using monitoring data to make a difference: cornwall ...

A-M. Moon, N. Pontee, M. Clemo, J. Rogers. Monitoring to make a difference: Cornwall dune management. FCRM09, 30th June - 2nd July 2009, Telford International Centre.

USING MONITORING DATA TO MAKE A DIFFERENCE: CORNWALL DUNE MANAGEMENT Anne-Marie Moon1,Dr Nigel Pontee1, Dr Jonathan Rogers2 and Robert Webbon1 Martin Clemo3 1Halcrow

Group Ltd, Burderop Park, Swindon, UK Group Ltd, Sowton, Exeter, EX2 7LB 3Restormel Borough Council, Cornwall, UK

2Halcrow

KEY WORDS: Monitoring Data, Sand Dunes, Management ABSTRACT In order to fulfil their role as a natural coastal defence, the various pressures imposed on sand dunes must be managed in a sustainable fashion. This paper describes the use of monitoring data collected as part of the South West Regional Coastal Monitoring Programme (SWRCMP) in the development of a management strategy for coastal dunes in Cornwall. The main elements of the work are: •

A Sand Dune Inventory;

•

A Best Practice Management Guide; and,

•

Two pilot Dune Management Plans.

In the development of the Sand Dune Inventory, monitoring data provided information on the location and existing state of the 25 sand dune sites in Cornwall. Of the 25 sites identified the data was then used to identify nine representative sites for further study. This was based on the dune characteristics and main drivers at each site. Studies were undertaken on these nine sites to provide more detailed investigation into the coastal processes and impacts affecting the sand dunes, to aid the development of the Best Practice Management Guide. The SWRCMP monitoring data formed the basis of the analysis for these detailed studies, which included assessment of the sediment budget and storm response. In the final stage of the project two pilot Dune Management Plans were developed based on the detailed studies and analysis of additional monitoring data available. The development of the Dune Management Plans was used to test and refine the Best Practice Management Guide. As the SWRCMP continues and a longer data set is compiled, this information can be integrated into future periodic reviews of the two Dune Management Plans.

INTRODUCTION Coastal sand dunes and their associated natural beaches are one of the most important natural resources in Cornwall for their: coast defence function; tourism value; and, habitat value. Coastal dunes in Cornwall comprise 15% of the overall area of coastal dune complexes in the UK. In Cornwall there are 25 dune systems which vary in size; from small pocket beach systems of less than a hectare in size; to large dune systems up to 550 hectares in size. The majority of these dune systems are currently eroding. In order to fulfil their role as natural coastal defences, and to provide important tourism resources, the various pressures imposed on the dunes by natural processes, such as sea level rise and wave action; as well as those imposed by tourism, such as trampling; need to be managed in a sustainable way.

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This paper outlines the use of monitoring data collected as part of the South West Regional Coastal Monitoring Programme (SWRCMP) in the development of a management strategy for coastal dunes in Cornwall. The strategy involves studies of a number of representative sites in Cornwall, and a review of management techniques from elsewhere. The main elements of the Strategy are: • A Sand Dune Inventory; • A Best Practice Management Guide; and, • Pilot Dune Management Plans. Following collation of data for the Sand Dune Inventory, detailed studies of representative sites were carried out using data from the SWRCMP. The data analysis provided an understanding of the dune systems to date. This analysis aided the development of the best practice management guide. More detailed analysis was then carried out to develop the pilot beach management plans. As the monitoring scheme continues into the future it will be able to provide information for future reviews of the management plans and implemented techniques. MONITORING DATA A regional monitoring programme for the Southwest of England commenced in Late 2006, entitled the Southwest Regional Coastal Monitoring Programme (SWRCMP) (CCO 2007). The data from this ongoing programme was used by the Cornwall Sand Dune and Beach Management Strategy. The monitoring data collected includes: • • • • • •

LiDAR data – once every 1 to 5 years; topographic survey data profiles – 1,2 or 3 per year; topographic baseline surveys – once every 5 years, 2 years or annual; hydrographic surveys – once every 5 years; ortho-rectified photography – once every 5 years; and, non ortho-rectified photography – 3 yearly

