Treatise on Estuarine and Coastal Science

TREATISE ON ESTUARINE AND COASTAL SCIENCE EDITORS-IN-CHIEF

Eric Wolanski James Cook University and Australian Institute of Marine Science, Townsville, QLD, Australia

Donald McLusky University of Stirling, Stirling, United Kingdom VOLUME 3

ESTUARINE AND COASTAL GEOLOGY AND GEOMORPHOLOGY VOLUME EDITORS

BW Flemming Senckenberg Institute, Germany

JD Hansom University of Glasgow, UK

AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO Academic Press is an imprint of Elsevier

3.01

Estuarine and Coastal Geology and Geomorphology – A Synthesis

BW Flemming, Senckenberg, Wilhelmshaven, Germany JD Hansom, University of Glasgow, Glasgow, UK © 2011 Elsevier Inc. All rights reserved.

3.01.1 3.01.2 References

Rationale Scope

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Abstract Coastal geodiversity, the geological and geomorphological processes and landforms of estuaries and coasts, is of vital importance as providing not only protection from marine processes such as storm waves but also the foundations for sustaining the integrity and biodiversity of ecosystems along our shores. Since the characteristics and functioning of coastal and estuarine ecosystems are addressed in subsequent volumes of the treatise, it is important now to contextualize coastal geodiversity as a key component in our understanding of how estuaries and coasts function, be it in relation to sandy beaches, tidal mud flats, or rocky coasts and in a range of latitudes from the tropical to polar shores. This volume, thus, sets out to capture some of the wide range of coastal geodiversity and set it within a framework that will be of relevance to the subsequent volumes of this treatise. In spite of this, we are aware that not all coastal contexts are represented here, and in future editions of the treatise we intend to take the opportunity to fill any gaps identified.

3.01.1 Rationale Over past centuries, the coastal zones of the world have progres sively become the most densely populated areas on the Earth, the rate of population expansion having steadily increased over time. Today, nearly 60% of humankind lives within 50 km of the coast and this rapidly increasing trend (e.g., Pernetta, 1994) has been, and is still, putting enormous pressure on coastal environ ments and their natural resources. In the context of climate change and the predicted acceleration of sea-level rise (IPCC, 2007), the environmental problems experienced today will inevitably increase in the foreseeable future. In addition, new and perhaps unforeseen problems will emerge that will require responsible action if the effects of natural hazards are to be mitigated and large-scale social disasters avoided, and although unrelated to climate the 2004 Indian Ocean and 2011 Japan tsunami events are sad cases to the contrary. Catastrophic events, in particular, call for rapid and well-coordinated responses. An important basis for such action is the availability of integrated coastal zone management plans that have been adapted to the specific requirements of the different coastal regions of the world (Krishnamurthy et al., 2008). Such plans need a firm basis in science (GESAMP, 1996) and the purpose of Volume 3 of this treatise, therefore, is to provide the basic scientific framework for a comprehensive understanding of the physical nature and functioning of estu aries and coasts in terms of geological setting, geomorphological expression, hydrodynamic processes, and sedimentary character istics and responses.

3.01.2 Scope Volume 3 of this treatise deals with geological and geomor phological aspects at global scales presented in 11 separate chapters. Chapter 3.02 focuses on the geology, morphology,

and sedimentology of estuaries and coasts. Together with the weathering products of igneous rocks, sedimentary rocks form the main sources of sediment supplied to the coasts of the world by glaciers, rivers, and erosion of the coast itself. It is estimated that between 20 � 109 to 70 t of sediment is deliv ered to the coast every year, part of which is stored along the coast in estuaries, deltas, and beaches (Figure 1). Estuaries show a marked tripartite longitudinal zonation that is inde pendent of tidal range. Both lower and upper estuaries are sandy and/or gravely, bioclastic material being restricted to the lower part and generally increasing in quantity toward the sea. Middle estuaries, by contrast, consist of muddy sediment formed by flocculation processes in the mixing zone between saltwater and freshwater. While the majority of beaches consist of gravels and sand, including various proportions of bioclastic material, some coasts remain muddy because wave action is unable to remove the large amounts of fine-grained sediment supplied by some rivers. The morphodynamic beha vior of beaches is finely tuned to local grain size and wave climate, beach slope generally increasing with increasing grain size, but decreasing with higher wave energy so that, for any given grain size, high-energy beaches tend to have flatter slopes than low-energy beaches. Chapter 3.03 deals with sea-level variations (Figure 2). The response of coastal geomorphic types and landforms to sea-level rise is complex, coastal forms being a product of interactions between energy and material, with step-wise adaptations to external forcing. Sea-level rise as a driver of coastal change needs to be seen within the context of other processes and determinants, particularly in relation to waves, tides, storms, and sediment supply. A range of coastal types is examined (tidal environments, beaches, barriers, deltas, cliffs and platforms, dunes, and coral atolls) to evaluate their response to sea-level rise and to provide an understanding of the likely geomorphic consequences of sea-level rise. It is recognized that sea level has a part to play in coastal change

