standard compiled by the International Standards Organization (ISO) or the Federal Government Data Committee (FGDC). The FGDC metadata standard for GIS data is a subset of the ISO standard, and is an adequate template for most GIS related metadata documentation. It is not specific to government data. Detailed information about ISO standards can be found at http://www.iso.org/iso/home.html.Detailed information about the FGDC standard can be found at May 2015

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http://www.fgdc.gov. Metadata is an important aspect of GIS data which deserves attention. Metadata describes the data and tracks its creation and evolution, along with important information such as originators, purpose, date of acquisition and processing steps. Data analysis involves using GIS tools and functionality to derive value added data and knowledge from one or more initial datasets. Examples can include deriving slope from elevation data; deriving landslide susceptibility from elevation, slope, hydrology and surface type; or difference mapping by analyzing the morphological changes occurring in a specific area using data collected from multiple year surveys. Data visualization includes preparation of data for effective screen display or cartographic representation on a digital or hardcopy paper map. Examples of common digital map formats include PDF, JPEG or MrSID formats. Hardcopy maps are usually generated on a small format printer, large format plotter, or professional offset printing. Data dissemination involves the distribution of data in formats that are useful and usable by the client interested in exploring or working with the data. Dissemination can range from distributing a fixed, finished digital or paper map to distributing a complete digital workspace which is dynamic, interactive and includes the data used for the visualizations as part of the distribution package. More specific dissemination strategies will be covered later in Section 2.

2.6 GIS for renewable energy geologic/geophysical site assessment Using a GIS for geologic/geophysical site characterization in this paper focuses on the data management and data dissemination aspects of GIS functionality. Many organizations have well developed analytical and cartographic GIS skills, but data management and dissemination are still problematic in many organizations, mostly due to lack of resources and expertise. Successful data management and dissemination of GIS data depends on many factors: • What type of data are to be managed? • Who will manage that data? • Where will the data be stored? • What is the appropriate system for the size of your organization? • What backup systems are in place? • Who are the clients requiring access to your data? • What mechanisms do the clients use to access data?

2.7 Spatial data types managed by GIS Data acquired on field surveys must be transformed into GIS consumable data types for storage within a file based computer server or a relational database management system (RDBMS). GIS data types fall into two broad categories, vector and raster. Vector data are geometric based data in zero, one or two dimensions. Zero dimension vector data are called points. One dimensional vector data are called lines. Two dimensional vector data are called polygons. Point, line and polygon vector data can be rendered in three dimensions using specialized software for 3-D viewing and analysis. Examples of this kind of software include ESRI’s 3D Analyst, IVS Fledermaus and Google Earth. Data can be represented in the “fourth” dimension using software specializing in time series analysis, which are offered by most of the major GIS vendors. Rasters are cell-based data which form continuous surfaces or lattices. One can think of raster cells as pixels, but pixels as a term is used only by the media industry and the general public. It is not a term used by the GIS community. Raster data in GISs can represent thematic data such as elevation, temperature, geophysical properties and any other themes which can be represented as a surface, or May 2015

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image data such as satellite imagery or aerial photography. What differentiates raster imagery from standard images is the property of georeferencing, where each cell in the image has a spatial location value which corresponds to its position on the surface of the earth. Tabular data is not a spatial data type. It is a table of attributes which can be associated with spatial data types through joins and relates based on common attribute fields. Using relational joins to associate tables with spatial data allows better organization of attribute data, allows customized views of the data to be created, and prevents the need to create large cumbersome tables of data which contain so many fields they are difficult to view and work with. Related tables are the most efficient way to associate attributes with spatial data within a GIS database management system. Georeferencing is the essential property of all GIS data, both vector and raster. A full understanding of georeferencing requires knowledge of map projections and spatial reference systems, which is beyond the scope of Section 2. GIS data in general will be in a decimal degree coordinate system (latitude/ longitude) or a planimetric coordinate system (Mercator, UTM, Lambert Conformal, Albers Equal Area, etc.). The coordinate system for each dataset must be clearly defined in order for it to be used effectively in GIS. Most GIS have the capability to project data “on the fly” for visual display, meaning that as long as a projection is defined for each dataset, all data will be shown visually in a common projection even if the underlying projections are different from each other. However, if you are manipulating datasets physically with each other using GIS, typically when deriving new data or performing analysis, then it is important for all data used in the manipulation or analysis to be physically in the same coordinate system, which may require reprojection of several datasets into a selected common projection. Selecting a common projection depends on the shape and size of the area in which you are working in. For example, mapping in Canada at regional scales is typically done in Universal Transverse Mercator (UTM), while mapping in Canada at local scales are typically Lambert Conformal. When working with raster-type data, it is important to have the underlying projections in a planimetric system rather than a decimal-degree system, or undesirable anomalies may appear when processing the rasters.

2.8 Data management roles Successful spatial data management within an organization depends on having staff dedicated specifically to managing the data. The data manager or data management team is responsible for the following: • Safe storage, backup and logical organization of the data; • Security of the data or “gate keeping”; • Quality control (QC) and documentation of the data (metadata standards); • Efficient and timely data retrieval; • Client support and troubleshooting of the database or file system. Data management can be performed by one or more people, depending on the size and resources of the organization. Some organizations may have an IT specialist taking care of the computer server hardware and operating system, a database specialist to take care of the database application programming and software maintenance, and an information management specialist to load and retrieve the actual data from the system. Small organizations will have talented individuals performing all three main data management roles.

2.9 Storage and retrieval of GIS data After data are processed into appropriate GIS data types they must be stored in a location which is secure, dedicated to data storage and backed-up on a regular basis. If data are to be served to an organization as a whole and disseminated to external clients then it is recommended that a computer May 2015

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server or a network of servers be available and dedicated to the storage and retrieval of GIS data. It is not recommended that corporate GIS data be stored on computers dedicated to other tasks or that run other applications other than those needed to maintain the integrity of the GIS data. Storage of the GIS data on a computer server can be built in two general ways, either in a file-based system or in a database management system. Both have pros and cons associated with their implementation and maintenance, and which way to go depends upon the size of an organization and the funding and staff available to maintain the system. While Section 2 is concerned only with spatial GIS data types, a GIS is often only one component of a much larger data management system within large organizations and agencies. File-based systems for GISs are directories of folders on a network computer server which contain spatial data types in formats that can be used directly by the GIS. Vector data formats typically can include shapefiles, Microsoft Access geodatabase feature classes and Autodesk DXF formats. Raster data formats can typically include ESRI GRID, USGS DEM, GeoTIFF, and JPEG2000. The directories can be organized by theme, by location, or by data type depending on the needs of the organization. File-based system implementations are less expensive, easier to implement, and require less staff to maintain. They are less robust than a database management system, and experience has shown that it is more difficult to search and retrieve data from a file-based system. Database management systems for GISs store all data, i.e., vector, raster and tabular, within a relational database such as Oracle, Informix, or SQL Server. Database management systems are more expensive to implement than a file-based system, require specialized expertise, and must be maintained on a regular basis. However, for larger organizations with the available resources a RDBMS is robust, powerful, flexible and able to interface with a wide variety of applications from GIS clients to web-based mapping services.

2.10 GIS data management system requirements At the most simple level of GIS implementation, a computer with the following components is required: a standard operating system like Windows, Unix, Linux or MacOS; GIS client software; and enough disk space on the computer to hold data either internally or through connected USB drives. This is the basic configuration for stand-alone systems and is suitable for individual entrepreneurs. Using this type of implementation within a multi-person organization can quickly lead to data redundancy, data loss and lack of quality control. Multi-person organizations are best suited to client/server architectures where the corporate GIS data are stored on a common network server and accessed by the GIS software installed on client computers. For small organizations it is generally more economical to use a file-based system. However, the data server should have a redundant array (RAID) or a backup computer which mirrors the primary server in case of data loss. Synchronizing the primary and backup servers is the responsibility of the IT specialist or data manager. A local area network is also required for this type of GIS implementation. For large multi-person organizations, file-based systems may become difficult to manage and the most appropriate level of GIS implementation includes an RDBMS like Oracle or SQL Server. This type of implementation requires not only GIS client software and a network computer server, but also RDBMS software and middle-tier software which allow communication between the GIS clients and the RDBMS. An example of such middle-tier software is ESRI’s Spatial Database Engine (SDE) which is included in their ArcGIS Server package. Additional staff are required to maintain such a system, the most important being the database administrator who is responsible for the maintenance, programming, upgrade and administration of the database itself. This role can also include the data manager, who is responsible for gate keeping, storage and retrieval of the GIS data to and from the database. May 2015

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2.11 GIS data dissemination strategies GIS data dissemination, like GIS data management, can have several levels of implementation from simple to complex. The simplest level of dissemination is stand-alone through a medium such as DVD or distributed zip files. This type of dissemination can be accomplished by all levels of GIS implementation, whether it is a stand-alone system or a RDBMS driven system over a network. After creating and completing a GIS project and its requisite cartography and analysis, the project map is bundled, along with its supporting data and software for installing a read-only application of the GIS, onto a medium such as a DVD. An example of such read only GIS software is ArcReader from ESRI. Although DVD-based GIS project discs are easy to produce, they suffer from limited storage capacity, physical deterioration over time, and are in fact an obsolete technology in light of hand-held devices and internet-based mapping services. Also, the client is forced to do some work in installing the GIS read-only software application. A slight improvement over DVD limitations is to produce a zip file containing the GIS project and reader software and distributing it to clients via the internet. In this case the organization requires a web page from which the clients can interface and download the project. This allows distribution of larger sets of data but downloading speed may be hampered by bandwidth limitations and slow networks. The next level of dissemination implementation is to distribute the GIS data on a file-based system through a download protocol such as FTP. This is a good strategy for distributing the data quickly and directories can be organized logically for dissemination as they are for managing the data. A web developer is necessary to create the portal from which the FTP directories are accessed. The main disadvantage of this dissemination strategy is that the data are given to the client without any layout or symbolization of the data, so the client must have GIS software on hand in order to display and manipulate the data meaningfully. There is also an issue where the client symbolization of the data after download may not be the way it was intended to be shown by the data originators. This method of GIS data dissemination differs from the previously described method of packaging the data along with GIS reader software, where the author prepares and symbolizes the data in a read-only format. File-based download protocols are a good way to disseminate data to GIS enabled agencies or expert agencies who already know what to do with the data they download. The most sophisticated and complex way of GIS dissemination is through internet-based portals containing web mapping services (WMSs). WMSs are mapping interfaces which connect to data from one or more sources and deliver the data in one mapping view on the screen to the client fully symbolized as the data owner intended it to be. The best and most ubiquitous example of a full WMS is Google Earth, which connects to millions of data sources and displays them on the virtual surface of the earth. The advent of Google Earth has actually simplified WMS for GIS data holders for distribution over the internet. The interface and system is maintained by the Google Corporation, and data holders simply connect their data sources to the Google server via the Google Earth interface. This method of dissemination should not be used for private or sensitive data due to the public nature of Google Earth. Data dissemination portals with WMS can be custom developed for an organization, and the web page can be password protected so that it is accessed only by authorized clients. This requires web-expertise and programming technology to be available to the host organization. Many organizations cannot afford a full complement of specialists in IT, information management, GISs, database management and web programming and must offload roles in the GIS management-dissemination work flow wherever possible. Internet-based dissemination strategies through either FTP or WMSs have the advantage of keeping the data close to the source and being able to update the data when necessary. WMS dissemination strategies have the added advantages of minimizing the work required by the client, symbolizing the

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data to the owner’s specification, and not requiring the client to have GIS software on their local computer. WMSs are generally visual and read-only, but these applications can be extended to become Web Feature Services (WFSs), which allow download of the data visualized by the WMS. WMSs also minimize data redundancy. For example, all ESRI GIS clients can access worldwide satellite imagery by connecting to ESRI’s cascading satellite imagery WMS, so clients do not have to purchase or keep satellite imagery locally if all they need is a high quality imagery background for their maps and data. A Custom WMS/WFS is being developed for the Nova Scotia tidal energy industry and is anticipated to be publically accessible by the end of 2015. The resulting website will consolidate, display and provide tools to manipulate tidal-energy-related spatial data from existing, ongoing and future research projects in the region. For further information please contact the Fundy Ocean Research Center for Energy (FORCE).

3 Geophysical methods 3.1 Multibeam sonar Multibeam sonars are sophisticated echo sounding devices used to measure water depth. The seafloor is insonified by an array of narrow beams formed from a single or dual transducer system. Because the array of beams can be configured to overlap, complete coverage of the seabed can be achieved in an efficient swath mapping process (Figure 3.1). Multibeam sonar surveys are presently standard practice for hydrographic surveying and bathymetric charting and are employed by the Canadian Hydrographic Service, which is certified to International Hydrographic Organization (IHO) (2008, 2009) and ISO 9001:2000 (Quality Management Systems) standards. It is recommended that all surveys be conducted to IHO (2008) standards. Accurate multibeam bathymetric surveying requires accurate ship positioning and ship motion sensing as well as knowledge of the sound velocity profile in the water column. More detailed descriptions of the principles and methodologies of multibeam surveying can be found in Courtney and Shaw (2000), Coggan et al. (2007) and Hughes-Clarke (in press).

Figure 3.1 Schematic illustration of multibeam sonar method. The seabed is insonified by a swath of narrow beams along a line perpendicular to the heading, each one providing a direct measurement of water depth. As the ship advances, continuous and overlapping coverage along a broad corridor is achieved.

Note: Courtesy of John Hughes-Clarke.

Collection of multibeam data can be problematic in areas of high tidal currents. May 2015

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For seabed characterization, two multibeam products are useful: bathymetry and acoustic backscatter.

3.1.1 Bathymetry In water depths less than 100 m, modern multibeam systems can provide bathymetric resolution of 10 cm or less at a spatial resolution of 3 m or less. These data can be gridded into digital elevation models that can be displayed as contour maps, shaded relief images or 3-D renderings (Figure 3.2). Detailed bathymetric information is directly valuable for placement of bottom structures and cables, for example to avoid placement on steep slopes or spanning across small scale relief features. For marine geological interpretation, these detailed bathymetric products serve much the same purpose as topographic maps and air photos in terrestrial geological mapping. They provide a geomorphological base map from which geologic landforms can be interpreted directly from the seabed relief. For example, in the example shown in Figure 3.2, bedrock outcrops and large subaqueous dunes can be identified. In other regions, glacial landforms such as moraines, eskers and drumlins can be easily mapped. Submarine landslides and other geohazards can be identified. These landform interpretations provide primary information on which more detailed geological characterization can be based. Interpretation of bathymetric data in GIS allows for examination of the data at the full range of scales available. Zoomed-in images can provide seabed texture information that can be interpreted to provide information on the geologic formations occurring at the seabed. Detailed interpretations at this scale are often aided by additional information from acoustic backscatter maps (see next Section).

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Figure 3.2 Examples of how digital elevation models can be displayed as contour maps (top), shaded relief images (middle) and 3-D renderings (bottom), Boundary Pass area, Gulf Islands, BC.

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A further application of multibeam bathymetric mapping is to monitor changes in the seabed morphology over time-scales relevant to seabed infrastructure deployment. Although many geologic landforms are stable over long time-scales, features such as subaqueous dunes (Duffy and Hughes, 2005), scour troughs (Shaw et al., 2012), channels and submarine landslides (Hill, 2012) can show significant morphologic changes at interannual or shorter time scales. Two or more survey data sets can be quantitatively compared using GIS to produce difference maps that highlight bathymetric change over the time period between surveys (Figure 3.3). For this kind of analysis, care has to be taken to understand the achievable resolution of the sonar systems being used and the errors associated with both the data acquisition and the subsequent gridding procedures. Hughes-Clarke (in press) provides a detailed discussion of the issues surrounding comparison surveys.

3.1.2 Acoustic backscatter strength Multibeam data sets can be processed to obtain information on the strength and characteristics of the seabed reflection. These reflectivity characteristics depend in gross terms on the beam angle, the scattering roughness of the seabed and the reflection/absorption characteristics of the substrate (Courtney and Shaw, 2000; Hughes-Clarke, 2012). In principle, the backscatter intensity can be related to parameters such as sediment grain size or geotechnical properties and could therefore be used to interpret the nature of the geologic substrate. However, in practice, numerous complexities in both the beam and seabed characteristics prevent simple relationships between these parameters and the backscatter intensity (Hughes-Clarke, 2012). Nevertheless, multibeam backscatter maps have great value for seabed characterization (Figure 3.4).

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Figure 3.3 Example of difference map showing active movement of subaqueous dunes, Roberts Bank, Fraser delta, BC. (a) shaded relief image showing subaqueous dunes in 2001; (b) shaded relief image showing subaqueous dunes in 2007; significant changes in the form of some dunes can be discerned; (c) map showing difference in bathymetry between 2001 and 2007.

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Figure 3.4 Multibeam backscatter for the same 2007 as in Figure 3.3. Lighter areas show higher bottom backscatter indicating a coarser or harder bottom. In this case, the high backscatter appears in the troughs between subaqueous dunes, suggesting that the trough sediments are coarser grained sand and gravel lag deposits. Darker areas correspond to dune crests, which are sandier.

There are two approaches to interpretation of multibeam backscatter data: qualitative and quantitative. The qualitative approach uses backscatter strength in an integrated fashion with other data sets, notably sample and sub-bottom geophysical data to interpret the seabed geology. Quantitative classification techniques based on backscatter have been developed for both single-beam and multibeam sonars (Pace and Dyer, 1979; Pace and Gao, 1988; Milvang et al., 1993; Preston et al., 2001; Hamilton, 2005) and automated commercial software is available. Use of quantitative classification can reduce the amount of interpretation bias, although some methods still require a degree of supervision. Any seabed classification, qualitative or quantitative, should be supplemented by a ground-truthing program of bottom grab samples and/or bottom photographs or video so that the different classes can be equated to real seabed characteristics. It should, however, be noted that very subtle class distinctions identified by acoustic methods may be difficult to groundtruth adequately (Preston and Kirlin, 2003). Figure 3.5 provides an example of a seabed classified using an automated system.