The programme was designed on a risk-based analysis - the more vulnerable or eroding sites are monitored more frequently than those which are known to be stable. The availability of this monitoring data enables data to be analysed to enable successful management of beach and dune systems. This will enable studies to be carried out without the costs of carrying out specific survey work, and as the collected dataset lengthens over time longer term trends will be able to be identified; these cannot be derived from short term surveys carried out specifically for individual studies. SAND DUNE INVENTORY The Sand Dune Inventory was developed to collate existing data on Cornish Dune Sites. Key data requirements which could be answered, in whole or in part, through the SWRCMP data included: Dune Location The identification of dune sites was initially carried out using Mapping and Aerial Photographs provided by the monitoring programme. This was followed up with a visit to each of the dune sites. Dune Area The total area of dune system at each site was measured from aerial photographs using GIS software. This could be assessed much more accurately than if OS mapping were used. The dune area was further assessed and divided into inactive and active dune area. The active dune area was defined as the currently mobile and dynamic portion of the dunes that is likely to require management. The inactive dune area was defined as the area of dune that has become immobile, either due to human actions, or the development of thick, permanent vegetation cover. It was clear from aerial photographs where the dune system became fixed in this way. As well as identifying the potential area of dunes requiring management, this information is also important in assessing the dune dynamics and ability to react to both natural and anthropogenic pressures. Backshore Characteristics 2

The backshore characteristics are important in assessing the ability of the dunes to move and adapt to pressures. Built up areas behind the dunes, or steeply sloping land can restrict roll back processes. The aerial photographs enabled identification of dune sites limited by backshore characteristics such as tourist developments. Key Morphological Features Key morphological features of the dunes were identified from aerial photographs. Features such as blow outs, cliffing, the location of streams running through the dunes and bare sand along trampled footpaths were easily identified. Key features were noted and provide focus for future site visits and studies. Coastal Setting The coastal setting was again identified through aerial photography and mapping and further developed following site visits by coastal scientists and engineers. Site Specific Management Issues Questionnaires completed by local dune managers and literature reviews provided information on the key pressures at each dune site. Using aerial photography and topographic profile data this was assessed further through identification of the impacts of key pressures. Bare sand tracks through the dunes and large scale erosion at specific locations are clearly seen in aerial photographs as an overall view of the whole dune system can be gained, which is not possible when a person is on the ground.

DETAILED STUDIES AND BEST PRACTICE MANAGEMENT GUIDE A number of detailed studies were undertaken to provide more detailed investigation into the coastal processes and impacts affecting the study sites, and aid the development of the best practice guidance. The SWRCMP monitoring data formed the basis of the analysis for the detailed studies. Using the Dune Inventory, nine sites were chosen for detailed study. The sites were chosen based upon the dune and beach characteristics, past evolution, anthropogenic pressures and geographical location. The sites were chosen to be representative of the variety of dune sites and pressures around the coast of Cornwall. The nine study sites chosen are shown in Figure 1.

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Summerleaze Beach

Constantin\e Bay

Fistral Beach Perranporth

Par Sands

Harvays Towans Praa Sands

Marazion Poldhu Cove

Figure 1 – Location of 9 representative detailed study sites This section of the paper describes the use of the monitoring data in carrying out the detailed studies to understand the dune systems and enable the development of appropriate management techniques. Sediment Budget Dune and beach management issues are frequently related to the sediment budget, such as the loss of material due to wave erosion or the build up of aeolian sand on roads that requires removal. An understanding of the sources and sinks and the sediment transport pathways is therefore integral to the development of effective beach and dune management strategies (Figure 2). A clear understanding of how, when and where sediment is moving both to and from, enables the problems at a site to be understood. Consideration of future climate change impacts on the sediment budget is also needed to ensure that proposed management schemes are sustainable in the long term. All aspects of the SWRCMP data were used to contribute to the understanding of the sediment budget. Aerial photographs were first used to digitise the position of the dune front based on visible vegetation and stream positions. Aerial photographs from 2000 provided by the Environment Agency (EA) were compared with SWRCMP aerial photographs from 2006/2007, where available. The digitised positions were overlain to observe changes in the dune front and stream position over the 6 year period. (Figure 3). This data provided information on the contemporary changes in dune position which were then also be compared with changes indicated by historic mapping analysis. As repeat aerial photographs are collected through the SWRCMP longer term recession or accretion of the dune front can be identified.