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Figure 1 View of Kogel Bay, eastern seaboard of False Bay, Cape Province, South Africa. The coast of Kogel Bay is a good example of sediment sources and sinks in close proximity. Sand and gravel is deliv ered to the shore by gravitational processes supported by drainage from the steep hinterland comprising quartzite of the Table Mountain Group (gray color) and weathered Cape Granite (yellowish brown color). Note the small blind estuary along the beach receiving seawater by overwash processes. Photograph courtesy of Natalia Flemming.

for a wide variety of coastal environments over different timescales, but its significance must be evaluated relative to other drivers of coastal evolution, especially sediment supply and extreme events. Such understanding provides the basis for coastal management strategies in human responses to sea-level rise. Chapter 3.04 deals with wave-dominated coasts (Figure 3). The term ‘wave-dominated coast’ is applied to coasts with an abundance of sediment, primarily sand but including gravel and cobbles, and where contemporary coastal evolution is shaped through erosion, transport and deposition of sediment

Figure 2 Climate change will bring about significant changes at the coast as a result of sea-level rise and storms. Understanding how coasts respond to these drivers that have different time and length scales is of critical importance for sustainable coastal communities and economies. Here, in Santa Cruz, California, the city is built on a series of ancient raised beaches where old beach sediments, made up of loose sands, overlie a relict hard rock shore platform. Knowledge of the differential response of these two materials to storm waves focuses efforts for coastal protection not at the toe of the cliff but where the more easily eroded ancient beach deposits overlie the rock platform. Photograph courtesy of Andrew Plater.

Figure 3 View looking east of the nearshore beach and dune system at Greenwich Dunes on the northeast coast of Prince Edward Island, Canada, taken on 17 December 2007. A storm with strong winds from the north west the previous day has flattened the beach and eroded the embryo dunes. Falling temperatures have resulted in freezing of the beach and ice is beginning to form at the seaward edge of the foreshore. The inner nearshore bar has been flattened by the storm resulting in a wide inner surf zone. Wave breaking is occurring on the second nearshore bar with the landward trough marked by a distinct zone where there is no wave breaking. Winds have now switched to the south so that spray from breaking waves is carried offshore. The picture was taken by a monitoring camera set on a mast about 13 m above the beach. Photograph courtesy of Robin Davidson-Arnott.

by waves, and wave-generated currents. These coasts are also influenced by tides and tidal currents, but they play a subor dinate role compared to waves. Waves generated by strong local winds are characterized by short periods, whereas long-period swell waves along open ocean coasts are gener ated far out to sea. Waves may break directly on the beach or, with gently sloping beaches and large waves, some distance offshore, forming a surf zone between the breaker line and the beach. The average form of the beach and nearshore is con trolled by sediment size and by the characteristics of the wave climate, changes in the profile occurring in response to chan ging wave conditions on a timescale of hours to months. In addition to the sand and gravel beaches found along main land coasts, longshore transport of sediment may result in the formation of spits and bars, including barrier islands. These barriers are separated from the mainland by a lagoon, bay, or marsh, with the lagoon or bay being connected to the open sea by a tidal inlet or inlets cutting through the barrier. Barriers are subject to rapid change during storms when storm surges accompanied by large waves erode coastal dunes and overtop the barriers to form overwash fans. The stability of all mainland beach and barrier coasts is also influ enced by sea-level change. Sea-level rise over the next century is likely to lead to an increase in the proportion of wave-dominated coasts that are subject to increased erosion and landward transgression. Chapter 3.05 deals with river-dominated coasts (Figure 4). Many rivers deliver substantial amounts of sediment to the coast and, as a consequence, their mouths comprise a dynamic assemblage of landforms depending on wave climate, tidal range, and river discharge. Large Asian rivers and those on the tectonically active islands of Oceania supply more than 70% of the sediment reaching the oceans. Estuaries occur where


Figure 4 River-dominated coasts are typically characterized by deltaic depositional systems in low wave-energy environments, here exemplified by the Macquarie River delta, Australia. Photograph courtesy of Colin Woodroffe.