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Figure 3.5 Seabed classification of Simrad EM3000 multibeam data from Patricia Bay, Vancouver Island, BC, Canada. The automated classification was carried out using QTC MULTIVIEW, commercial software developed by Quester Tangent.

Note: Courtesy of the Canadian Hydrographic Service.

3.2 Sidescan sonar Sidescan sonar is an acoustic technique used for high resolution imaging of the seafloor, which was first developed in the 1950s (Blondel and Murton, 1997; Lurton, 2002). An acoustic pulse is emitted towards the seafloor across a broad angle perpendicular to the path of the sonar (see Figure 3.6). Reflections from the seabed are received and recorded as a function of time, which is related to the angular distance from the sonar. As the sonar advances through the water, a swath of backscatter data is accumulated along the ship’s track (Figure 3.7). The echo data are similar to backscatter data collected by multibeam sonar systems. Despite advances in multibeam sonar technology, sidescan sonars still provide the advantage of higher resolution (as fine as cm-scale in the across-track direction), particularly in deeper water where the sidescan can be towed close to the seabed. For marine renewable energy applications in relatively shallow water, applications for sidescan sonar systems include detailed characterization of the local seabed (e.g., for detailed engineering design or habitat mapping), identification of obstacles (e.g., smaller bedforms, boulders, wrecks or other anthropogenic materials) and monitoring of bottom structures (e.g., evaluating scour or sediment accumulation around structures, moorings and cables).

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Figure 3.6 Principles of sidescan sonar surveying

Note: Courtesy of StarFish Seabed Imaging Systems (http://www.starfishsonar.com/technology/sidescan-sonar. htm).

Interferometric sidescan sonar is a relatively recent innovation that allows bathymetric as well as backscatter information to be obtained from towed sidescan systems (Lurton, 2002). The incorporation of a second antenna in the sidescan fish allows for measurement of phase differences between the two receivers. This can be advantageous, particularly in shallow water where the wider angle of the sidescan system compared to multibeam systems makes for more efficient bottom coverage. However, there are also shortcomings including the potential for ambiguity in the depth measurements and sensitivity to ambient noise. The collection of sidescan sonar data can be difficult in regions of high currents as sidescan sensors are normally towed behind a vessel connected by a cable and are subject to small movements from currents and turbulence. However, the benefits are great as the systems are normally towed much closer to the seabed than vessel mounted survey equipment. This can produce very high resolution imagery able to detect seabed features and sediment textures to 10 cm resolution, often essential information for micro-siting bottom mounted devices. The operation of sidescan sonar systems in new areas of high currents is best approached by trial and error test surveys to determine the best direction for survey lines and time of survey relative to tides and currents. It may mean that in some areas, only a few hours of daily operation can provide the best data. These limits need to be considered in planning surveys.

3.2.1 Field acquisition Details on surveying practices can be found in Coggan et al. (2007). Sidescan sonar systems typically involve towing the sonar towfish at a relatively low elevation above the seabed and therefore require skill and experience to operate. A critical consideration is the geo-positioning of the sidescan data. Past practice has been to position the towfish using the cable layback distance and assumptions about the towing angles. This technique can lead to sizeable errors in positioning the towfish and consequently of the backscatter reflections. Such errors can be acceptable for general interpretation or monitoring, particularly if independent reference points such as bottom structures are available. For correctly geopositioned data that can be easily integrated into a GIS project, accurate positioning of the towfish is necessary. Ultra-short-baseline acoustic tracking systems that provide range and angle information between the towfish and a ship-mounted transponder are typically used for this purpose. May 2015

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3.2.2 Data processing The raw sidescan data requires several corrections to enable true-scale and amplitude-balanced map presentation. Time variable gain (TVG) is applied to correct the reflection amplitude for signal loss due to the cross-track range. Slant range correction is applied to take into account the incidence angle (in the vertical plane) of the reflection and convert slant range to a true horizontal range. Finally, the offset due to the two-way travel time from the transducer to the seabed (i.e., removal of the water column) is effected by using a bottom tracking routine that picks the first strong seafloor reflection. The resultant image is scale-corrected and horizontally positioned and can be mosaic (Figure 3.7).

Figure 3.7 Sidescan sonar mosaic over the Point Grey ocean disposal site near Vancouver, BC.

3.2.3 Visualization and interpretation Geo-positioned sidescan sonar data, both bathymetric data if available and echo intensity, can be imported as raster and image files into GISs. The sidescan echo intensity data have similar characteristics to multibeam backscatter data and can be integrated into geological interpretation in a similar fashion. If 3-D bathymetric models are available, the sidescan data can be draped onto the seabed surface to visualize the relationship between bathymetry and backscatter intensity. Software is also available for automatic classification of sidescan data.

3.3 Sub-bottom acoustic profiling Information on the geological units lying beneath the seafloor is important for determining foundation conditions, determining the lateral continuity of such conditions and identifying sub-seabed geohazards such as faults, shallow gas and weak layers. Sub-bottom acoustic profiling is a standard method for characterizing sub-seabed geology. This technique uses relatively high energy acoustic signals which penetrate into the seafloor and are reflected back from layers of different acoustic impedance May 2015

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(= velocity of sound x density of the medium) (Figure 3.8). In a manner similar to echo-sounding, the two-way travel time of the return signal is used to determine the depth below seafloor of the reflections. Sub-bottom profiling is typically carried out in advance of coring or sampling, so accurate site-specific information on the acoustic velocity of the sub-bottom is rarely available. Sub-bottom profiles may therefore be displayed using a two-way travel time scale or using an approximate depth scale based on an assumed velocity model for the sub-bottom.

Figure 3.8 Principles of sub-bottom profiling

Sub-bottom acoustic systems use acoustic sources in the audible range (frequency of 1 to 6000 Hz) and the choice of source is a compromise between resolution and penetration. As a rule of thumb, the higher the frequency, the higher the resolution, but the lower the penetration that can be obtained. The higher frequency, controlled waveform sources such as 3.5 kHz and chirp sonar are referred to as sonar systems whereas the lower frequency sources such as airguns, sparkers and water guns are considered to be seismic systems. For deeper penetration, higher energy seismic sources must be used, which generally requires a greater mobilization effort and therefore cost. In addition, higher energy sources bring a higher risk of impact on marine mammals so that regulatory requirements may be more stringent. Mosher and Simpkin (1999) provide a review of the different types of seismic sources and systems, including their advantages and limitations. These are briefly summarized below.

3.3.1 Acoustic systems 3.3.1.1 3.5 kHz profiler This controlled waveform source can be operated from a hull-mounted or side-mounted transducer. The technology has largely been replaced by chirp systems (see below) but the robustness of the older systems means that many are still in operation. The 3.5 kHz profiler is generally adaptable for use in a May 2015

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wide range of water depths through control of power and pulse length. The topside units typically consist of a transceiver unit, graphic recorder or monitor and data storage media. Analogue output from these systems may be digitized and archived in digital format.

3.3.1.2 Chirp sonar In chirp systems, instead of using a single frequency, the transmitting pulse sweeps over a range of frequencies and pattern recognition techniques are used to recognize coherent reflected returns. The result is that chirp systems can generally provide higher resolution for the same pulse length compared to single frequency systems. Figure 3.9 is an example of a chirp sub-bottom profile. There is presently a wide range of companies that supply chirp sub-bottom profilers, either as stand-alone systems or combined with sidescan sonar. These systems are typically supplied as complete packages with customized transceivers and digital recording and playback systems.

Figure 3.9 Chirp sonar profile from Manitounouk Sound, northern Québec, showing approximately 40 m of sub-bottom penetration through fine-grained sediments to a hard basal reflector interpreted as bedrock or glacial till. Unit 1 is a glaciomarine unit, while units 2 and 3 are marine basin-filling deposits.

Note: From Hill et al. (1999) © Canadian Science.

3.3.1.3 Boomers Boomers are electro-mechanical systems that impart a pressure pulse into the water through the electromagnetic separation of two large diameter metal plates. They typically operate in the 500 to 20 000 Hz range. This large bandwidth provides a balance between high power at low frequencies and relatively high resolution at the higher frequencies. Boomer systems require high voltage power supplies and the sources are towed behind the ship. The towfish or catamaran housing the boomer source may also contain a small internal hydrophone (receiver) array. Alternatively, or in addition, a May 2015

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larger external hydrophone array may be towed in an oil-filled “eel”. Topside hardware includes a firing box, a receiving unit and digital recording and playback devices. Figure 3.10 is an example of a boomer seismic profile.

Figure 3.10 Huntec Deep Tow (boomer) System profile from Ballenas Basin in the Strait of Georgia, BC showing almost 90 m of penetration through ice-contact and iceproximal glaciomarine deposits (Unit 2), distal glaciomarine deposits (unit 3), marine basin-fill deposits (unit 4), and overlying bedrock (unit 1).

Note: From Picard et al. (2006).

3.3.1.4 Sparkers Sparker systems use a high-voltage electrical arc to create a bubble of vaporized water which then collapses (Figure 3.11). This process generates a low-frequency pressure pulse with a moderate bandwidth in the 2000 to 5000 Hz range. Sparker systems can achieve greater penetration than boomers but typically have lower resolution (Figure 3.11). Topside hardware is very similar to boomers.

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Figure 3.11 Sparker seismic profile, St. Lawrence Estuary, Québec

Note: From Bellefleur et al. (2006).

3.3.1.5 Air guns Air guns are mechanical systems that release compressed air to create a sharp initial pulse followed by a few oscillatory pulses as the resultant bubble collapses. Air gun seismic sources provide high energy output at a predominantly low frequency, which achieves greater penetration through acoustically hard units than lower energy systems (Figure 3.12). The bubble pulse related to collapse of the compressed air bubble tends to produce unwanted artifact reflections that can confuse interpretation. Various configurations of air guns have been designed to shape the seismic source wave and minimize this problem. Generated injection (GI) guns, which contain a second chamber, are commonly used. The second chamber is released with a slight delay, which tends to dampen the bubble pulse. In addition to the airgun source, airgun systems consist of a compressor or air tanks, a long hydrophone array and topside electronics similar to other seismic systems. Airguns can be run as single units or as multiple gun arrays, depending on requirements. Large multichannel seismic surveys are often run with numerous airguns.

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Figure 3.12 Airgun seismic profile from the Strait of Georgia, BC. Profile shows more than 200 m of acoustic penetration through a unit interpreted as glacial till, which is typically coarse-grained and indurated.

Note: Courtesy of M. Riedel.

3.3.2 Field acquisition As with most geophysical data collected at sea, the quality of sub-bottom data depends on the design of the field study, the skill of the shipboard personnel and the conditions under which the survey was carried out. Data quality can be severely impacted by poor weather conditions that introduce excessive transducer and receiver motions. Interpretation of sub-bottom data depends greatly on the lateral continuity and coherence of reflectors. While swell compensation algorithms can be applied to data collected under moderate sea, larger motions degrade data coherency to the point where the data cannot be interpreted. Poor weather is a fact of life in marine surveys, so sufficient contingency time needs to be built into any survey plan. Sub-bottom surveys are best planned on the basis of existing bathymetric surveys. If the survey can be staged after a multibeam survey, the morphological information provided by the bathymetric survey can guide the planning of survey line orientation and spacing. It is often efficient to obtain chirp sonar data at the same time as a multibeam survey because it involves relatively little set-up. While chirp sonar data may not be ideal for characterizing the hard seafloors that are typically associated with marine renewable energy projects, the chirp data can nevertheless help to plan the choice of equipment and the survey design. Sub-bottom data are collected so that the stratigraphic interpretations can be integrated into the geological characterization of the study area. Surveys should therefore aim for comprehensive coverage of the area, typically by laying out an orthogonal grid of survey lines at a line-spacing appropriate for the geological setting. The orthogonal layout enables the correlation of reflectors by systematic checking at crossing points and delineation of the three-dimensional form of strata and the bounding surfaces between reflector packages.

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Seismic surveys with boomers, sparkers and airguns can be conducted using single channel or multichannel receivers. Single channel receivers or “streamers” combine the signal from numerous hydrophones in a short horizontal array whereas multichannel systems record signals from each hydrophone spaced in a suitable geometric pattern. Although more costly and complex to operate, multichannel seismic systems offer the advantage of advanced signal processing that allows for considerable improvement in the signal to noise ratio, reduction of unwanted artefacts in the data and thus clearer detailing of geological structures (Bellefleur et al., 2006).

3.3.3 Data processing The standard format for storing sub-bottom and seismic data is SEG-Y, developed by the Society of Exploration Geophysicists, SEG (Norris and Faichney, 2002). SEG-Y is an open standard and interpretations vary with usage. Commercial data acquisition and storage software associated with specific products may include significant variations of the format. However, most specialized data presentation and processing software packages are capable of reading and manipulating a wide variety of commercial SEG-Y formats. In Canada, existing analog sub-bottom and seismic profiles collected by the Geological Survey of Canada in the pre-digital age have been made available through NRCan’s GeoGratis server (http://geogratis.ca) in JPEG2000 format. These sets can be viewed in commercial JPEG2000 viewers or freeware. Software is also freely available to convert these large scanned images into SEG-Y format (at ftp://ftp.nrcan.gc.ca/ gsc/courtney/index.htm). Some data sets, when acquired by knowledgeable experts in the field, require little or no postacquisition processing and can be imported directly into seismic interpretation software packages. Site characterization sub-bottom data are often acquired in single-channel mode, whereas for deeper exploration seismic surveys multi-channel systems are used. Site characterization data are also generally much higher resolution than seismic data collected for oil and gas exploration. While many seismic processing and interpretation packages are designed for multichannel exploration and may therefore provide advance processing features for multichannel data that are not applicable to single channel subbottom data, most can be used for simple processing and visualization of high resolution sub-bottom profiles. A variety of data processing techniques are available for the optimization of the data quality for interpretation. At the simplest level, these are aimed at enhancing the signal to noise ratio and providing a clear image of the sub-bottom. A first order interpretation of sub-bottom profiles can yield information on the stratification of geological units below the seafloor. In addition, it is possible to analyze the amplitude, frequency and phase of reflection signals to obtain more refined physical information on the geological units. Details of processing techniques are beyond the scope of this Guide, but several reference books are available on the subject (e.g., Yilmaz, 2001). Information from acoustic/seismic interpretation can be incorporated into geological maps where they outcrop at the seafloor or individual units can be mapped independently. Seismic interpretation software can be used to track individual horizons, which can be compiled into customized maps useful for geotechnical investigations including maps of depth to particular horizons or deposit thickness (isopach) maps (Figure 3.13). Such maps are limited in resolution by the line-spacing of the survey.

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Note: From Josenhans and Zevenhuizen (1989) © Canadian Science Publishing.

Figure 3.13 Isopach maps of Quaternary formations on the Labrador Shelf

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4 Seabed sampling 4.1 Grab sampling Grab samplers are used to take surficial samples of the seabed and come in a variety of designs and sizes (Figure 4.1). Grab samplers take a somewhat disturbed sample of the bottom sediment suitable for grain size measurements or general characterization of the seafloor sediment, but are not generally suitable for making geotechnical measurements. The choice of grab sampler is very much dependent on the nature of the seafloor. Coggan et al. (2007) describe a number of such samplers (Van Veen, Ponar, Day and Hamon grabs) used for the purpose of habitat characterization. These types close passively using the wireline tension during the pull-up phase of the operation. They are typically suitable for relatively soft muddy substrates, but tend not to retain coarse sediments. One exception is the very large IKU grab, which requires heavy duty wire, A-frame and lifting capacity to operate (Figure 4.1). However, it is capable of taking samples in very coarse grained sediment including cobble lags and glacial till. For smaller samples of coarse-grained sediment such as sand and gravel, the GSC has had considerable success with the spring-loaded Shipek grab (Figure 4.1). This sampler weighs in excess of 50 kg and requires a mechanical winch and boom or A-frame to deploy.

Figure 4.1 Grab samplers commonly used in marine geoscience today.

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4.2 Coring Core sampling is used to obtain samples for stratigraphic, sedimentologic and geotechnical analysis. For these purposes, obtaining a core sample as undisturbed as possible is generally the objective. Generally, the higher the ratio between core diameter and the corer’s annular thickness, the lower the degree of disturbance. However, because of weight considerations, the dimensions of a corer are usually also a trade-off between length and diameter.

4.2.1 Gravity corers Gravity corers are relatively simple devices consisting of a weighted core barrel with a tapered cutter, one-way valve and core catcher (Figure 4.2). The core intrudes into a plastic liner, which is removed from the core barrel, capped and stored for later processing. Friction along the core margins and the pressure that builds above the core during penetration have the effect of compressing the core during the sampling process. Increasing the width of the corer can reduce this effect. Gravity cores are usually successful in unconsolidated mud but penetration can be low if the material is relatively consolidated or coarse grained.

Figure 4.2 Schematic of a gravity corer

Note: From Caleb McClennen, Sea Education Association, www.sea.edu.

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4.2.2 Piston corers Piston corers are similar to gravity corers but include a piston that is drawn through the core liner as it penetrates into the sediment (Figure 4.3). This requires a more complex set up with a trigger weight or corer, a trigger arm and a measured loop of wire between the two. Piston corers generally succeed in obtaining longer (10 m or more) and relatively undisturbed cores that are more suitable for physical property measurements than gravity cores. Piston coring is only possible from sizeable vessels equipped with an A-frame, crane and winches. Like gravity corers, they are limited to sampling fine grained, cohesive sediments. Whereas they are capable of penetrating thin sand beds, thick beds are generally disturbed and liquefied by the coring process. A particular issue with piston corers is that the bow wave of the corer approaching the seabed can flush away soft sediment so that core top may be missed in the sampling. Gravity corers are often used as trigger weights for this reason because their relatively slow penetration means that they generally do retain the core top. This can then be used as a crosscheck with the piston core.