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Onshore wave transport Sand Mining Longshore Transport Cliff Erosion

Beach Dune Erosion

Aeolian Transport

Longshore Transport

Dunes

Fluvial Sediments

Inputs (+) Outputs (-

Figure 2 – Schematic sediment budget for a beach dune system

Figure 3 – Aerial photograph analysis: Digitisation of the dune front at Fistral Beach Beach profiles were measured at each site by Halcrow Group Ltd in November or December 2006, to provide a baseline position for the dunes. Since then SWRCMP have carried out topographic surveys for many of the detailed study sites on a twice yearly basis. In total there were 4 topographic profile 5

surveys available for analysis at each detailed study site at the time of the analysis. Cross-Sectional Areas (CSAs) were calculated for each profile, using SANDS, based on a pre-defined Master Profile (Figure 4). For the purposes of the study the bottom of each Master Profile was set as the MLWS level, whilst the landward extent was defined by the least-landward survey, in order to include as much data as possible within the analysis. For each profile, the Master Profile was split into two sections one covering the beach part of the profile, and one covering the dunes. This allowed the analysis to look at variability in the CSA of the beach and dunes independently, and to investigate how the two systems interacted at each site. Dune Section

Beach Section

= Master Profile = Measured Profile

Landward Boundary – Start of Foredune Calculated Beach CSA

Lower Boundary MLWS

Figure 4 – Description of Master Profile for the beach sections Annual topographic profile data from the same season of consecutive years were compared to identify annual changes in the beach and dune profile. As only one to two year’s worth of data were available this currently provides an idea of only the short term changes in the dune system. Comparison of the twice yearly survey data enabled identification of seasonal changes in the beach and dune system. This seasonal analysis indicated whether sediment was lost during the stormier winter periods, or whether it was stored within the sediment system and returned to the dunes in summer months when conditions were calmer. An idea of the dynamic interaction between the beach and dunes was also gained. The process of material being released from the dunes into the beach in winter, for example, was identified as the dune profile eroding while the beach profile accreted. Analysing the monitoring data in this way provided an understanding of the health of the dune system. Combined with other information such as historical mapping analysis, site visits and information from other studies an understanding was gained of the overall sediment budget. It was identified whether there is sediment available for accretion or whether the site is eroding because sediment is lost from the system and not replaced. Understanding these processes enables appropriate management techniques to be identified and inappropriate techniques to be filtered out. Sand trapping, for example, would not be appropriate at sites with a negative sediment budget where there is little or no mobile sand available to be trapped. Storm Response

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Storms play an important role in determining both short and long term dune behaviour. The form of the beach and dunes following a storm event provided important information on the management techniques required. Storm wave erosion is a key problem at exposed sites on the Cornish coast, such as Fistral Beach, where large volumes of sand can be lost as the result of a single storm. By assessing the storm impacts and the storm response of the beach it was assessed whether management should be implemented to minimise the impacts of storm waves or aid post storm recovery. SWRCMP includes post storm response surveys to record the beach profiles within one tidal cycle of a storm event at selected beaches. Photographs are also taken at the location of the beach profiles. Comparing pre and post storm surveys, and also later surveys, indicated the change in profile resulting from the storm as well as the ability of the beach to recover. Photographs taken at profile locations also provided an idea of the impact of storm conditions upon the dune vegetation and the methods of failure/ erosion such as slumping of the dune face. As more post storm data is collected, storm thresholds for damage to the beach can be identified. Wave buoys located around the Cornwall coast as part of the SWRCMP also provide real time information on inshore wave conditions. These were used within the SWRCMP to identify storm conditions to initiate post storm surveys. This data was used within this study to assess the magnitude of the storm which the beach and dunes were subject to prior to the post storm survey. DUNE MANAGEMENT PLANS The Best Practice guidance was tested and refined through the production of two pilot Dune Management Plans (DMPs). The DMPs provide a long term approach (50-100 years) to management of the beach dune system. They include guidance for future monitoring, beach and dune action levels and the nature of actions required to sustain the overall management objectives if those levels are reached. These DMPs are based on the existing information collated in the Dune Inventory, monitoring data available from the SWRCMP, and further site visits. The creation of the two worked examples of DMPs will allow other users to use the Management Guide and Dune Inventory to produce DMPs for their sites. Each DMP includes a geomorphic assessment of the likely beach-dune evolution over the next 50 to 100 years. This was completed through geomorphological appraisal based upon a review of SWRCMP monitoring data through the detailed studies and the review of existing collated information. The following were considered; • the physical nature of the beach and dunes; • the natural forces impacting upon the beach and dunes; • beach and dune response to extreme events; and, • the influence of coastal defence structures present. Additionally, a detailed assessment of the uses and pressures on the dune sites provides the basis to define clear management objectives and identify the most appropriate management techniques. The DMPs will be reviewed periodically. During this time, as the SWRCMP continues, the monitoring dataset will lengthen and start to provide information on longer term trends within the dynamic beach and dune system.