preexisting topography has not been completely infilled, whereas deltas commonly protrude from the coast, a classical example being the prominent triangular delta at the mouth of the Nile, the river bifurcating into distributaries before dischar ging into the sea. Detailed studies of the Mississippi Delta have provided chronological and stratigraphic frameworks for river-dominated depositional systems. As deltas are also influ enced by waves and tides, delta morphology has been successfully classified in relation to the relative dominance of river, wave, and tide influences. However, these processes can vary spatially and temporally in both the active delta and the abandoned delta plains that were deposited during former phases of delta progradation. This is very clearly demonstrated by the mega-deltas of Asia, but can also be recognized in smaller contexts around the Australian coast. Human activities have impacted many rivers, increasing sediment load on some and decreasing the flux in others through damming and diver sion. Such catchment impacts, together with rapid urbanization on delta plains, accentuate the threats posed by natural hazards and climate change. Chapter 3.06 deals with the morphodynamics of tide-dominated coasts, in particular those characterized by tidal flats. Tidal flats are common in sedimentary environments where the tidal range is large relative to wave height, although tidal flats can also occur where the mean range is less than 1 m, as long as mean wave heights are much smaller. In the absence of waves and mangroves or tidal marsh, the low-bed-slope environments that develop consist of sediments that are exposed subaerially between the lowest and highest astronomical tide. This chapter attempts to synthesize the morphodynamics of tidal flats of all grain sizes along both sheltered and open coasts and builds on earlier work by incorporating the additional morphodynamic understanding gained over the past 15 years. This includes a review of numerical models that improves the understanding of net transport of finer sediment due to asym metries in tidal forcing. These models predict that if offshore concentration is held fixed but tidal range is varied, the width of tidally dominated flats at morphological equilibrium will remain more or less constant and nearly independent of tidal range. Tidal asymmetry can result in the development of spec tacular tidal bores (Figure 5).

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Figure 5 Tidal bore near the town of Qiantang located in the funnel-shaped section of Hangzhou Bay, China. With heights of up to 9 m and traveling speeds of up to 40 km h−1, it is one of the largest tidal bores in the world. Tidal bores preferentially develop in large macrotidal estu aries or bays characterized by a marked landward convergence of the opposing shorelines, thereby causing dramatic increases in the height of the tidal wave, while friction at the seabed slows it down to the point that the crest of the tidal wave begins to overtake its base, causing it to break. Photograph by Burg Flemming.

Chapter 3.07 concerns itself with coastal cliffs and rock coasts in general (Figure 6). A high proportion of the world’s coasts is composed of rock and is found in almost every type of morphogenetic environment. Even where rocks do not dom inate the coastal scenery, they may provide the framework for depositional features, including pocket beaches between rock headlands, tombolos connecting rock islands and promon tories, and beaches on shore platforms backed by rock cliffs. This chapter provides an up-to-date review of the erosional processes on rock coasts including the several types of mechan ical wave erosion, weathering processes, biological activities, and mass movements. Significant advances have been made in our understanding of rock coast morphodynamics and evolution as a result, in part, of the use of specialized field and laboratory equipment, mathematical modeling, and remote-sensing techniques to measure rates of erosion. In

Figure 6 The granitic El Arco de Cabo San Lucas located at the extreme southern tip of Mexico’s Baja California peninsula is a good example of a plunging rocky cliff coast. Photograph courtesy of Alan Trenhaile.

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spite of our increased understanding of rock coasts, problems remain in determining their age and mode of development, the role of inheritance, the effect of rising sea level, and the relative importance in time and space of the myriad processes that operate according to climatic and geological conditions. Chapter 3.08 deals with dune coasts. A number of botanical and geomorphological classifications of coastal dunes exist, but many remain cumbersome and difficult to use. This chapter recognizes four main coastal dune types – foredunes, blowouts, parabolic dunes, and transgressive dune fields (composed of older established dunes) – but does not include dunes that have been partly or wholly built, modified or seriously impacted by humans. The morphological and sedimentological results of variations in flow dynamics over various types of dunes are reviewed in light of recent advances with particular emphasis on blowouts and parabolic dunes in the context of initiation, morphology, flow dynamics, and evolution. At a larger scale, transgressive dune fields are aeolian sand deposits formed by the downwind or alongshore movement of sand over vegetated or semi-vegetated terrain, sometimes bypassing rocky headlands (Figure 7). They range from small dune fields of a few hundreds of meters in alongshore and landward extent to very large fields with wavelengths of 300–5500 m and heights of 20–450 m. Transgressive dune fields are well developed on high wind and wave energy coasts with significant sediment supply and also on coasts with limited vegetation growth in semiarid, arid, arctic, and subarctic regions. Transgressive dune fields may be completely vegetated (postformation), partially vegetated, or largely unvege tated (fully active) and have also been termed ‘mobile’ or ‘migratory’ dunes. Transgressive sand sheets also occur and may form a landform unit or type within transgressive dune fields. Models of beach–dune interactions are reviewed focusing on potential aeolian sediment transport and foredune morphology, foredune ecology, foredune erosion processes, dune field devel opment, and the long–term role of sediment supply and sea level. Chapter 3.09 concentrates on glaciated coasts (Figure 8). These encompass all those coasts that are now or have been