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Figure 4.3 Left: Piston corer in preparation. Right: Schematic of piston corer operation. When the gravity corer hits the seabed, the tripping arm releases the piston corer to free fall to the seabed. As it penetrates, a piston is drawn through the core liner, minimizing the compressive stresses on the core.

Note: After Woods Hole Oceanographic Institution.

4.2.3 Vibracorers When coarse grained sediments are encountered, core samples may be obtained using vibracorers (Figure 4.4). These devices use a vibrating head to penetrate the corer into the sediment. The requirement of an electrical power supply and umbilical usually limits operations to relatively shallow water depths. Various designs are available, including those that retain the core in plastic liner. Other designs use a cement-mixer type vibrating head attached to an aluminium core barrel into which the core intrudes. For water depths greater than a few metres, the vibracorer needs to be supported by a tripod frame, so that the weight of the vibrating head does not tip the corer over.

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Figure 4.4 Geological Survey of Canada vibracorer.

4.2.4 Box corers Box corers are shallow penetration devices that obtain a large undisturbed sample of the top 50 cm or so of the sediment surface (Figure 4.5). They are only useable in soft muddy substrates. They may be used for detailed analysis of sedimentary structures (including burrow traces), benthic faunal analysis or sedimentation rate determination using Pb-210 or Be-7 dating. Generally large and weighty, they are limited to use on larger vessels with A-frames, cranes and winches.

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Figure 4.5 Box corer

Note: From Hannes Grobe, Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany.

4.2.5 Core storage Cores should be opened and physical properties measured at the earliest possible opportunity. On larger vessels, it may be possible to conduct measurements on board the ship, but if the cores need to be preserved and transported to shore they should be capped, tightly sealed and stored vertically in order to preserve physical properties. Standard practice is to use electrical tape to attach the core cap, and paraffin or beeswax can be used to provide an extra vapour seal. For long-term storage, cores should be kept in refrigerated storage below 4 °C, but not be allowed to freeze. Freezing the cores will cause the pore water to expand and alter the physical properties of the sediment.

4.2.6 Core processing and automated core logging Commercial laboratory systems are available that allow geophysical logging of the whole round core or of the split core, typically on a tracked system that automatically moves the core past the sensors (Weaver and Schultheiss, 1990; Holland et al., 2005; Figure 4.6). These data provide a permanent digital record of the core, whose properties and condition naturally deteriorate with time. The most common whole core measurements are P-wave velocity (useful for comparing core properties with sub-bottom or seismic profiles), gamma (137Cs) density (bulk density), and magnetic susceptibility (can reflect subtle May 2015

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variations of sediment provenance). Some systems also allow for 3-D X-ray computed tomographic (CT) imaging of the whole round cores.

Figure 4.6 Multi sensor core logger, Royal Roads University, Victoria, BC.

Gravity, piston and vibracorers are split lengthwise into two equal halves. Unless the core has been collected to provide material for whole round testing (e.g., consolidation or triaxial testing), one of the split halves should be preserved as an archive. The other half is designated as a working half from which sub-samples can be taken or destructive testing carried out. Once split, the half-core can be rescanned and subjected to line-scan imaging, colour spectrophotometry, natural gamma spectrometry, X-ray fluorescence spectrometry and near-infrared and visible spectrophotometry. If X-ray CT imaging is not conducted on the whole round core, the half cores or thinner slabs from the core can be X-rayed.

4.2.7 Core description A full description of the core lithology should be conducted immediately after the split core has been opened (and scanned if available). Ideally, core descriptions are completed with the geophysical and Xray data at hand. The core descriptions should record the important geological and geotechnical characteristics of the sediment. Professional engineering bodies provide standards for soil description (e.g., ASTM D2488-09a), which can be applied to marine sediments if the core has been collected for engineering characterization. Geological core description is not standardized and terminology differs significantly (and sometimes confusingly) from engineering practice. Geological description focuses on the stratigraphic and sedimentologic characteristics of the sediment. Table 4.1 provides an overview of the key elements of geologic core description.

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Table 4.1 Elements of geological core description Characteristic

Terminology

Bedding

Lithologic units: sharp, diffuse bed boundaries; bed thickness. Grading: fining up, coarsening-up

Colour

Use standard colour chart e.g., Munsell

Grain size

Use Wentworth scale; clay, silt, sand, gravel

Sorting

Well-sorted, poorly sorted

Sedimentary structures

Lamination, cross-lamination, ripple lamination, cross-bedding

Bioturbation

Burrows; ichnological species. Index of bioturbation

Carbonate content

Terrigenous, marl, carbonate ooze, bioclastic

Diagenetic features

Veins, concretions

Macrofossils

Bivalve shells: fragments, whole /articulated valves, orientation (life position or not); gastropods; brachiopods; sponge fragments and spicules; fish fragments, bones, scales; plant and wood fragments

Core disturbance

Soupy, voids, gas cracks, fractures, flow-in disturbance index

Geologic and geotechnical core descriptions can be displayed in graphic logs. Both freeware and commercial software are available for displaying descriptive data combined with core log data.

4.2.8 Geotechnical properties Split working half cores can be sub-sampled for a range of additional geotechnical measurements, including grain size, water content, bulk density, consolidation tests and triaxial tests. Shear vane measurements and other strength tests can be conducted on the working half. Standard ASTM engineering protocols should be followed for these tests.

4.3 Geochronology Geologic interpretation of Quaternary deposits is aided by absolute age dating of samples using radioisotopes. A variety of isotopes with different half-lives exist and the most commonly used are described below.

4.3.1 14C (Radiocarbon) 14C

has a half-life of 5730 ± 40 years. It is naturally present in the atmosphere and oceans and is therefore absorbed into organic tissue. Its presence in organic matter preserved in sediments makes it useful for age determination of Holocene and Late Pleistocene deposits (Arnold and Libby, 1949). The Radiocarbon website (http://www.radiocarbon.org/) is an important resource that includes information on the method, keeps up-to-date lists of active radiocarbon dating laboratories around the world and links to calculators for various corrections and calibrations. Care must be taken in selection of samples for radiocarbon age dating. In the past, bulk organic carbon diffusely present in marine sediment was used to obtain radiocarbon dates. However, such material is potentially subject to reworking and may contain a proportion of carbon much older than the depositional layer in which it is found and would therefore provide a date much older than the time of deposition. Standard modern practice is to isolate organic matter such as wood, shells or microfossil

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tests that can be reliably inferred to be formed in situ at the time of the deposit (e.g., a bivalve shell found in growth position in a core; a peat deposit formed in a marsh environment) or at least have had a short transport time compared to the age of the sediment (e.g., twigs and wood). Conventional radiocarbon age dating is carried out on samples weighing 50 g or more, but modern accelerator mass spectrometry (AMS) techniques can produce ages on samples as small as a few tens of milligrams. This allows, for example, for radiocarbon age dating of calcareous microfossils such as foraminifera. Care must be taken in handling the samples in order to avoid contamination from other organic materials, e.g., glue on mounting slides etc. Both conventional and AMS techniques are available at many laboratories including commercial laboratories. Various corrections can be applied to radiocarbon ages and it is important to be explicit about the convention being used when reporting dates. The conventional radiocarbon ages reported in most labs include a standard error, are referenced to 1950 AD and include the assumption that all C14 reservoirs have remained constant with time. All radiocarbon laboratories provide unique lab numbers to each radiocarbon date and it is standard practice to include this reference number when reporting the date. Ages measured on samples of marine organisms generally give ages that are several hundreds of years too old because the oceans represent a large carbon reservoir. A geographically variable correction is required for this “marine reservoir effect” (Stuiver and Braziunas, 1993). Databases and software are available for making this correction (http://www.radiocarbon.org/Info/index.html#intro). Considerable efforts have been made by radiocarbon researchers to calibrate radiocarbon ages against other methods, notably tree rings and corals (Reimer et al., 2004; Fairbanks et al., 2005). These researchers also provide calibration software to convert conventional radiocarbon ages to calibrated ages, expressed in calendar years (http://www.radiocarbon.org/Info/index.html#intro).

4.3.2 210Pb The use of 210Pb, which has a half life of 22 years, for age dating of marine sediments up to 100 years old has been carried out since the 1970s (Robbins, 1976). 210Pb results from radioactive decay of 214Po either in the sediments themselves or, more importantly, in the atmosphere and water flowing into the ocean. The relatively short half-life makes 210Pb useful for estimating sedimentation rates and contaminant fluxes. For site investigations, this technique can be important for determining background sedimentation rates or changes over the time periods over which MRE structures are deployed. The standard technique is to collect a box- or gravity-core and to measure 210Pb activity on a set of evenly spaced sub-samples from the core top down. Below a surface mixed layer, 210Pb activity will typically decrease down core and the sedimentation rate can be calculated from the slope of the log excess 210Pb activity vs. depth (Lavelle and Massoth, 1985; Lavelle et al., 1986; Johannessen et al., 2003). Similarly to radiocarbon dating, there are a number of academic and commercial laboratories that provide the appropriate analytical services for 210Pb dating.

4.3.3 7Be 7Be,

with a half-life of 53.3 days, is another air and water borne radioisotope that can be used to study sedimentation rates (Sommerfield et al., 1999). In this case, the very short half-life makes 7Be useful for identifying seasonal sedimentation events that have occurred within a few months of the sampling. It could therefore be applied to analysis of changes in sedimentation pattern due to emplacement of structures.

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The sampling technique is similar to that for 210Pb and, theoretically, sedimentation rates could be calculated if a sufficient number of samples were available. However, more typically sedimentation events are only a few centimetres thick and the 7Be activity drops off rapidly in underlying sediments.

4.4 Microfossil analysis Quaternary sediments may contain variable concentrations of microfossils that can provide proxy information on the environmental conditions at the time of deposition (Table 4.2) (Hillaire-Marcel and de Vernal, 2007). The presence or absence of particular species can indicate whether marine or nonmarine conditions existed at the time of deposition, or more commonly, the overall assemblage of species in a sample can provide an indication of surface or near bottom temperature and/or salinity. This can be useful for understanding the original depositional environment of a deposit (e.g., glaciomarine vs. marine), which in turn aids understanding of the physical properties and past history of the deposits.

Table 4.2 Common microfossils used in analysis of continental shelf cores Common Name

Classification

Description

Foraminifera

Protozoa

Planktonic and benthic forms. Calcareous tests. Benthic are more abundant in shelf sediments. Assemblage information can be used to provide environmental information e.g., temperature and salinity of bottom waters (Sen Gupta, 2002).

Dinoflagellate cysts

Protozoa

Occupy a wide range of marine environments. Cysts are resistant to dissolution and therefore well preserved. Transfer functions exist to interpret past T, S and ice conditions (e.g., de Vernal and Hillaire-Marcel, 2000; de Vernal et al., 2001).

Diatoms

Algae (Eukaryotes)

Marine and freshwater algae with silica frustules that are well preserved in sediments. Common phytoplankton in shelf waters with high productivity. Can be used as proxies for pH, temperature, salinity, nutrient concentration (Smol and Stoermer, 2010).

Pollen

Plants

Terrestrial plant pollen and spores transported to the marine environment by wind and water. Pollen assemblages generally assumed to be representative of the local vegetation cover and can be used for stratigraphic correlation within a limited region and paleoclimate interpretation.

4.5 Bottom photography and video Valuable information on seabed characteristics can be obtained with camera systems, especially where sampling is difficult, such as on hard, gravelly seafloor. Numerous configurations of bottom still and video cameras, as well as lighting options, are available. These include frame-mounted systems triggered automatically by a weighted trigger arm, video and still cameras mounted to grab samplers, remotely-operated vehicles (ROVs) and autonomous underwater vehicles (AUVs). Most systems capture images of the seafloor covering areas in the order of 1 m2 (Figure 4.7) and include either scale bars or scaled objects in the field of view. Images should be georeferenced.

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Figure 4.7 Bottom photograph taken on Amphitrite Bank, B.C., a site of potential wave energy, showing gravel cobbles and boulders. Compass and vane approximately 40 cm in length.

Typical uses of bottom photography include the analysis of bottom sediment grain size and benthic habitat classification. Images can be orthorectified through the use of laser spot patterns (Pilgrim et al., 2000) and automatically classified using digital image analysis techniques.

5 Geological mapping 5.1 Geological map flow This Section describes a typical work flow for producing geological maps from geologic and geophysical data. Some of the content is modified from an internal GSC report (Shaw et al., 2007). The general work flow for seabed mapping is shown in Figure 5.1. In many real world situations, geological maps are produced in incremental stages as more data become available. In these cases, the geological map flow represents an iterative loop where new data cycle through the map flow and more refined versions of the map are produced.

5.1.1 Data acquisition Multibeam bathymetry data provide the base maps for marine geological mapping but other types of geophysical, geological and oceanographic data may be compiled to aid interpretation of seabed geology. Details of the acquisition methods are described in Sections 2 and 3 above. Good data management practices, as described in Section 2 make compilation and integration of data sets an efficient and practical process.

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5.1.2 Data processing Raw multibeam data sets contain numerous erroneous values and artifacts that need to be removed both to avoid misinterpretation and to enhance the aesthetic quality of the maps. Appendices A and B of IHO (2008) provide detailed Guidelines for Quality Control and Guidelines for Data Processing of bathymetric data. As IHO standards are primarily designed to improve the safety of navigation, they are generally in excess of what would be required for geologic interpretation. If data are acquired from national agencies complying with IHO (2008), they can be considered adequate for defining seabed relief and construction of geological base maps. If the surveys are independently conducted, it is recommended that the data are processed to the IHO (2008) standard.

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Figure 5.1 Typical work flow for multibeam sonar mapping of the seabed.

Guidelines for QC and processing of multibeam backscatter data and other geophysical systems are not generally formalized. However, for all data types, several general principles should be observed: • Organizations should develop data processing protocols that define or conform to a set standard.

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• Original (raw) data should be conserved before processing and processed files should be conserved separately. • A record of data processing steps should be kept.

5.1.3 Data archiving All data used in geological map production should be archived and available for inspection by subsequent users. Refer to Section 2.9 for more details of data storage methods and to Section 2.5 for guidelines related to metadata. Complete records of metadata should accompany each data set and each map version.

5.1.4 Creation of imagery for the maps A variety of map products can be generated to summarize the geological conditions of a site but standard practice would be to produce the following as a minimum: (1) sun-illuminated (or shaded relief) seafloor topography; (2) backscatter strength; and (3) surficial geology.

5.1.4.1 Sun-illuminated seafloor topography Corrected bathymetry data are imported as x,y,z files into a GIS system and gridded. Multibeam data from water depths less than 100 m can usually be gridded at 5 m or less horizontal spacing. While fine grid scales can result in large file sizes to be manipulated, higher resolution provides more detailed geomorphic information. When data sets from different surveys are merged and compiled into a single grid, the data sets need to be normalized to a common datum. Most multibeam data (and all CHS data) are referenced to lowest low water, which is controlled and defined by tide gauge measurements. Where significant overlap between data sets (desirable) is available, most GIS systems provide mosaicing tools and typically a choice of methods for smoothing and averaging points. The more and larger the data sets to merge are, the more processing time is required. It is recommended to merge data in stages, merging groups of adjacent grids, rather than attempt to merge large numbers of grids. The gridded file is then converted into a shaded relief image. Illumination for the shaded relief image has two components: the compass direction of illumination and the angle above the horizon. Illumination should be from the northern hemisphere, i.e., 270° (W) to 90° (E) and is chosen to highlight geomorphic features. Illumination across strike is better than along strike for this purpose. In the past, illumination was along track lines to minimize the refraction artifacts. Today, data quality is higher and refraction artifacts are less pronounced. The angle above the horizon varies, but it is commonly 30° to 40°. Semi-transparent colour shading representing bathymetry can be overlain onto the shaded relief image. A variety of colour schemes are available in most GIS packages. The rainbow (Figure 5.2) and Haxby (Figure 5.3) palettes are effective and are commonly used. The colour scheme should be applied with histogram equalization, to maximize the change in colour across the greatest changes in water depth. Isobaths (or depth contours) can also be generated in GIS systems and layered over the shaded relief (Figure 5.4). A suitable isobath interval must be decided and should depend on the depth range in the map. Too many isobaths are a distraction whereas too few provide insufficient depth information. In some instances the interval between isobaths on a single map can vary from gently-sloping, shallow regions to steeply-sloping, deep regions. Before a contouring routine is applied, it is commonly useful to re-grid the DTM to a lower resolution in order to create smoother contours. Extracting contours using a high-resolution grid (especially in areas of low relief) can result in crenulated contours that distract the eye and detract from the general appearance of the map.

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Figure 5.2 The Rainbow palette, with depth represented according to the colour spectrum: red, orange, yellow, green, blue, indigo, and violet. Red is shallow water and violet is deep.

Figure 5.3 The Haxby palette colours contain more white compared to the Rainbow palette, making them more muted, or pastel, in appearance.

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Figure 5.4 Bathymetric contours superimposed on coloured shaded relief bathymetry (Boundary Pass, near Saturna Island, B.C.).

5.1.4.2 Images of backscatter strength When a backscatter strength grid has been created, the data are histogram equalized to optimize the binning of the grid cell data into colour intervals, and an appropriate colour scheme is applied. Backscatter strength is typically presented in either gray-scale or two-tone colour schemes (Figure 5.5). The coloured backscatter may then be combined with the gray-scale shaded relief from the elevation model as a transparent overlay, which has the advantage of relating backscatter variations to the topography (Figure 5.5). However, because the underlying shaded relief image uses shading to represent visually shadows, interpretation of subtle changes in backscatter intensity can be impeded by the shadowing. It is recommended that detailed interpretation should include examination of the nontransparent backscatter layer and discretion should be used in map presentation using transparent overlay.