FUTURE APPLICATIONS OF MONITORING DATA The SWRCMP is planned to continue into the future. As more data becomes available this can be integrated into future reviews of DMPs. The one to two year data set available at the time of this study was able to provide a greater understanding of the beach dune systems than if no monitoring data were available. This data however only provided information on recent short term changes in the beach dune system. As the monitoring programme continues, more data will be collected and further work can be carried out. This section outlines some of the ways in which future monitoring data can be used and integrated into the DMPs reviews.. Through application of dune management alongside detailed monitoring of the site, adaptive management can be implemented. This can be done through assessment of the monitoring data to evaluate the effectiveness of the management techniques in place. Management can then be modified and refined as required to develop a sustainable long-term management approach. 7

The data used in this study enabled identification of short term trends in the behaviour of the dune and beach system. As monitoring continues, and a longer database is developed, longer term trends can be indentified. From this it can be assessed whether the trends identified within this study, such as erosion, will continue in the longer term requiring management, or whether they are short term trends, with recovery periods in which accretion will occur and hence less or different management will be required. This can be incorporated into the DMP reviews as the monitoring data set develops. Where possible the DMP identifies trigger levels at which intervention will be required to aid recovery of the beach dune system. Ongoing monitoring of beach and dune levels will indicate when levels approach or exceed the trigger level, indicating that action is required. Once longer term monitoring data is available and more information is gained on the recovery of the beach dune system, the trigger levels can be reassessed and revised if required. At the time of this study only one set of LiDAR data was available. As the monitoring programme continues and repeat LiDAR is available this can be used to provide elevation data further into the dune system. This can be used alongside aerial photography to monitor specific features within the system such as the development of blow outs. Only one set of hydrographic data was also available at the time of this study. As repeat hydrographic surveys are carried out the changes in offshore bathymetry can be analysed to provide further information on offshore sediment movements, adding to the sediment budget analysis. CONCLUSION Coastal sand dunes and their associated natural beaches are one of the most important natural resources in Cornwall for their: coast defence function; tourism value; and, habitat value. A management strategy for coastal dunes in Cornwall was developed based on the use of monitoring data collected as part of the South West Regional Coastal Monitoring Programme (SWRCMP). The strategy involved studies of a number of representative sites in Cornwall, and a review of management techniques from elsewhere. Within this project monitoring data made a difference in the following ways: Allowed identification of the 25 dune systems around the Cornwall Coast; Enabled identification of dune characteristics; Enabled identification of key morphological features; Enabled identification of key pressures and impacts at each site; Contributed to the understanding of the Sediment Budget through assessment of annual and seasonal trends in breach profile data; • Enabled analysis of post storm data to provide a greater understanding of storm response; and, • Contributed to the development of 2 DMPs to plan long term management of the Dune systems; • In the future, the continuation of the monitoring programme will enable longer term trends to be assessed and will contribute to periodic updates of the DMPs. • • • • •

The use of SWRCMP data reduced the need for survey work to be included within this study. The use of regional monitoring data therefore provided significant cost savings without compromising the data collection and quality. As the monitoring programme continues and longer term data sets are available, this will provide further benefits to future studies as any project specific data tends to be over short time periods and hence unable to provide information on longer term changes to the beach/ dune system.

References BTCV (2005) Sand Dunes – A Practical Handbook, BTCV, Doncaster, pp109. Burgess K, Jay H, Hutchison I, Balson P, Ash J (2001) Futurecoast: Assessing Future Coast Evolution. 36th MAFF Conference of River and Coastal Engineers Carter, R.W.G (1988) Coastal Environments, Academic Press, Northern Ireland, pp617

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Channel Coastal Observatory (2007) Regional Strategic Coastal Monitoring Programme Website (Accessed 2007) www.channelcoast.org CIRIA, 1996. Beach Management Manual, CIRIA Report 153, Construction Industry Research and Information Association, UK Fisher, E (2001) Coastal Sand Dunes, An Evolutionary Industry, MSc Thesis, The University of Southampton (unpublished). Halcrow (2002) Futurecoast CD, Produce by Halcrow Group Ltd, Swindon UK, as part of the Futurecoast study for DEFRA 2002. Halcrow (2007) SANDS Database Website (Accessed 2009) http://www.halcrow.com/software/solutions/sands_home.asp Pye, K. (Ed) (1994) Sediment Transport and Depositional Processes, Blackwell Scientific Publications, Cambridge, pp397 Pye, K.(2007) Sand Dune Processes and Management for Flood and Coastal Defence, defra project FD 1302, Available from defra website (Accessed 2007) www.defra.gov.uk Scottish National Heritage (2000) A Guide to Managing Coastal Erosion in Beach/Dune Systems, SNH, Scotland, pp128.

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