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Figure 5.5 Depiction of backscatter strength. Top left: sun-illuminated seafloor topography; top right: backscatter for the same area; bottom left: backscatter in the ocean colour scheme; bottom right: backscatter in the ocean scheme draped over the gray elevation model. The result of step 4 is a greater appreciation of the correlation between backscatter and bathymetry.

One significant issue with respect to backscatter maps is that backscatter response is highly dependent on the multibeam system used and the exact system configuration, making it difficult to compare or merge data sets from different surveys. If data from more than one survey are used, the different surveys may have been conducted with different transducer settings so that the range of the raw data sets will be very different. To date, simple corrections for this problem have not been found and trial and error methods are used to minimize the aesthetic differences. Typically, the joins between different data sets remain visible. Care is required in interpreting backscatter variations across joins.

5.1.4.3 Surficial geology The surficial geology map is a representation of the geological character of the seabed based on geomorphology, surficial materials and sub-bottom stratigraphy, interpreted from geophysical surveys and sampling. There are several different approaches to presenting surficial geology depending on the objectives of the study, the richness of data available and the level of time and effort put into analysis. Several of the approaches require integration of sub-bottom data into a geological interpretation of the surficial units. From a surficial geology mapping perspective, only those units that outcrop at the seabed would be mappable. Because of Holocene sea level rise, the seabed may have been reworked during marine transgression, leaving a transgressive deposit of varying thickness. Judgement must be used whether to map the transgressive deposit or the underlying unit as the predominant seabed unit.

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5.1.4.3.1 Pragmatic approach In this approach features observed on multibeam imagery are delineated, described and interpreted in a relatively ad hoc fashion in order to illustrate particular points. For example, areas of gas venting or fields of subaqueous dunes can be shaded to specifically highlight them. The map may be accompanied by explanatory text and include illustrations such as 3-D views of the imagery or bottom photographs. An example of this map type is Fader et al. (2001). This is an effective way to add understanding to the bathymetric information. On the other hand, it does not lend itself to a systematic approach when dealing with diverse map regions.

5.1.4.3.2 Surficial materials approach This type of map shows the distribution of seafloor textures as revealed by sampling and groundtruthing by cameras, ROVs, sidescan sonar systems, etc. Textures can be presented in terms of mean or median grain size, or percentage of individual size fractions (e.g., % gravel, % sand, % silt, % clay). Depending on the data available, this kind of map can be more or less related to spatial geophysical data such as sidescan sonar or multibeam backscatter. Where sample station density is relatively high, spatial variations of grain size data can be presented as gridded and contoured or colour-coded raster data (e.g., Barrie and Currie, 2000). (See Figure 5.6.) This may be sufficient for indicating regional trends but is generally insufficient for site specific studies. Where multibeam backscatter or sidescan sonar data are available, these higher resolution mapping techniques should take precedence in delineating textural unit boundaries, with sample data being used as groundtruth to define the textural characteristics of the acoustic units.

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Figure 5.6 Grain size distribution in the Strait of Georgia, B.C. based on contouring of a dense network of bottom grab samples.

5.1.4.3.3 Formations approach In this approach, lithologically similar stratigraphic units or formations are mapped as surficially outcropping units. A geological formation is defined as “a body of rock strata… which is unified with respect to adjacent strata by consisting dominantly of a certain lithologic type or combination of types or by possessing other unifying lithologic features” (American Geological Institute, 1974). Mapping on Atlantic Canadian shelves by the Geological Survey of Canada (e.g., Fader et al., 1977, 1982; King, 1970; MacLean and King, 1971; Josenhans et al., 1986; Cameron and King, 2010) has resulted in formational frameworks that have regional validity (Table 5.1). Time equivalent formations on the Scotian Shelf and the Grand Banks have different names, being defined locally, as is common practice in land geology.

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Table 5.1 Quaternary surficial formations on the Scotian and Newfoundland shelves, and their acoustic character, material composition and genesis (after Fader and miller, 1986) Scotian Shelf

Grand Banks of Newfoundland

La Have Clay

Placentia Clay

Sable Island Sand and Gravel

Grand Banks Sand and Gravel

Sambro Sand

Adolphus Sand

Emerald Silt

Downing Silt

Scotian Shelf Drift

Grand banks Drift

Acoustic character and material interpretation Acoustically transparent mud: Pleistocene-Holocene basin deposits Acoustically stratified – incoherent sand and gravel: Transgressive deposits in water depth < 100 m Acoustically stratified fine sand: Sub-littoral deposit, below 100 m Acoustically stratified gravelly sandy mud: Glaciomarine sediment Acoustically incoherent gravelly sandy mud: Till

The advantage of using a formation framework is that mapped units have lithologic similarity and therefore are, at least to the first order, likely to have similar geotechnical properties. The disadvantages are that units defined in this way can be time transgressive and important variations in geotechnical properties may be missed. For example, a modern mud deposit (< 10 000 years BP) may overlay a mud deposited during the last interglacial period (> 120 000 years BP). Having similar lithologies, these two chronologically distinct muds would be classed as a single formation, but because of the intermediate period of glaciation, the latter may be much denser and overconsolidated compared to the former.

5.1.4.3.4 Genetic approach In this approach, depositional units with common interpreted genesis and depositional environment are distinguished and mapped. This approach is similar to the formations approach, but is not bound by formation nomenclature. Syvitski (1991a, 1994) developed a methodology for seismic interpretation of glaciated regions characterized by complex relative sea-level histories, dominated by isostatic rebound, and the sustained presence of ice in the coastal areas. Seismic stratigraphic units are defined as “successive intervals traced on the basis of acoustic attributes, bedding styles, and/or unit geometry” (Syvitski and Praeg, 1989). Syvitski’s recognition of more complex processes, and in particular, of paraglacial processes (paraglacial referring to a period when ice was close to the coast and relative sea level was dropping rapidly) is reflected in more complex stratigraphic sequences than on the banks and by an absence of formational names. Based on the vertical stratigraphic sequence of deposits from twenty of the world’s glaciated shelves, a complete deglacial sequence consists of some or all of the following: (1) ice-contact (icedeposited and/or ice-loaded sediments); (2) ice-proximal sediments; (3) ice-distal sediments; (4) paraglacial coastal sediments; and (5) post-glacial sediments (Table 5.2 and Figure 5.7). In this scheme there is no attempt to force the data into a formal formation-based stratigraphic scheme.

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Table 5.2 A genetic stratigraphic scheme, based on Syvitski (1991a, 1994) 1. Ice-contact sediments

Ice-loaded glaciomarine diamicton and till that may record the maximum extent of glaciation

2. Ice-proximal (outwash) sands, diamictons and muds

Sediments that reflect dynamics near an ice margin

3. Ice-distal muds and diamicton

Sediments that record the marine influence on sedimentation, hundreds of km from an ice margin

4. Paraglacial coast/nearshore gravels, sands and muds

Sediments that record the terrestrial ablation of ice sheets

5. Post-glacial sands/muds and gravelly sandy lags

Sediment that record modern ocean and terrestrial conditions away from the influence of ice sheets

Figure 5.7 Air gun seismic profile from Lake Melville, Labrador showing typical genetic units from a glaciated terrain: Ice-contact diamicton (1); ice-proximal sand and mud (3/4); ice-distal mud (2 and 10); paraglacial deltaic sand and mud (12); postglacial mud, sand and gravel (13).

Note: From Syvitski and Lee (1997).

5.1.4.3.5 Sequence stratigraphic (allostratigraphic) approach In land-based geological mapping, the traditional formation approach to stratigraphy has been largely replaced by “sequence-“ or “allo-stratigraphy” where depositional units are bounded by unconformity surfaces representing significant sea level changes within a time stratigraphic framework (Vail et al., 1977; North American Commission on Stratigraphic Nomenclature, 1983). Such units (“systems tracts”) represent discrete packages of sediment deposited under similar sea level conditions (i.e., highstand, lowstand, falling stage or transgressive). This approach is similar to the genetic approach in that it is based on the identification of seismic stratigraphic units but the emphasis is placed on recognizing bounding unconformities whose significance relates to sea level change.

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In shallow, high resolution sub-bottom/seismic interpretation, similar principles can be applied although the number of stratigraphic packages will be limited because of the relatively short period of time likely represented in the relatively shallow acoustic penetration. Furthermore, in Canadian offshore regions, the sea level history is complicated by the effects of glacial loading and the associated isostatic adjustment of the earth’s crust. Therefore, bounding discontinuities are related to relative sea level change factoring in both water level and crustal movements, which will be locally dependent on the timing and extent of glacial advance and retreat. Global (eustatic) sea level change can therefore not be inferred from bounding discontinuities in Canadian settings nor used as correlation horizons beyond the local region. Despite this caveat, the same methodology can be applied to high resolution seismic interpretation in the Canadian offshore. Some of the recommended interpretation techniques are summarized briefly below. The first step in defining acoustic stratigraphy is to identify stratal terminations, where one reflector terminates against another (Mitchum et al., 1977) (Figure 5.8). These include the following: • Erosional truncation – where a reflector terminates against an overlying erosion surface, usually identified from its relief.

Figure 5.8 Stratal termination terminology TOPLAP

(OVERLYING UNCONFORMITY)

OFFLAP

ON LAP

ONLAP DOWNLAP

TRUNCATION

(UNDERLYING UNCONFORMITY)

INTERNAL CONVERGENCE

Note: From Mitchum et al. (1977); courtesy American Association of Petroleum Geologists.

• Toplap – where an inclined reflector terminates against an overlying, lower angle surface without erosion, therefore interpreted to be sedimentary in origin. • Downlap – termination of a reflector onto an underlying surface where the dip of the underlying reflector is less than that of the terminating reflectors; typically represents progradation of the upper unit. • Onlap – termination of a reflector onto an underlying surface where the dip of the underlying horizon is greater than that of the terminating reflectors; typical of basin fill. These stratal terminations define packages of reflectors with a common depositional origin, which can be interpreted based on acoustic character and stratigraphic relationships. Acoustic character includes all attributes of the reflectors that fall within a single package, including reflector amplitude (strong to weak), geometry of stratification (transparent/unstratified, well-stratified; horizontal, dipping reflectors etc.; Table 5.3; Mitchum et al., 1977). These combined characteristics are used to define an acoustic or seismic facies, which can be interpreted in terms of their depositional environment or process of deposition (e.g., delta foresets; basin fill turbidites; glacial till). Because seismic facies represent distinct responses to the acoustic signal, they are likely to have distinct physical properties and so can be used to map geotechnical conditions in the sub-surface. Commonly, several contemporaneous seismic facies can occur laterally adjacent to each with either gradual or relatively abrupt transitions. These facies associations can be used to further interpret depositional environments. May 2015

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Table 5.3 Parameters used to define seismic facies (after Mitchum et al., 1977) Reflection terminations Lapout Baselap onlap downlap Toplap Truncation Erosional Structural Concordance (no termination)

Reflection configurations Principal stratal configuration Parallel Subparallel Divergent Prograding clinoforms sigmoid oblique complex sigmoid-oblique shingled hummocky clinoform Chaotic Reflection-free Modifying terms: even, wavy, regular, irregular, uniform, variable, hummocky, lenticular, disrupted, contorted.

External forms Sheet Drape Wedge Bank Lens Mound Fill

If bounding unconformities are present, they can be used to separate packages of seismic facies or (in sequence stratigraphy terminology) systems tracts that are intimately related to relative sea level changes (Catuneanu, 2006). The four primary systems tracts are as follows: • Lowstand systems tract (LST); • Transgressive systems tract (TST); • Highstand systems tract (HST); • Falling-stage systems tract (FSST) Catuneanu (2006) provides a detailed text book on the sequence stratigraphic method. A recurring problem has been non-standardization in approach and different terminology is used by different workers. Catuneanu et al. (2009) provide an attempt to standardize the approach and the terminology. From a site characterization perspective, these packages can be interpreted in terms of their age and depositional history. The sequence stratigraphic approach has been used most commonly on unglaciated continental shelves. For example, Allen and Posamentier (1993) describe the stratigraphy of the Gironde Estuary, identifying lowstand and highstand systems tracts bounded by disconformable surfaces related to falling then rising sea level. Tesson et al. (1993) describes stratal patterns in seismic reflection profiles from the outer shelf of the European continental margin. In this example, discrete wedge-shaped sediments are bounded by disconformities resulting from cyclic changes of sea level over the late Quaternary. In southern Canadian continental shelf settings, it is common to observe a transgressive flooding or erosion surface related to the transgression that occurred after the Late Wisconsinan (last) ice sheet retreated between 21 ky and 12 ky before present. This surface typically separates late Wisconsinan lowstand deposits, including glacial and non-marine deposits, from younger marine deposits. Although it is rare for high-resolution seismic systems to resolve older deposits related to previous glacial and sea level fluctuations, there are examples, e.g., in the St. Lawrence Estuary, where a more complex stratigraphy can be resolved. In glaciated regions, sea level changes are complicated by the effects of glacio-isostatic depression and rebound. Relative sea level may vary over relatively short length scales adding complexity to the interpretation. Recognition of bounding unconformities that relate to sea level is nevertheless important information and can aid in the interpretation of surficial deposits.

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5.2 Seascape mapping The advent of multibeam sonar as a geological mapping tool in the mid-1990s led to a new approach to seabed mapping, resulting in maps of 1) shaded seafloor relief; 2) backscatter strength; and 3) surficial geology. Because geologists had long been aware of the links between surficial geology and benthic communities (Maritime Testing, 1985), the multibeam sonar revolution provided an opportunity to go one step further, and illustrate that link in the form of a ‘Seascape’ map.

5.2.1 Definition of a seascape The definition of a seascape is based on the Australian Land-System approach, developed to manage agricultural land. To adequately understand the land and its use and management it was thought necessary to understand the relationships between soils and the soil parent materials, climate, topography, and the influence of different kinds of management that derive from the uses to which the land may be put. Land-systems are “areas, or groups of areas, throughout which there is a recurring pattern of topography, soils, and vegetation” (Christian and Stewart, 1953). A good example of the land system approach is the mapping of Bougainville and Buka Islands, Territory of Papua and New Guinea, by Scott et al. (1967). By analogy, the definition used here is as follows: “Seascapes are underwater landscapes with recurring patterns of geomorphology, texture, and biota.”

5.2.1.1 Geomorphology The geology of the sea floor integrates geological processes over time and tends to determine the distribution of textures and biota within a region. Geomorphic units are delineated through analysis of shaded-relief elevation models derived from gridded multibeam bathymetry data. This reveals relief and slope characteristics with a vertical accuracy of ± 0.1 m. Analysis should be performed on the digital elevation model (DEM) and not on a .tiff or .jpg image. The DEM should be analyzed by use of shading from varying azimuths, deriving cross sections, creating slope maps, etc.

5.2.1.2 Texture While texture conventionally refers to grain size at the seafloor, we expand the definition to include properties at depth. A critical component of delineating texture is the backscatter strength map, a loose proxy for sea-floor texture. Commonly this parameter is bi-modal: areas of high backscatter strength correspond to bedrock and gravel at the sea floor, and areas of low backscatter strength correspond to sand, muddy sand, and mud. While backscatter strength is thus an imperfect proxy, it is relatively easy to sub-divide the backscatter signal by considering the geomorphology. For example, an apparently uniform area of backscatter may be sub-divided into areas with low relief imprinted by iceberg scours (glaciomarine mud) and areas of high, rugged relief (bedrock). It is necessary to understand sediment properties at depth, particularly for engineering applications. This is particularly important where winnowing has created surface lag veneers unrepresentative of properties at depth. Sub-bottom profiler data are critical to the seascape approach. Data may already be available for a region, but more often, and preferably, targeted surveys yield the necessary information. In Canadian settings, surficial units have well-defined characteristics: postglacial mud and sandy mud (acoustically transparent); glaciomarine mud (strong acoustic stratification in a draped style); and ice-contact sediment (acoustically incoherent). Grab sampling and cores provide groundtruthing of the acoustic data.

5.2.1.3 Biota Unlike in habitat mapping, the processes of assessing the biota has not been systematic in the seascape approach, and indeed, the approach assumes that geomorphology and texture are the dominant determinants. However, there are potential cases where the biotic component may be the more May 2015

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determinant factor for defining a seascape unit. Examples would be where the seafloor is occupied by sponge reefs, as in some areas of British Columbia coastal waters, or oyster beds, in shallow water estuaries. While acoustic mapping methods can be used to identify some biotic communities such as sponge reefs, sea floor photography and video are the ideal tools for determining the biological characteristics of the geomorphic/textural classes. In general hard, immobile substrates have attached biota, in contrast to sandy and muddy seafloors that may contain infauna.

5.2.1.4 Interpretation Because seascape units are defined primarily on geomorphology and texture, it is usually possible to place a genetic interpretation on the unit. This interpretation therefore integrates the genetic approach described in Section 5.1.4.3.4. In Canadian waters, this typically includes recognition of units deposited under glacial, paraglacial or post-glacial conditions.

5.2.2 Example of a seascape map An example of a seascape map is shown in Figure 5.9. Placentia Bay, a large embayment on the south coast of Newfoundland, was covered by a series of five 1:50 000 scale maps of seafloor shaded relief and backscatter strength. The entire region was depicted in a 1:250 000 scale seascape map containing 15 seascape units, comprised of two principal groups: five sub-littoral seascapes and eight deep water seascapes in addition to two anthropogenic seascapes (dredged areas and spoil areas). The seascape units were delineated in a Geographic Information System (GIS) and polygons created. These were draped over the shaded relief model for the bay, derived from a 10 m grid of the multibeam sonar data. The processes that condition the sea floor were different in shallow water verses deep, hence the grouping of the seascapes into the two main classes. Figure 5.10 shows the legend for the sub-littoral seascapes. Each seascape is described in terms of morphology, texture, and biota. A short piece of commentary (italics) summarises genesis of the seascape unit, outlines modern processes, or adds information about textural properties.

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Figure 5.9 Seascape map for Placentia Bay, Newfoundland. In addition to classifying the sea floor, the map contains images of bathymetry, backscatter, current circulation, and explanatory text.

Note: From Shaw et al. (2011).

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Figure 5.10 Extract from Open File 6683 showing legends for the five sub-littoral seascapes in Placentia Bay.

5.3 Seabed habitat mapping Marine habitat mapping consists of “plotting the distribution and extent of habitats to create a map with complete coverage of the seabed showing distinct boundaries separating adjacent habitats” (MESH, 2008). The term “habitat” is defined as “…both the physical and environmental conditions that support a particular biological community together with the community itself” (MESH, 2008). The May 2015

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mapping of marine biological communities evolved from the longstanding practice of mapping the geology of the seabed (see Section 5.1). Early examples of marine habitat mapping were based on acoustic data like sidescan sonar (McRea et al., 1999) and multibeam sonar (Kostylev et al., 2001) but incorporated biological information gleaned from seafloor photography and videography and sediment sampling. Recognition that the boundaries of seafloor habitats are often gradational in nature has led to an alternative definition of marine habitat mapping: “The use of spatially continuous environmental data sets to represent and predict biological patterns on the seafloor (in a continuous or discontinuous manner)” (Brown et al., 2011). Marine habitat mapping is undertaken for a number of reasons, including (1) to support government spatial marine planning, management, and decision making; (2) to support and underpin the design of marine protected areas; (3) to conduct scientific research programs aimed at generating knowledge of benthic ecosystems and seafloor geology; and (4) to conduct living and non-living seabed resource assessments for economic and management purposes (Harris and Baker, 2011). Marine habitat mapping is carried out worldwide, with some countries embracing national seafloor mapping programmes, such as Norway (www.mareano.no), Ireland (http://www.marine.ie/Home/sitearea/areas-activity/seabed-mapping/seabed-mapping), and the countries bordering the Baltic Sea (www.balance-eu.org). Providing a forum to advance marine habitat mapping is GeoHab (www.geohab. org), an international organization of marine scientists conducting research using a range of mapping technologies and techniques in the study of benthic habitats and ecosystems. The marine habitat mapping scientific literature has blossomed over the past decade, with an early fisheries-focused compendium of Barnes and Thomas (2005) being superseded by GeoHab-related publications including Todd and Greene (2007), Heap and Harris (2011) and Harris and Baker (2011). Each of these volumes contains a wealth of examples of habitat mapping around the globe.

5.3.1 Habitat mapping schemes There is no universally recognized and accepted habitat mapping scheme. Over the past two decades, hierarchical habitat classification schemes have been devised (Table 5.4) for the management of the seabed and for planning seabed use. This nested scale approach enables mappers to design effective habitat mapping programs by choosing the appropriate suite of survey equipment for the spatial task. For example, vessel size and equipment (and cost) would be quite different for mounting a one-day survey of a small coastal inlet versus a multi-month regional deep ocean survey. The reader is referred to the references in Table 5.4 for detailed descriptions of the marine habitat classification schemes.

5.3.1.1 Examples of map products The aforementioned volumes contain a broad spectrum of habitat maps. There are almost as many types of habitat maps as there are authors. Only two habitat maps are reproduced here to highlight how different spatial scales can be mapped. German Bank is located off the southern end of Nova Scotia on the Scotia Shelf in the eastern Gulf of Maine. Multibeam sonar bathymetric data were collected over 5320 km2, followed by sediment samples, seafloor imagery and analyses of biological data, including bycatch of Fisheries and Oceans Canada research surveys (Todd and Kostylev, 2011). The benthic habitat map of German Bank is shown in Figure 5.11. Oceanographic factors which were found important for structuring benthic fauna on the bank (oxygen saturation, temperature variability, water stratification and chlorophyll-a concentration) are strongly related to water depth. These factors vary smoothly over the bank and it is difficult and May 2015

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meaningless to use the gradients in each of them for boundary definition at the scale of this study. The approach taken for German Bank was to designate the water depth of 100 m as an approximate boundary which separates high from low values for all of these oceanographic factors. These two depth classes, termed shallow and deep, were combined with the four seabed geology classes of bedrock, till, mud and sand to produce the resulting eight habitat types which were found relevant to benthic fauna on the bank.

Table 5.4 Comparison, on the basis of length scale, of hierarchical marine habitat classification schemes.

Note: Modified after Harris (2011).

This map of the distribution of habitats on German Bank provides a template for mapping and managing the bank’s habitats. Knowledge of the life histories and ecology of individual species combined with historical fisheries data could be used with substrate characteristics to predict locations on the bank that favour the reproduction and survival of species. These predictions would be a valuable tool for developing research and management strategies for the bank’s environments.

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Figure 5.11 Benthic habitat map of German Bank, Gulf of Maine, off southwest Nova Scotia.

On a much more limited spatial scale (< 80 km2), Stanton Bank, located between Scotland and Northern Ireland, was studied using multibeam sonar, photographic and videographic imagery, and sediment samples (McGonigle et al., 2009). Moving toward an objective approach to habitat mapping, statistical analyses using commercially available software produced a habitat map with six classes (Figure 5.12). Habitat A is characterised by bioturbated muddy substrata containing burrowing megafauna. Habitat A* is typified by a higher sand content than A, although the two have similar faunal assemblage structures. Habitat A** is coarser grade of unconsolidated sediment than A and A*, and it supports characteristically different infaunal assemblages. Habitat B is mixed substrate, dominated by soft sediment interspersed with pebbles and cobbles and occasional boulders and bedrock outcrops. Habitat C is characterised by the predominance of bedrock and boulders with associated epifaunal community assemblages, most notably deep sponge communities. Habitat D is typified by hard substrates dominated by mixed boulders and cobbles, with assemblage diversity dominated by deep sponge communities similar to habitat C.

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Figure 5.12 Benthic habitat map of Stanton Bank, Irish Sea.

6 Geohazard analysis 6.1 Principles of hazard and risk analysis A variety of natural geohazards are present in the marine environment and can affect marine renewable energy installations. An assessment of the risk associated with these natural geohazards is therefore important from both economic and environmental perspectives. Hazard analysis is the first stage of risk analysis and involves identifying the natural hazard phenomenon and mechanism, the location of the hazard and the probability of occurrence of the hazard event. While hazard analysis can be limited to determining the worst case scenarios, risk mitigation planning, design standards and insurance pricing all require a more probabilistic approach (Sorenson et al., 2012). The hazard analysis is followed by a consequence analysis (an assessment of the elements at risk and their vulnerability) and an integrated risk analysis (that weighs the probability of hazard occurrence against the consequences). Hazard analysis consists of identifying the following: • The location, timing, magnitude and severity of past occurrences of the hazard. Aspects of this are most commonly achieved through geologic mapping of the seabed and identification of the geomorphic manifestation of the phenomenon. • The frequency of past occurrences. This can usually only be inferred from a detailed local study and understanding of the phenomenon. • The probability of a future event of a certain magnitude to occur. • The local site effects that may amplify the effect of the event on a structure. May 2015

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In this Section, some of the principal geohazards present in the marine environment are described and some indications of how to assess the frequency of occurrence and local site effects are given.

6.2 Earthquake hazard Earthquake shaking represents a potential hazard to marine renewable energy infrastructure through the displacement of energy conversion devices, disturbance of the foundation conditions (e.g., through liquefaction) and stresses on transmission cables and connectors. The CSA Group (2008) standard for offshore structures (CAN/CSA-S471-04) requires a “probabilistic evaluation of expected ground motions”, evaluation of the “potential for amplification of ground motions by soil deposits” and evaluation of the “potential for related earthquake effects” such as liquefaction and slumping. Earthquake hazard assessment is focused on the ground motions (shaking) associated with an earthquake and should include both a characterization of potential earthquake sources, seismic wave propagation and the local ground conditions that might amplify the incoming seismic waves.

6.2.1 Seismic source characterization Source characterization involves documentation of the location and magnitude of past earthquakes in the region of interest making use of nationally held earthquake catalogues that store data on past earthquake events. Natural Resources Canada, through the Geological Survey of Canada supports a network of seismometers that detect earthquake events. Earthquakes are located and published on a publicly available database through the internet (http://www.earthquakescanada.nrcan.gc.ca/). The earthquake database includes historical earthquakes that date back to 1627 and a map of their distribution (Figure 6.1) providing a direct indication of earthquake occurrences around the country. The Earthquakes Canada website also makes tools available to customize map products and event listings for client specified regions or time periods. GIS projects can be linked to database available on the Earthquakes Canada website so that updated information is always included.

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Figure 6.1 Historical seismicity in Canada.

Note: From Earthquakes Canada website.

6.2.2 Ground motion characterization Earthquake ground motion at a particular location depends on the location and magnitude of the earthquake and the geologic substrate through which the seismic waves travel. Some geologic conditions can cause amplification or de-amplification of the seismic waves, but the degree is dependent on the wave period and the strength of shaking. Resonance effects can cause structures to vibrate violently at particular frequencies. Information on shear wave velocities and layer thicknesses of sub-seabed geological units are required to characterize potential ground motions. Boore (2006) has summarized the most commonly used methods for characterizing shear wave velocities. These include invasive methods such as borehole and seismic cone penetrometer measurements and non-invasive methods that involve the placement of arrays of recording instruments on the seabed (Table 6.1). Molnar et al. (2007) provides a comparison of some of these methods.

Table 6.1 Methods to determine subsurface shear-wave velocity Source: Molnar et al., 2007; after Boore, 2006). Invasive methods

Non-invasive methods

a)

a) b)

b)

Surface source i) Receiver in borehole ii) Receiver in cone penetrometer (SCPT) Downhole source i) Suspension PS logger ii) Crosshole

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Single station (h/v) Multiple statioms i) Active sources (linear spread of receivers) 1) SASW 2) CSWS /MASW (Multiple array) ii) Passive sources (2D array of receivers) 1) Frequency-wave number (FK) 2) Spatial autocorrelation (SPAC) 3) ReMi (receivers in line) iii) Combined active and passive source

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The National Building Code of Canada (2010) contains guidelines for the design of earthquake resistant buildings and uses a series of National Earthquake Hazard Maps (e.g., Figure 6.2) and ground motion parameters for selected cities based on data and research by the Geological Survey of Canada. The Earthquakes Canada website provides access to these maps and to ground motion parameters for creating design spectra for any location in Canada. The National Building Code of Canada (2010) uses the time-averaged shear-wave velocity to 30 m (VS30) as a practical metric for classification of sites with respect to potential amplification or de-amplification. However, the frequency dependence of seismic amplification and strength of shaking represent large uncertainties when this relatively simple parameter is used.

Figure 6.2 Seismic hazard map (Spectral acceleration for a period of 0.2 seconds at a probability of 2%/50 years for firm ground conditions (NBCC soil class C)). Spectral acceleration is contoured in g.

6.2.3 Mapping active faults Given the relatively short record length (in geological terms), the assessment of the hazard of earthquakes with return periods of more than 50 years is difficult. In some locations such as the Cascadia margin off the west coast of Canada and the U.S., geological records of tsunami deposits and event deposits in marine cores have been used to estimate great earthquake return periods. Such information should be taken into account where available. Particular attention in site investigations should be paid to the identification and mapping of faults that either intersect the seafloor or are visible in sub-bottom seismic records. Faults represent the location of potential slip planes, which can generate earthquakes. The presence of such features at a site indicates the possibility of an earthquake with a potentially long return period. If the displacement of the fault is close to the seabed, even a smaller earthquake could be quite damaging. An example of such an earthquake is the 6.3 ML earthquake close to Christchurch, New Zealand on February 22, 2011. In this case the earthquake was located within 5 km of the surface and vertical ground-motions exceeded 2g at some locations (Bradley and Cubrinovski, 2011). The event occurred on a previously unrecognized fault with limited historical seismicity indicating that the return period of such an earthquake would have been in the order of thousands of years. An example of how active faults can be identified on the seabed is given in Barrie and Hill (2004). In this case, linear arrangements of gas escape features (pockmarks; see Section 6.4) and escarpments were May 2015

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visible in multibeam sonar bathymetry maps (Figure 6.3). Examination of sub-bottom seismic profiles crossing these features showed that the seabed and underlying Holocene sediments were displaced by a fault. Consideration of potential fault movement and sedimentation rates, the age of the fault was estimated at less than 1500 years. Detailed investigation of faults mapped at a site can therefore yield more precise information about the timing of past movement on the fault.

Figure 6.3 Shaded relief multibeam image from the Strait of Georgia showing linear chains of pockmarks that are related to underlying fault structures.

6.2.4 Earthquake hazard analysis A specific methodology, termed probabilistic seismic hazard analysis (PSHA), has been developed for valuating seismic hazard (Cornell, 1968; McGuire 2004). PSHA is “a methodology that calculates the likelihood that some measure of earthquake ground motion will be exceeded at some site during some specified time interval in the future” (Hanks et al., 2009). The methodology involves characterization of the seismic source information, as described above in Section 6.2.1, and characterization of the ground motion effects in terms of the propagation and amplification of earthquake ground motion. The latter requires knowledge of the sub-seabed geology because seismic amplification is a function of layer thickness and seismic velocity. Large uncertainties exist in quantifying the required input information and in the United States, Senior Seismic Hazard Analysis Committee (SSHAC) (1997) provides one set of guidelines for this quantification. Hanks et al. (2009) evaluate and discuss the implementation of these guidelines.

6.3 Tsunami hazard The documentation in recent years of the devastating effects of tsunamis in coastal areas emphasizes the need to evaluate the potential for a damaging tsunami at any coastal site. Tsunamis are long period, low amplitude surface waves that are generated by sudden displacements of the seabed associated with large earthquakes or submarine slides. Leonard et al. (2010) compiled an extensive bibliography of May 2015

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the tsunami hazard literature and included an overview of the tsunami hazard in Canada. The amplitude or height of a tsunami wave is highly dependent on water depth. Although tsunami waves have amplitudes typically in the order of tens of centimetres in deep water, they increase in height as they shoal into shallow water and are affected by the shape of the coastline. Severe tsunamis can reach heights of many metres and form a very high energy surge that can run up onto the shore and inland for many hundreds of metres or even kilometres depending on the local topography. This water then flows back into the sea, typically entraining enormous amounts of debris from flattened buildings, vehicles and other materials. Coastal and nearshore infrastructure exposed to potential tsunamis are at risk to damage from flooding and from the surging wave and debris as well as from strong, complex currents in the nearshore. Similar to other natural hazards, an evaluation of tsunami hazard can be conducted through knowledge of potential tsunami sources and the analysis of past events to put it into a probabilistic framework (e.g., Leonard et al., 2014). The following sources of tsunami should be considered:

6.3.1 Earthquake sources Large subduction zone earthquakes such as the M=9 Tohoku (Japan) earthquake (March 11, 2011) can generate significant tsunamis. Canada’s Pacific coast is exposed both to tsunamis originating on the Cascadia subduction zone, located immediately off the west coast of Vancouver Island, to tsunamis originating off the west coast of Haida Gwaii and to far-field earthquakes on other circum-Pacific active margins. Studies of tsunami deposits along the coastline of British Columbia, Washington and Oregon suggest an average recurrence interval for Cascadia earthquakes of 500 years and individual recurrence intervals of 200 to 800 years (Atwater et al., 2005; Goldfinger et al., 2010; Leonard et al., 2010). The last great earthquake on the Cascadia margin was in 1700 (Satake et al., 1996). The last great earthquake off Haida Gwaii was in 2012 (James et al., 2013). Tsunamis from far-field megathrust earthquakes can also be significant, depending on distance from the source and propagation direction. A tsunami originating from the M=9.2 1964 Alaska earthquake caused significant damage in Port Alberni, B.C. (White 1966; Thomson 1981). Numerical modeling simulations indicate that a similar-sized earthquake in the Caribbean could cause damaging tsunami waves along the Atlantic coast of Canada (Knight, 2006; Atlantic and Gulf of Mexico Tsunami Hazard Assessment Group, 2008; Leonard et al., 2014). Shallower crustal earthquakes can also lead to seafloor displacements and tsunamis. Because their frequency of occurrence is relatively low, there are no well documented cases of earthquake generated tsunamis from shallow crustal faults, although a local earthquake generated a tsunami in Puget Sound about 1000 years ago (Atwater and Moore, 1992) and the M=7.3 1946 earthquake on Vancouver Island triggered submarine slope failures that generated local tsunamis (Hodgson 1946; Rogers and Hasegawa 1978; Mosher et al., 2004).

6.3.2 Landslide sources Numerical modeling of submarine landslides suggest that large submarine landslides are capable of generating significant tsunamis (Synolakis et al., 2001; Rabinovich et al., 2003; Fine et al., 2005). Because the slides are often triggered by earthquakes, the submarine slide origin is often obscured until detailed analysis is undertaken. The M = 7.2 1929 Grand Banks earthquake triggered a large submarine slide that generated a tsunami along the southern Newfoundland coast (Fine et al., 2005). On the west coast of Canada, a 1975 submarine slide in Kitimat Arm not related to an earthquake generated an 8 m tsunami (Murty 1979; Skvortsov and Bornhold 2007). Subaerial rockslides that slide into the ocean can also cause devastating tsunamis (Bornhold et al., 2007).

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6.3.3 Evidence of past tsunamis The sedimentary deposits of tsunamis have been found in coastal marshes and lakes (Atwater, 1987; 1992; Clague and Bobrowsky, 1994a,b; Clague et al., 1999; 2000). These deposits can be used to determine a chronology of past events. Clague et al. (2000) describe the criteria used to identify tsunami deposits (Figure 6.4).

Figure 6.4 Criteria for identification of coastal tsunami deposits

Note: After Clague et al. (2000).

6.3.4 Evaluating tsunami hazard Knowledge of the exposure of a site to sources of tsunami generation and the record of past tsunami events can be combined to evaluate the probability of a tsunami hazard. Numerical modeling of tsunami propagation can be used to assist evaluation of the site exposure. In the absence of direct evidence for past tsunamis, modeling can be used to evaluate the potential hazard from specific earthquake or landslide events (e.g., Rabinovich et al., 2003; Fine et al., 2005). Probabilistic methodology can be applied to evaluate the hazard (Geist and Parsons, 2006; Power et al., 2007; Thio et al., 2007; Sorensen et al., 2012; Leonard et al., 2014). Geist and Parsons (2006) provide an overview of probabilistic methods. Leonard et al. (2014) have evaluated both local and distant tsunami sources and produced the first probabilistic national tsunami analysis for Canada. Power et al. (2007) May 2015

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describe a methodology applied to a national hazard assessment in New Zealand. Sorensen et al. (2012) describe a similar method for the Mediterranean Sea. These authors use the existing catalogue of historical earthquake events to generate a synthetic catalogue of events for a statistically robust time period, from which they in turn generate tsunami propagation models. The resulting database was then used to generate site specific probabilities of tsunamis of different sizes, regional maps showing the distribution of the tsunami hazard and specific travel times of potential tsunamis at different sites.

6.4 Gas venting hazard Overpressured gas generated in the sub-surface can escape through the seabed and in some cases cause disruption of the seabed in the form of crater-like pockmarks (King and MacLean; 1970; Hovland and Judd, 1988; Hovland et al., 2002; Judd and Hovland, 2007) or mud volcanoes (Paull et al., 2007, Milkov, 2000). In most cases on continental shelves, the gas is a mixture of methane, higher hydrocarbons and hydrogen sulphide, which can be explosive and/or noxious and therefore represent a safety hazard where a structure or drilling related to structure emplacement provides a conduit for the gas to reach the atmosphere. Whereas emplacement of renewable energy converters does not involve the inherent risks associated with oil and gas drilling activities, it is still important to identify and avoid areas of potential gas venting. The presence of gas in the sub-bottom creates overpressures that can be released when disturbed.

6.4.1 Identifying subsurface gas The presence of gas in the sub-surface can be determined through detailed analysis of seismic reflection profiles and particularly seismic reflection attributes (Schumacher and Abrams, 1996; Avseth et al., 2005; Chopra and Marfurt, 2007). The presence of high amplitude anomalies or “bright spots” has long been used in seismic exploration as an indicator of gas presence. The negative acoustic impedance contrast between an overlying layer and a gas-bearing, low velocity layer gives a strong negative amplitude anomaly followed by relatively high absorption of higher frequencies. In high resolution seismic profiles, shallow gas typically appears as a diffuse reflection that masks underlying stratigraphy (Figure 6.5). Subsurface gas reflectors are sometimes seen as vertical plumes that approach or intersect with the seabed and may be directly related to a gas venting structure at the seabed (Figure 6.5).

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Figure 6.5 Gas as a diffuse reflector in high resolution seismic profile, Strait of Georgia. Vertical “plumes” of gas are visible approaching the seabed beneath a pockmark.

6.4.2 Identifying gas venting Gas venting can be difficult to identify in areas where bedrock outcrops or there is little sediment cover. However, because gas tends to escape through faults and fractures in the rock, such features should be examined in more detail if they are visible in multibeam or sidescan sonar imagery. Where there is substantial sediment cover, gas venting is indicated by two types of features: seabed pockmarks and mud volcanoes. However, gas venting can also occur when there is no macro-scale seafloor expression (Judd and Hovland, 2007).

6.4.2.1 Pockmarks Seafloor pockmarks are conical crater-like depressions present on the seafloor in many regions of the world (Figure 6.6). They were first discovered in post-glacial sediments of the Scotian Shelf by King and MacLean (1970) and have since been documented in estuarine, continental shelf and deep sea environments around the world (Hovland and Judd, 1988). Globally, they occur in water depths from < 20 m to > 4000 m, have diameters ranging from 1 to 300 m and can be up to 30 m deep (Hovland and Judd, 1988). They are found on both sandy and muddy bottoms in both glaciated and non-glaciated regions, commonly associated with methane and/or higher hydrocarbon gas seeps, carbonate hardgrounds and bioherms. Pockmarks have been convincingly linked to fluid escape at the sea bed (Hovland et al., 2002). Many of the occurrences appear to be associated with shallow methane gas generated in situ by bacterial reduction of organic matter (e.g., Kelley et al., 1994). However, others have been shown to be related to hydrocarbons of deeper, thermogenic origin or groundwater (Whiticar and Werner, 1981). The physical mechanism for escape of the fluids is less well understood (Hovland et al., 2002). Several reports have noted the association between pockmarks and earthquakes (Clifton et al., 1971; Field and Jennings, May 2015

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1987; Hasiotis et al., 1996). Barrie and Hill (2004) have noted the association between pockmarks and active seabed faults (Figure 6.3).

Figure 6.6 Shaded relief multibeam image of English Bay, at the mouth of Vancouver Harbour, B.C. showing a spectacular field of more than 200 pockmarks ranging from 15 to 100 m in diameter and from 5 to 15 m in depth.

Proving the presence or absence of pockmarks requires full bottom coverage multibeam or sidescan sonar images. Grids of widely-spaced chirp sonar or seismic profiles may be insufficient to observe them. However, sub-bottom profiles across pockmarks are invaluable for evaluation of the gas venting hazard.

6.4.2.2 Mud volcanoes Mud volcanoes are defined as “topographically expressed seafloor edifice(s) from which mud and fluid (water, brine, gas, oil) flow or erupt” (Milkov, 2000). Other terms such as mud mounds, mud diapirs and “pingo-like features (PLFs)” (Paull et al., 2007) are used to describe similar features. Mud volcanoes are also best documented by a combination of multibeam (or sidescan) imagery and sub-bottom sonar or seismic data. The typical expression of mud volcanoes is a central pinnacle of unstratified sedimentary material surrounded by a moat that is generally lower than the surrounding seafloor (Figure 6.7). Most, but not all such features are associated with release of gas from beneath the seabed (Hovland and Judd, 1988). For example, Paull et al. (2007) propose that PLFs in the Beaufort Sea are related to dissociation

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of gas hydrates in the sub-surface. The associated overpressures drive the mud and gas upwards to the seabed.

Figure 6.7 3-D multibeam sonar image of mud volcanoes on the Beaufort Shelf. The features range up to 20 m high and 300 m across. The linear gouges also apparent from the image are caused by grounded sea ice.

6.4.3 Determining the activity of gas venting Some pockmarks and mud volcanoes can be clearly related to “plumes” of sub-bottom gas while others appear to have no immediate relationship to sub-bottom gas. The most direct way to determine whether active venting is occurring is to observe the gas escaping at the seabed through underwater acoustic or photography/video methods. Acoustic methods provide a means of remote sensing of streams of gas bubbles emanating from the seafloor. The strong acoustic backscatter presented by gas bubbles in the water column makes bubble streams visible in both single beam sounder and chirp sonar records. Recent advances in extracting water column information from multibeam sonar systems potentially allows for systematic mapping of venting gas (Hughes-Clarke, 2006; Schneider von Deimling et al., 2007; Paull et al., 2011) (Figure 6.8).

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Figure 6.8 Image of gas plumes in the water column of the Beaufort Sea collected using a multibeam sonar.

Note: From Paull et al. (2011).

Underwater photography/video surveying is practical when targets such as pockmarks or mud volcanoes have been identified. The typical field of view of camera systems is limited to a few square metres, so that comprehensive surveying is inefficient. Nevertheless, underwater photography/video can provide direct evidence of gas venting (Hovland, 2007). Such systems range in sophistication from simple drop cameras to sled or AUV-mounted cameras capable of high resolution imagery (Mitchell and Coggan, 2007). The intermittent nature of gas escape means that indirect methods of detecting activity may need to be used. Re-surveying with multibeam sonar can be used to identify newly-formed bathymetric features like pockmarks or mud volcanoes. Re-surveying techniques are discussed in more detail in Section 6.5.2.2.

6.5 Slope stability, submarine landslides and other mass movements Submarine landslides and related mass movements, including debris flows and turbidity currents are common in marine environments on slopes as moderate as 1o (Middleton and Hampton, 1976; Edgers and Karlsrud, 1982; Booth and O’Leary, 1991; Booth et al., 1993; Hampton et al., 1996; Masson et al., 2006). Where present, they are a significant potential hazard to marine infrastructure, both through direct interaction with the structure or in the case of large slides through generation of an associated tsunami. For example, a relatively small submarine slide (8 x 106 m3) on the Var Delta in 1979 resulted in several deaths of construction workers at the Nice airport where land area slid into the sea. This generated a tsunami of 11 m that inundated the coastal town of Antibes and transformed into a turbidity current that broke telephone cables up to 105 km from the original slide location (Habib 1994; Mulder et al., 1997; Assier-Rzadkiewicz et al., 2000). Submarine slides are known to be triggered by seismic events (Rogers, 1980) so that the slide risks are compounded with other risks associated with seismic events. May 2015

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Submarine slides and mass movements can occur in a range of environments from coastal to deep sea and at a range of scales orders of magnitude larger than on land. The Storegga Slide, which “involved some 3000 km3 of sediment, affected 95 000 km2 of the Norwegian continental slope and basin and had a runout distance of around 800 km” (Haflidason et al., 2004; Masson et al., 2006), is one of the largest submarine slides to be documented. For marine renewable energy installations in coastal and shelf settings, the risk of submarine slope failure is highest in areas close to river deltas and in high slope areas such as fjord margins. Deltas are regions where rivers entering the ocean deposit large volumes of sediment and typically over-steepen the slope at their mouths. Submarine slides are therefore common features of delta fronts (Prior et al., 1981; 1982; McKenna and Luternauer, 1987; McKenna et al., 1992). Fjords are drowned glaciated valleys characterized by steep walls and, in certain cases, deposits of sensitive clays, making them particularly vulnerable to slope failure (Syvitski and Schafer, 1996; Locat et al., 2003a, b). Mass movements comprise a range of processes with the common attributes of being initiated by slope failure and being driven downslope by gravity. Mulder and Cochonat (1996) recognize 13 varieties of mass movement that can be broadly grouped into slides/slumps, plastic flows, and turbidity currents (Figure 6.9). Slides and slumps consist of detached blocks of sediment (or rock) that have moved downslope with relatively minimal disruption of the internal stratification. Plastic flows, including debris flows, are in a generalized sense generated when slides begin to disaggregate and entrain water from the overlying water column, although the details of this entrainment and mixing are poorly understood. When the failed sediment is predominantly fine-grained and cohesive, they transform into viscous, matrix-supported debris flows whereas if the original sediment is more sandy and non-cohesive, they form fluid supported grain flows. In both cases, the downslope motion is relatively slow and approximates viscous laminar flow. When additional entrainment of water occurs the flow transforms into a turbulent suspension known as a turbidity current (Middleton and Hampton, 1976; Parker et al., 1986). If the slope angle is sufficient, the turbulent flow becomes “auto-suspending” with turbulent motions maintaining the sediment in suspension. The high concentration of suspended sediment gives the flow excess density and it accelerates downslope. The tendency for the flow to disperse is counteracted by erosion and resuspension of the eroded sediment at the base of the flow as long as the slope angle is sufficient to maintain the flow’s downslope velocity. Turbidity currents are known to transport sediments hundreds of kilometres down continental margins into the deep ocean (Wynn et al., 2002). Deposition occurs when the slope angle decreases past a critical value for the flow, at which point dispersion of the suspension into the water column exceeds the rate of sediment supply from basal erosion. The turbidity current loses momentum and the sediment load is dropped.

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Figure 6.9 Classification of submarine mass movements. Cohesive material structure with distinct sides. Few displacemen ts. Motion do not due to pore fluid.

MASS SLIDE

No failure surface Low rate of deformation

CREEPING

Isolated decimetre-sized to hect ometre-sized elements (not included in a matrix)

BLOCKor SLAB GLIDING ROCKAVALANCHE TIONAL z/l < 0.15 TRANSLA SLIDE

Well distinct failure surface. Structur e greater than 100 m

SLIDE ROTATIONAL SLIDE

0.15 < z/l < 0.33

Motion support ed by matrix strength Laminar regime and c ≥ 0.09

Structure with indistinct sides Triggering and motion due t o pore fluid

MASS FLOW

Motion support ed by fluid GRAVITY FLOW

DEBRIS FL OW

Interstitial fluid has a prominent role in the triggering. Fluid and sedimen t are mixed. Interstitial fluid has a prominent role during the tr ansport ation. Sediment “floats” in the fluid layer. Low density, thin fine deposits

Turbulent regime and c < 0.09

TURBIDITY CURRENT

LIQUEFIED FLOW

Main grain size: clays and silts.

FLUIDIZED FLOW

Main grain size: greater than silts.

Possible ignition

MUD FL OW SILT FLOW SAND FLOW GRAIN FL OW

LOW DENSITYTURBIDITY CURRENT

Progressive ignition

HIGH DENSITY TURBIDITY CURRENT

High-density Catastrophic ignition

TURBID CLOUD

Selfsupport

Note: From Mulder and Cochonat (1996); courtesy SEPM, Society for Sedimentary Geology.

6.5.1 Identifying submarine slides and mass movement deposits Submarine slides and mass movements can be identified in multibeam bathymetric images, sidescan sonar images and in seismic profiles. The majority of such features have been identified on continental slopes in deep water regions unlikely to be developed for marine renewable energy. However, coastal basins and inlets have similar if smaller features. Their preserved morphologies can be linked to the processes of mass transport and deposition (Mulder and Cochonat, 1996). A simple slide deposit may be preserved as a mounded, coherently stratified mass, visible as a bathymetric feature, and may be traced up slope to a slide scar (Figure 6.10). Plastic flows (typically debris flows) are characterized by a mounded hummocky surface and incoherent or transparent internal stratification (Figure 6.10).

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Figure 6.10 Shaded relief image of Kitimat Arm, B.C. showing submarine slides and debris flows.

Fully dispersed turbidity currents tend to form channel-fan systems visible in multibeam bathymetry and sub-bottom profiles (Figure 6.11). However such flows can distribute sediment over very long distances leaving more subtle deposits in the form of broad lobes and horizontally stratified beds that infill antecedent topography.

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Figure 6.11 Shaded relief multibeam image of the Main Channel of the Fraser River delta showing well-developed submarine channel terminating in a depositional lobe.

Note: After Hill (2012).

6.5.2 Determining the activity of submarine slides and mass movements As with other hazards, information on the probability of slope failure events can substantially reduce risk and uncertainty. However, there have been few systematic attempts to quantify this probability for submarine slope failure. Three main techniques can be used to assess this probability: • Estimating past frequency of events through stratigraphic studies • Measuring contemporary changes in seabed bathymetry • Geotechnical evaluation

6.5.2.1 Estimating past frequency Stratigraphic studies can be used to develop an understanding of when and how frequently submarine failure events have occurred in the past. The analysis and age dating of piston cores that penetrate into or below slide or mass transport deposits can be used to determine minimum and maximum ages for the event. This typically requires radiocarbon or 210Pb dating of bivalve shell or foraminiferal material in marine sediments above or below the mass movement deposit. Where multiple events can be observed in seismic profiles, strategic coring can yield a chronology of past events (Piper et al., 2003).

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Such research can determine whether past events occurred under specific conditions (e.g., lowstand of sea level) or at a particular return frequency. The latter can be evaluated against the chronology of potential trigger events (e.g., earthquakes).

6.5.2.2 Measuring contemporary change Many submarine slides and mass movements are of sufficient scale that re-surveying seabed regions with multibeam sonar can determine if slope failures have occurred over the time period between surveys. Individual survey data can be gridded, imported into a GIS and then quantitatively compared using differencing algorithms. Using this technique, Hill (2012) demonstrated that small slope failures occurred at an inter-annual time scale on the Fraser Delta slope seaward of the main distributary channel. Hughes-Clarke (2012a, b) completed daily surveys of the Squamish Delta slope and documented numerous failures on a daily time scale. When contemplating construction of difference maps from different surveys, it is important to conduct a systematic evaluation of all the potential errors (Hughes-Clarke et al., 2009, 2011). Errors of several tens of centimetres can result from positioning, sonar and gridding issues. This is generally not problematic when detecting submarine slide and mass movement features many metres in height but can be significant if attempting to measure more subtle morphological changes or sedimentation (Kammerer et al., 1998).

6.5.2.3 Geotechnical evaluation The civil engineering community has developed formal methods and guidelines for geotechnical evaluation of slope stability on land and these can be applied in principle to submarine slope stability (e. g., US Army Corps of Engineers, 2003; Wang et al., 2012). Slope stability analysis typically includes detailed site investigation and various analytical techniques (e.g., limit equilibrium method). Site investigation includes geologic setting, lithology, stratigraphy, empirical measurements of soil strength, and estimates of past overburden pressure. An analysis of potential conditioning of the site including factors such as sediment loading, over-steepening and groundwater seepage as well as of the potential triggers of slope failure (e.g., earthquakes) should also be included where data are available.

7 Current scour and sedimentation Because marine renewable energy converters are generally placed in areas of high wave and/or tidal current energy, the impacts of erosion, sediment transport and sedimentation are likely to be significant. A solid understanding of the sediment dynamics of the site and the hazard they represent are therefore essential both for ensuring the safety of the structure and assessing the long range environmental impacts. In most development regions, the shear stresses imposed by currents and wave motions, will be sufficient to move sediment in the sand and gravel size range. Gravity structures placed on such a mobile seabed will be susceptible to scour and erosion thus leading to instability and possible foundering of the structure. Structures placed on stable bedrock could also be damaged by sand transported in suspension and movement of cobbles and boulders on the seabed. Arrays of tidal or wave converters removing energy from the environment could lead to changes in the tidal range and regional circulation of water, resulting in corresponding changes in sediment transport and sedimentation patterns, leading in turn to potential impacts on coastal and marine habitats.

7.1 Principles of sediment erosion, transport, and deposition 7.1.1 Grain size classification and analysis Sediment of different grain sizes are formed by in situ weathering of rocks and subsequent abrasion in high energy fluvial and marine environments. Grain size is conventionally classified according to the

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standard Wentworth scale (Figure 7.1). Size classes in this scale are defined on the basis of grain diameter using a logarithmic (log2) or phi (ϕ) scale (Wentworth, 1922; Krumbein and Sloss, 1963). The main grain size classes are gravel (including boulders and cobbles), sand, silt and clay. Various techniques are available for the measurement of grain size (Syvitski, 1991b) including sieving and settling tubes for sand-sized sediment and x-ray- and laser-scattering techniques for fine-grained sediment (McCave and Syvitski, 1991). The options for coarse–grained sediments are limited to sieving and counting techniques, the latter based either on direct sampling or image analysis from photography (Adams, 1979; Butler et al., 2001; Sime and Ferguson, 2003). While there are few examples of rigorous grain size analysis of coarse grained sediment in the marine environment, there is an extensive literature on measurement of gravel bed grain size in rivers (Church and Kellerhals, 1978; Hey and Thorne, 1983; Church et al., 1987) that can be adapted for marine samples and/or image analysis.

Figure 7.1 Wentworth grain size scale.

Note: From Farrell et al. (2012); courtesy SEPM, Society for Sedimentary Geology.

Most sediments consist of a mix of size classes and are therefore best defined statistically as a grain size distribution (Figure 7.2a). A similar grain size classification for soils is included in ISO 14688-1 (International Organization for Standardization, 2002). Where major size classes overlap, descriptive terminology based on the relative proportions of major size classes can be used (e.g., Shepard, 1954; Folk, 1954) (Figure 7.2b,c).

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Figure 7.2 Grain size presentation methods: a) size distribution (histogram and curve); b) Shepard's ternary diagrams; c) Folk's ternary diagrams. Source: United States Geological Survey.

7.1.2 Thresholds of sediment erosion and deposition The response of sediment grains to the motion of an overlying water column depends on a set of water and grain properties, including the shear stress imposed by the moving water, the density and viscosity of the water and the grain diameter and density (Middleton and Southard, 1978). Laboratory experiments have demonstrated that for constant water and grain properties, the threshold at which grains of a particular size begin to be transported by flowing water depends primarily on the bed shear stress, which is in turn related to the current speed and the bottom boundary layer structure (Middleton and Southard, 1978; van Rijn, 1993). This can be represented on the Shields curve where the initiation of movement of grains in flume experiments is plotted as a function of two dimensionless variables, the dimensionless bed shear stress (left) and the grain Reynolds number (right) (Middleton and Southard, 1978; Figure 7.3): Equation (1)

where τ0I =I bed shear stress u*I =I shear velocity = dsI

=I grain diameter

νI

=I fluid kinematic viscosity

λsI

=I grain density

λI

=I fluid density

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Figure 7.3 The Shields diagram, criterion for initiation of sediment motion.

The advantage of this dimensionless plot is its great generality. Other graphs representing the initiation of sediment movement can be compiled for specific conditions of water depth, temperature or grain density. However, because they apply to restricted conditions, these should be used with caution when estimating the initiation of grain motion for marine sediments. In general, a critical shear stress value can thus be defined for the threshold of sediment movement as a function of grain size. Stronger currents are required to move coarser grains. Once in motion, grains may move as bedload, by rolling or saltating along the bed, or as suspended load. Sediment grains are maintained in suspension by the fluid turbulence. The threshold for suspended load transport is generally higher than for bedload transport so that movement is initiated as bedload but as the bed shear stress increases, more grains go into suspension until the sediment transport is predominantly by suspension. However, very fine sand and finer grains do not go through a bedload phase but are suspended immediately once the threshold is exceeded. Bagnold (1966) indicated that this threshold occurred when Equation (2)

where ws is the grain settling velocity. In sediment transport calculations, it is important to distinguish non-cohesive sediments (typically sand and gravel) for which grains respond individually to the imposed shear stress, and cohesive grains (silt and clay) for which static intergranular forces inhibit single grain transport. Cohesive sediment tend to erode as aggregates of individual grains and may deposit similarly as aggregates or flocs (Kranck, 1975). May 2015

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7.1.3 Sediment transport equations Evaluation of sediment transport requires knowledge of the bottom shear stress, and thus current speed and bottom boundary layer structure, as well as the grain size of the bottom sediment. For unidirectional flow, the bottom boundary layer is generally tens of metres thick and is assumed to have a logarithmic velocity profile with a turbulent outer layer and viscous sub-layer close to the bottom (Van Rijn, 1989, 1993) (Figure 7.4). When significant wave energy is present, wave orbital motions may be combined with unidirectional currents. In this case, a thin (< 50 cm thick) wave boundary layer is embedded within the current boundary layer so that bottom sediments are subject to combined wave and current motions (Madsen and Grant, 1986).

Figure 7.4 Schematic of boundary layer characteristics where z = height above bed and u = horizontal velocity

Note: After van Rijn (1989).

Using the bottom shear stress calculated from the above theories, equations have been developed to predict the initiation and rate of sediment transport, notably Brown (1950), Yalin (1963) and Van Rijn (1993) for non-cohesive bedload transport; and Engelund and Hansen (1967) and Bagnold (1963) for the total load transport. Equations for erosion of cohesive sediments have been developed by Parchure and Mehta (1985), Amos et al. (1992) and Van Rijn (1993) and for deposition by Krone (1993). These formulations are typically offered as options in numerical models of sediment transport.

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7.1.4 Sediment transport under tidal flows Although at short time scales, tidal flows can be considered unidirectional, current speed and direction will typically show a complete 360 degree rotation over a single tidal cycle. This velocity rotation can be described in terms of a tidal ellipse (Figure 7.5). The form and sense of rotation of the tidal ellipse depends on local conditions, including location with respect to amphidromic points, seabed topography and water depth. Open water areas are typically characterized by more isotropic ellipses, whereas narrow constrictions show more rectilinear, bi-directional ellipses (Figure 7.6).

Figure 7.5 Example of a tidal current ellipse. Arrows indicate current speed and direction. H = high tide; H+1 = high tide + 1 hr; L = low tide etc.

Note: From the American Practical Navigator, United States Government.

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Figure 7.6 M2 tidal ellipses for measured surface currents in the English Channel and North Sea. Note the strongly bi-directional form of the ellipses in the restricted English Channel compared to the more open ellipses in the North Sea.

Note: Based on Davies and Furnes (1980).

Because sediment transport increases with current speed once the critical shear stress has been exceeded, the sediment transport under tidal flows similarly varies in rate and direction. For any location characterized by a tidal ellipse, the vector sum of the sediment transport rates over a single tidal current rotation will define the net sediment transport in a single direction. Shallow regions dominated by tidal flows therefore show distinctive patterns of sediment transport characterized by regions of sediment transport divergence and convergence (Figure 7.7). Regions of sediment transport divergence are known as “bedload partings” and typically show strong seabed erosion, whereas regions of sediment transport convergence are typically sediment rich and often characterized by well-formed subaqueous dunes (see Section 7.2). The restricted passages favoured by tidal energy companies because of their very strong tidal flows are commonly regions of sediment transport divergence and seabed erosion.

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Figure 7.7 Sand transport paths around the British Isles.

Note: After Stride (1973); ©MIT, published by The MIT Press.

7.1.5 Sediment transport under waves and oscillatory currents Compared to sediment transport under tidal flows, sediment transport under strong wave conditions is much harder to predict. First, there is the variability of the flows as a function of the wave climate, net sediment transport being dominated typically by extreme (storm) events. Added to this factor, there is the variability associated with shoaling wave asymmetry in shallow water. The instantaneous sediment transport direction oscillates back and forth in response to changes in the wave orbital velocity through a single wave cycle. In deep water, this oscillation is more or less symmetric so that there is little or no net sediment transport due to the waves. As waves shoal due to bottom friction and become more skewed, the forward stroke of the wave generates a higher velocity flow and sediment transport compared to the reverse stroke resulting in a net sediment transport in the direction of wave propagation (Swift and Thorne, 1991) (see Figure 7.8).

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Figure 7.8 Concept of the “littoral energy fence” where predominantly onshore sediment transport is favoured under shoaling waves due to the asymmetry of orbital velocities.

Note: After Swift and Thorne (1991).

Further complicating the sediment transport response to wave motions is the fact that strong wave conditions can be associated with tidal currents and are almost always associated with wind-driven currents, which may strongly enhance bed shear stress and influence the net sediment transport direction (Grant and Madsen, 1986; Amos and Judge, 1991; Li et al., 1997). At the shoreline, where waves break and storm surge set-up may occur, longshore currents, rip currents and downwelling currents are all likely to be significant. Whereas numerical models of sediment transport have been formulated to take many of these interacting processes, the complexity of sediment transport under strong combined waves and currents is such that results should be carefully validated through the acquisition of field data.

7.1.6 Bedforms and flow regimes Bottom roughness is an important factor in boundary layer dynamics and mobile beds are commonly observed to be characterized by bedforms. In laboratory flume experiments, under unidirectional flow, sand is seen to form a variety of bed forms each remaining stable over a particular range of flow velocities. Experimental data of this type have been compiled into flow regime diagrams such as Figure 7.9. With increasing velocity, sandy beds typically pass through a stage where ripples and/or dunes form. The differences between these forms are largely in scale, with dunes being generally an order of magnitude larger than ripples. As the velocity increases further these bedforms begin to become progressively rounded and flattened until eventually the bed becomes planar. At this upper plane bed flow regime, the bed is typically obscured by high concentrations of sediment in suspension or bedload movement. In flumes and shallow natural flows such as rivers, further increase in velocity will see an additional bedform stage where dune-like “antidunes” form in phase with standing waves at the water surface. The conditions for antidune formation do not generally exist in relatively deep marine waters.

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Figure 7.9 Bedform phase diagram showing the stability of bedforms under uniform steady flow in straight laboratory channels.

Note: After Leeder (1980).

7.2 Subaqueous dune classification Bedforms generated in laboratory flumes are limited in scale due to the practical limits on flow depth and wall effects. In the natural marine environment, much larger bedforms have been observed in sidescan and multibeam sonar images (Figure 7.10) and are very common in tidal environments.

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Figure 7.10 Shaded relief multibeam image of subaqueous dunes in Johnstone Strait, B.C.

These large bedforms, constructed of sand and/or gravel, have been given various labels, including megaripples, sand waves and bars, and several classification schemes have been developed. Ashley (1990), reporting on behalf of an expert panel, concluded that large-scale bedforms “occur as a continuum of sizes, not as discrete groups” (based on Flemming’s (1988) compilation of bedform dimensions) (Figure 7.11) and thus recommended the adoption of the single name “dune” for all such bedforms. Various descriptors can be added. For example, it is useful to use the term “subaqueous dune” to distinguish them from aeolian dunes formed by wind action on land. The expert panel suggested that subaqueous dunes could be adequately classified with primary descriptors of dimension and geometry (Table 7.1).

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Figure 7.11 Plot of height (H) vs spacing (L) of 1491 flow transverse subaqueous bedforms.

Note: From Flemming (1988) and Flemming and Bartholomä (2012).

Table 8.1 Recommended descriptors for subaqueous dune classification. Dimensional descriptor

Spacing

Height

Small

0.6 – 5 m

0.075 – 0.4 m

Medium

5 – 10 m

0.4 – 0.75 m

Large

10 – 100 m

0.75 - 5 m

Very Large

> 100 m

>5m

Shape descriptor

Characteristics

2-D

Dune crests and troughs parallel or sub-parallel. Geometry can be defined by a single transect perpendicular to flow.

3-D

Dune crests discontinuous. Geometry cannot be defined by a single transect perpendicular to flow.

Note: After Ashley (1990).

7.3 Methods for assessing bedform mobility In the high energy tidal and wave influenced environments typically sought for marine energy conversion, there is a high probability that any observed subaqueous dunes would be actively migrating. The migration of subaqueous dunes across a given locality implies the periodic deposition and erosion of several metres of sand or gravel over interannual or shorter time periods, presenting an obvious hazard to seabed structures and cables. While the presence of subaqueous dune fields is easily May 2015

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established using multibeam and/or sidescan sonar mapping techniques, the assessment of their mobility requires more detailed study.

7.3.1 Repetitive survey techniques Repetitive surveying by various sonar methods can be used to determine the rate and direction of dune migration. The fidelity of these techniques is dependent on the size of the bedforms relative to the survey accuracy, the latter being highly dependent on navigational accuracy and sonar beam characteristics. In the past, single beam echo-sounders and sidescan sonars have been successfully used to demonstrate bedform mobility and estimate rates of movement (Jones et al., 1965; Hawkins and Sebbage, 1972; Bokuniewicz et al., 1977; van den Berg, 1987; Harris, 1989; Berne et al., 1993; Kostaschuck and Best, 2005). However, modern multibeam echo-sounders provide much greater resolution and 3-D imaging of the bedforms, enabling more sophisticated qualitative and quantitative analysis (Duffy and Hughes-Clarke, 2005, 2012; Ernsten et al., 2005; Knaapen et al., 2005; Weinberg and Hebbeln, 2005; Daniell et al., 2008; Zorndt et al., 2011; Barnard et al., 2012; van Landeghem et al., 2012). Given that the migration rates of subaqueous dunes might be only relatively small compared to survey resolution (there are very few reliable figures on migration rates), high survey accuracy and precision are critically important and require detailed consideration of the beam characteristics, positioning and water column characteristics (Hughes-Clarke, 2012). Migration rates can be expressed in terms of crest displacement or volume displacement. For empirical evaluation of the hazard presented to structures or cables, the former may be adequate because knowledge of average migration rates would inform safe placement of structures and cables relative to the dune field and/or time scales over which dunes might influence a particular site. If greater understanding of the dynamics of the dune movement as a function of the flow field is required, volume displacement might be a more useful parameter for use in numerical modeling. The latter could, for example, be important in the prediction of the impacts of energy converter arrays on the dune field. Details of methods to measure dune displacement and bedload sediment transport using multibeam sonar are contained in Duffy and Hughes-Clarke, (2005; 2012).

7.3.2 Evaluation based on grain size and model predictions Since field surveys are expensive, there are often limitations on the number of repeat surveys that can be used to assess bedform mobility. Also, when bedform migration is oscillatory, no net migration occurs and the repeat survey approach will not be effective. In this case, model predictions of shear stress and sediment transport magnitude can be used to assess their mobility. Tidal current and wave data need to be collected either from instrumentation measurements (for sitespecific assessment) or from predictions of tidal and wave models (for regional assessment). Sediment grain size variations over bedforms should be evaluated through seabed sampling and/or imaging. For wave- or current-only cases, the wave and current parameters, together with grain size and water depth, can be used in a quadratic stress law (e.g., Dyer, 1986) to compute bed shear stress. For cases of combined waves and steady currents, the combined wave-current shear stress can be calculated using combined-flow bottom boundary layer theories (Grant and Madsen, 1986; Li and Amos, 2001). With knowledge of the grain size over the bedforms, the critical shear stress for initiation of bedload transport can be calculated (Miller et al., 1977). The prevailing bed shear stress is then compared with the critical shear stress over specific bedforms to determine how frequently the sediment on the bedform is mobilized. For the period that sediment is transported, the ratio of applied shear stress over the critical shear stress can be used to quantify the level of sediment transport. The combined knowledge of the frequency and magnitude of the mobilization of sediment on the bedform allows qualitative assessment of bedform mobility. No transport or weak transport with low frequency likely May 2015

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suggests bedforms are not active. Frequent and strong magnitude of sediment mobilization naturally indicate that bedforms are mobile. This approach has been used to assess mobility of various bedforms on Sable Island Bank, Scotian Shelf (Li et al., 2012), and is currently being applied in the Bay of Fundy under strong tidal current effects (Li et al., in press).

7.3.3 Numerical models of sediment erosion, transport and deposition There are several software packages that can be used to model sediment transport as described above. Some models are available through the US-based Community Surface Dynamics Modeling System (CSDMS) web site (http://csdms.colorado.edu/). This academic network provides open access to a broad range of surface dynamics numerical models and facilitates academic research including coupling and integration of the models. Other models are available commercially and through consulting companies.

7.4 Instrumentation for assessing sediment mobility Local, site specific measurements of sediment mobility are typically required for site assessment and environmental assessment. Empirical measurements are also required for numerical model validation and analysis. Because sediment transport is very difficult to measure directly, the assessment of sediment mobility requires measurement of a suite of parameters that determine bed shear stress and sediment transport direction. Consideration must be given to both the instruments required to make such measurements and the methods of data storage and retrieval. Williams (2012) provides a recent review of instruments used for research studies of sediment mobility. A comprehensive source of information is van Rijn (2007), which is also available online at the Coastal Wiki (http://www.coastalwiki.org/).

7.4.1 Benthic landers Measurement of sediment transport requires placing multiple instruments close to the seabed. Surfacebuoyed cable moorings are generally not ideal for this purpose and instruments are typically deployed on an instrument frame (or “benthic lander”; Figure 7.12). These are typically designed to withstand the strong hydrodynamic conditions into which they are deployed, to remain stable and stationary and to minimize effects on the flow by the platform and instruments themselves (Williams, 2012). Power supply and data storage are major considerations for stand-alone landers and compromises often have to be made between battery longevity, deployment time, data acquisition frequency and data storage. While individual instruments may log data into internal memory, central data loggers are often used to minimize issues with synchronization between instrument clocks. Whereas tidal flows recur at regular frequencies, wave conditions are more characterized by irregular frequency and duration as well as extremes. Modern data loggers can be programmed to sample at various frequencies, in burst mode and triggered by threshold conditions.

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Figure 7.12 Benthic lander designed for the Delta Dynamics Laboratory on the VENUS underwater observatory.

7.4.2 Cabled observatories Cabled observatories consist of instrument networks connected to seafloor cables that supply 24/7 electrical power from land sources and fibre-optic data transmission. The advantages of cabled observatories are that the continuous power supply eliminates power conservation concerns that limit data acquisition on stand-alone landers, and the fibre optic transmission of data enables continuous and real-time monitoring of instrument data through the internet. Canada has been a pioneer in developing submarine cable observatories through the VENUS and NEPTUNE Canada projects (Barnes and Tunnicliffe, 2008; Lintern and Hill, 2010) (Figure 7.13). While expensive to install at the site assessment stage, submarine observatories potentially provide a solution to long-term monitoring requirements for tidal and wave energy installations.

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Figure 7.13 The Victoria Experimental Network Under the Sea (VENUS) cabled observatory. A: Map showing location of cabled observatory; B and C: Data from the DDL site on the observatory.

Note: From Lintern and Hill (2010).

7.4.3 Instruments 7.4.3.1 Current meters A broad range of current meters are available on the market for measuring the speed and direction of flow. They generally fall under three categories: mechanical, electromagnetic and Doppler velocimeters. Mechanical current meters are rarely used for long-term deployments on landers or observatories due to problems with maintenance and fouling. Electromagnetic current meters (Figure 7.14a) provide point source measurements of flow based on the voltage induced by a conductor (seawater) moving through a magnetic field. These instruments are typically robust and can sample currents at rates up to 5 Hz.

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Figure 7.14 Examples of current meters: a) Interocean S4 Electromagnetic current meter (courtesy Interocean Systems Inc.); b) Teledyne RDI Sentinel Acoustic Doppler current profiler (ADCP).

Note: Courtesy of Teledyne RD Instruments.

Doppler velocimeters have become increasingly popular in recent years and measure the Doppler shift in frequency from reflections off moving particles in the water column. While laser Doppler velocimeters are used in research-level projects, the most common commercially-available current meters are acoustic Doppler velocimeters. In particular, the acoustic Doppler current profiler (ADCP) (Figure 7.14 b), which measures the vertical velocity profile through the water column by binning Doppler measurements for different return times, has become the instrument of choice for many marine applications. Some ADCP devices are capable of tracking surface elevation at a resolution where instantaneous wave conditions can be established, combining wave and current measuring capabilities in a single device and therefore reducing deployment costs related to site characterisation. The ADCP has the additional advantage of providing acoustic backscatter profiles that can be interpreted in terms of suspended sediment concentration (see Section 7.4.3.2). ADCPs may be attached to benthic lander or cabled observatory platforms pointing either upwards to measure the current profile through the water column or downwards to measure velocity distribution in the bottom boundary layer. For detailed measurements of the near-bed current profile, high frequency pulse-coherent Doppler velocity profilers have also been developed (Williams, 2012). When establishing sediment transport regimes it should be noted that variations in bathymetry and upstream turbulence can result in large changes in flow speed and direction for small changes in position, both horizontally and vertically. At energetic sites caution should be taken when inferring general conditions from local measurements.

7.4.3.2 Turbidity measurements Turbidity is a general term used to describe the clarity of water due to the presence of suspended particles. While micro-organisms and particulate organic matter can be significant sources of turbidity in ocean water, in dynamic sedimentary environments, turbidity is dominated by inorganic suspended

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sediment. Several options are available for measurement of turbidity, providing a proxy for suspended sediment concentration. Transmissometers measure the intensity of light directly transmitted from a fixed frequency source over a distance of a few centimetres (Figure 7.15a). The attenuation of the light as a result of particles in the water column means that the received intensity is inversely proportional to the suspended matter concentration. Data from a transmissometer may be expressed as beam transmittance, T, (% or decimal) or as the attenuation coefficient, C, where C = -ln (T). The relationship between C and the measured suspended sediment concentration is theoretically linear but great scatter is typically observed in field calibrations due to differences in the optical properties of particles (Bishop, 1986). Transmissometers may measure transmission across an open water space or pump water into an optical tube.

Figure 7.15 a) Seatech Transmissometer; b) D&A Optical Backscatter Sensor; c) OBS response for suspensions of different grain size (source Coastal Wiki website); d) time series plot of ADCP backscatter from the Delta Dynamics Laboratory, VENUS observatory showing sediment settling out of the Fraser River plume.

Optical backscatter sensors (OBS) measure the infra-red radiation reflected back from particles over a specific range of angles (Downing et al., 1981) (Figure 7.15b). The response of these instruments to suspended sediment concentration is very linear for particles of equal size, but varies greatly with grain size (Figure 7.15c). For this reason, calibration of sensors is generally carried out in the laboratory with samples of sediments from the study area. New multispectral sensors have been developed (Maffione and Dana, 1997) that make it possible to determine concentrations of suspended sediments of mixed particle size using simultaneous equations giving the responses of different wavelengths to different particle sizes (Green and Boon, 1993; Hatcher et al., 2000). May 2015

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Acoustic backscatter sensors (ABS), particularly ADCPs (see Section 7.4.3.1) use a similar approach, measuring the backscatter of acoustic impulses as a measure of turbidity. Whereas transmissometers and OBS’s provide measurements at fixed heights, ABS’s and ADCP’s are capable of providing profiles of suspended sediment concentration. Because of high rates of attenuation for radiation in the optical to infra-red range, optical sensors measure the turbidity within a few centimetres of the instrument. Because sound waves are less attenuated in the water column, ADCP backscatter provides measurements across metres to hundreds of metres from the instrument, depending on the sound frequency used (Figure 7.15d). This means that the backscatter measurement needs to be corrected for attenuation through the water column and due to suspended matter along the ray path. It also makes ADCP backscatter difficult to calibrate in the field. Hay (1983) and Thorne and Hanes (2002) provide reviews of the theory of acoustic measurements while Williams (2012) discusses ADCP backscatter measurements.

7.4.3.3 In Situ particle concentration and sediment settling velocity using LISST The Laser In-Situ Scattering and Transmissometery (LISST) instrument can be used to measure sediment concentrations and in situ grain settling velocities (Agrawal and Pottsmith, 2000) (Figure 7.16). The instrument acts as an in situ settling tube and consists of a closeable tube, which traps samples of the ambient water and then uses laser diffraction techniques to measure changes in the particle concentration with time. Using empirical values of grain settling velocities and assumptions about grain composition, the settling velocity information can be translated into grain size distributions. If suspended concentrations are high, inhibited or mass settling may occur, leading to unrealistic grain size estimates.

Figure 7.16 LISST-STX.

Note: Courtesy of Sequoia Scientific Inc.

7.4.3.4 Video or Time-lapse photography Bedform migration can be observed directly beneath lander or cabled observatory platforms using high resolution video and/or time lapse photography. This technique has been available since the 1970s (Summers et al., 1971), but high quality digital technology has greatly enhanced the possibilities for using this technique, which is routinely used in laboratory flume studies. Visibility can sometimes be a problem in dynamic regions because of high rates of suspended sediment transport. Photographic and

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video techniques combined with image analysis would be particularly useful for the determination of grain size and bedload movement on gravel bottoms (Drake et al., 1988; Buscombe et al., 2007).

7.4.3.5 Acoustic bed-scanning sonars Acoustic sector-scanning sonars, can also be used to image bedforms. These systems emit a broad (in the order of 30°) sonar beam and include a step motor that rotates the beam 360° around a vertical axis. Typically operating at frequencies between 600 kHz and 2 MHz, these systems can be mounted on lander or observatory platforms and set to point downwards and scan the seabed, providing circular sidescan-like images of the seabed backscatter (Figure 7.17). Time series of the images can be compiled to document bedform form, movement and evolution under varying current conditions (Traykovski et al., 1999). Although bed height information can be determined from scanning sonars (Lintern and Hill, 2010), detailed information on bedform dimensions can be obtained using profiling (or pencil beam) sonars that use narrow (1 to 2°) beams and rotate a user-determined fixed angle to provide high resolution profiles of the seabed topography. Typically both scanning and pencil beam sonars are used together to characterize bedforms and bed roughness for sediment transport calculations (see examples and Figures 7 and 8 in Williams, 2012). In this way ambiguity related to either bedform orientation and shape can be reduced.

Figure 7.17 Two sector-scanning sonar images collected on the Roberts Bank tidal flats at high water showing a rippled bed. The image on the right shows reduced roughness (ripple height) compared to the image on the left after a period of higher wave activity.

8 Seabed characteristics of marine renewable energy sites 8.1 Characterization of tidal energy sites Triton Consultants Ltd (2006) and Cornett (2006) mapped the most prospective tidal-power sites around Canada based on the identification of narrow tidal passages from CHS charts and estimates of energy flux through them (Figure 8.1). These passages constrict and intensify the tidal flow, producing May 2015

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maximum current speeds in excess of 12 knots (> 6 m s-1). A review of the sites identified by Triton (2006) as well as several examples world-wide reveal that these sites have many seabed characteristics in common (Figure 8.2).

Figure 8.1 Map of Canada showing prospective tidal energy sites (from coordinates provided by Triton Consultants Ltd.). Letters indicate locations of sites shown in Figure 8.2.

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Figure 8.2 Multibeam bathymetric images (shaded relief) of selected prospective tidal energy sites showing morphological similarities. Locations shown on Figure 8.1. A: Minas Passage, N.S.; B: Boundary Pas, B.C.; C: Placentia Bay, Nfld.

Shaw et al. (2012; in press; also Li et al., in press; Todd et al., in press) conducted one of the most detailed studies of a tidal energy site at the Fundy Ocean Research Center for Energy (FORCE) test site in Minas Passage, Bay of Fundy (Figure 8.2a). They identified two main seabed elements: a scour trough associated with the peak tidal flows and sediment banks associated with gyres that set up during the

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ebbing or flooding tide (Figure 8.3). The scour troughs are incised up to 25 m below the level of the surrounding seafloor and are characterized by rough seafloor topography.

Figure 8.3 Schematic map showing the principle morphological elements of the Minas Passage seabed shown in Figure 8.2A. The red square indicates the location of the FORCE test site.

The Minas Passage scour trough is incised through unconsolidated glaciomarine sediments and in its core region exposes Triassic bedrock. This bedrock is likely to consist of sandstones and conglomerates of the Wolfville and Blomidon Formations or basalts of the North Mountain Formation (Keppie 2000). These rocks have different physical characteristics and engineering properties that should be taken into consideration when siting a structure, including unit weight, Rock Quality Designation (ASTM Standard D6032-08), intact unconfined compressive strength, fracture spacing, bedding and fracture orientation. The sediment banks have a characteristically teardrop planform and are characterized by fields of large to very large sand and/or gravel dunes. Repeat multibeam surveys confirm that these dunes are actively migrating and bedform asymmetry indicates that the net sediment transport direction varies across the banks, signaling flood-dominance on one side and ebb-dominance on the other. Two other examples of potential tidal energy sites are shown in Figure 8.2 (B and C) and these same characteristics can be observed in both. In Boundary Pass, B.C., a scour trough has incised the seabed in the passage and a tear-drop shaped dune field is present on the west side of the scour trough (Figure 8.2B). Similarly at Placentia Bay, NL, a scour trough and dune field are present either side of the very narrow passage between Placentia Bay and Northeast Arm (Figure 8.2C).

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For site evaluation and eventual positioning of tidal energy converters, these general seabed conditions can be expected. Detailed studies to determine the rate of scour, if still active, and the mobility of the large sand and gravel bedforms will be important.

8.2 Characteristics of wave energy sites Canada’s wave energy potential is very large with substantial recoverable resources in the nearshore on both west and east coasts (Cornett, 2006) (Figure 8.4). Compared to tidal sites, however, wave sites are much less restricted to situations where the energy is concentrated into specific passages. It is therefore more difficult to generalize about the seabed morphology to be expected at potential wave sites. Typical sites with wave energy potential would consist of flat-topped banks with water depths less than 50 m. Two examples of such environments are presented here, one on the west coast, Amphitrite Bank, where coarse (gravel) sediments are characteristic and one on the east coast, Sable Island Bank, where sandy sediments are more abundant.

8.2.1 Gravel wave-dominated seabed Amphitrite Bank is located seaward of the town of Ucluelet on Vancouver Island (Figure 8.5) and is being considered as a potential wave energy test site by a consortium of companies and university researchers. The region of maximum wave height is located on the inshore edge of the bank, where it reaches its shallowest point (Figure 8.5). Whereas Amphitrite Bank has not been the subject of detailed study since Luternauer et al. (1986) and no multibeam bathymetric survey has been carried out, the few available sub-bottom profiles across the bank suggest that it may have had a glacial origin and its topography planed off by wave action (Figure 8.6a). A single sidescan sonar profile across the bank top shows that the surficial sediments are organized into shore perpendicular bands of higher and lower reflectivity, with the lower reflectivity bands being characterized by gravel ripples with average wavelengths in the order of 2 m (Figure 8.6b). This interpretation is confirmed by bottom photographs taken along the same transect (Figure 8.6c), which show a seabed characterized by cobble- to boulder-sized clasts in the areas of higher backscatter and better sorted, somewhat finer sized gravel in the lower backscatter bands. Notably, clasts of all sizes show little sign of encrustations by marine organisms, indicating that the gravel, cobbles and boulders must all be frequently remobilized in this very dynamic environment. Similar associations of shore-perpendicular bands and gravel ripples have been observed in other coarse-grained wave-dominated environments, particularly the inner shelf (Forbes et al., 1987). Wave energy converter sites located on coarse gravel bottoms can therefore be expected to experience the effects of very significant bedload sediment transport. Although there has been some research on gravel bedload transport indicating that the active layer may be several clasts thick (Frey and Church, 2011), there has been very little investigation of this phenomenon in the marine environment under the action of oscillatory currents, such as would be experienced at wave converter sites. This is an important knowledge gap that needs to be filled by fundamental research.

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Figure 8.4 Available wave energy in Canada's southern offshore: (a) off British Columbia; (b) off Nova Scotia and Newfoundland (Cornett, 2006).

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Figure 8.5 Amphitrite Bank, potential wave energy test site off Vancouver Island, B.C. showing location of sidescan sonar and camera station data of Figure 8.6.

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Figure 8.6 Seabed characteristics of Amphitrite Bank; a) air gun seismic profile showing flat wave-planed seabed overlying possible glacial deposits (dashed line marks top of possible till unit); b)sidescan sonar profile showing bands of more reflective (light) and less reflective (dark) seabed. The less reflective seabed is characterized by a texture that indicates the presence of gravel ripples; c) bottom photographs showing boulder, cobble and gravel grain sizes.

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8.2.2 Sandy wave-dominated seabed Sable Island Bank, off Nova Scotia is an example of a similar high-energy storm wave dominated environment where sand grain sizes are more predominant. In this case the sandy sediment is likely derived from glacial outwash sediments deposited under sea level lowstand conditions when the shelf was exposed. Sable Island Bank has been the subject of many years of seabed surveying and study (Amos and King, 1984; Hoogendoorn and Dalrymple, 1986; Amos and Nadeau, 1988; Li and King, 2007; Li et al., 2012). The bank is covered by subaqueous dunes of varying sizes from sand ridges to ripples (Figure 8.7) with the smaller forms superimposed on the larger ones.

Figure 8.7 Shaded relief bathymetry map of Sable Island Bank showing large sand-ridge scale subaqueous dunes (from Li et al., 2012).

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Figure 8.8 Sidescan sonar profiles of Sable Island Bank showing the range of smaller bedforms superimposed on the larger sand ridge morphology (from Li et al., 2012).

Sediment transport studies indicate that these bedforms are active under combined storm wave and current conditions (Li et al., 2012). Whereas their net movement over annual time scales appears to be small, they can migrate at several metres per year in opposing directions over shorter time scales. Wave energy converters and associated cables located in this sandy shelf environment would therefore be subject to frequent inundation and scouring due to the migration of bedforms. The largest bedforms (sand ridges) have crest to trough heights of up to 9.3 m so careful consideration should be given to wave converter siting and cable design in this environment. An understanding of the bedform dynamics at a prospective site would be crucial for reducing site risks.

9 Acknowledgements We are grateful to Chris Campbell (Ocean Renewables Canada), Andrew Cornett (National Research Council) and the late Mike Tarbotton (Triton Consultants Ltd.) for briefings on the marine renewable energy industry and resource assessment studies at the beginning of our work on this subject. This work was initiated with funds from the Government of Canada’s Clean Energy Fund. Completion of the work was facilitated by the understanding of Bernard Vigneault and Adrienne Jones.

10 References Adams, J., 1979. Gravel Size Analysis from Photographs. Journal of the Hydraulics Division, American Society of Civil Engineers,. 105, 1247-1255. Agrawal, Y.C. and Pottsmith, H.C., 2000. Instruments for particle size and settling velocity observations in sediment transport. Marine Geology, 168, 89-114. Allen, G.P. and Posamentier, H.W., 1993. Sequence stratigraphy and facies model of an incised valley fill: the Gironde estuary, France. Journal of Sedimentary Petrology, 63, 378-391. May 2015

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