Figure 7 The large and prograding transgressive dune field at Dona Juana on the Veracruz coast in Mexico displays a variety of dune land forms including deflation plains, gegenwalle (counter) ridges, transverse dune trailing ridges, blowouts and parabolic dunes, and barchanoidal transverse dunes with rainfall-filled interdunes. Flood-tolerant species are located in the lower parts with coastal matorral shrubs on older and drier parts, whereas burial-tolerant species dominate the mobile areas. Photograph courtesy of Patrick Hesp.

Figure 8 On glaciated coasts, small spits and numerous short and narrow barriers of mixed sand and gravel are common. They are often developed on a submergent irregular landscape with sediment sourced from localized backshore and reworked offshore glacigenic deposits, with partial supply from longshore. Near Seaforth on the Eastern Shore of Nova Scotia, spits and barriers connect closely spaced drumlins. Photograph courtesy of R.W.G. Carter.

affected by glacial ice at some time since the Last Glacial Maximum (LGM). In some situations, glaciation within the drainage basin may be sufficient to affect coastal processes, through proglacial or paraglacial reworking of glacigenic depos its and sediment transport to the shoreline. In others, the erosional imprint of glaciation dominates the coastal topogra phy. Glacial coasts are those where an existing ice front is in direct contact with seawater, such as occurs along the Antarctic coast where nearly 38% take the form of ice walls at the seaward limits of the Antarctic ice sheet. Proglacial coasts are those that are actively receiving outwash sediment such as proglacial deltas, fans, and braid plains, all of which are associated with shore-zone reworking. Paraglacial coasts occur on or close to formerly glaciated terrain where coastal morphology and evolu tion are influenced by the topographic imprint of glacial erosion or deposits, most importantly through the dominance of sedi ment supply from glacigenic deposits generated by fluvial or coastal reworking. The distribution of paraglacial coasts is approximately coincident with the limits of the LGM. More recently, the role of paraglacial effects has been recognized as a useful concept for understanding a broader range of surface processes and landforms that are directly conditioned by former glaciation and deglaciation in which sediment availability con ditioned by glaciation is the central unifying concept. Chapter 3.10 examines polar coasts (Figure 9). In contrast to glaciated coasts, polar coasts and subpolar coasts are dis tinctive because of extreme seasonality and the presence of various types of ice in the shore zone (predominantly tidewater glaciers, ice shelves, sea ice, and ground ice). Wave activity is effective mainly during the brief summers, yet imposes a strong morphological signature on most sedimentary coasts; however, some show little evidence of wave action and remain peren nially ice bound. Tidewater glaciers, grounded ice sheet margins, and ice shelves form ice coasts, which cover much of the Antarctic periphery and some parts of the Arctic, but many ice shelves in both hemispheres are in sharp decline due to climate change. The annual expansion and waning of sea-ice covers plays a variety of roles, as direct and indirect shore


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Figure 9 Polar coasts are characterized by the permanent or seasonal influence of ice in the sea, on the shore, and within the shore sediments as permafrost. Ice-push scours the nearshore and drives sediment onto the beach to form ice pileup of about 5 m high on the west coast of Banks Island, Northwest Territories, Canada. Note the delicate imprints of ice pileup higher on the beach. Photograph courtesy of Don Forbes.

protection (inhibiting waves or defending against them), as a constructive force pushing sediment landward and creating a variety of distinctive landforms, and as an erosional force that can be destructive to coastal infrastructure. Antarctic and sub-Antarctic coasts are dominated by rock or ice, and beaches are rare in Antarctica mainly due to an absence of meltwater streams. This restricts beach development to the few areas where glacial outwash transport of sediment occurs, notably the sub-Antarctic islands. The largest polar and subpolar coastal depositional systems occur across much of the Arctic where an extensive glacigenic and glacifluvial sediment supply is avail able. Where the coast is dominated by ice-rich sediments however, the effect of climate change on permafrost promotes enhanced rates of coastal erosion through combinations of thermal and mechanical processes. Chapter 3.11 is concerned with coastal and estuarine ero sion. Beach erosion and the loss or damage of shore-front properties are generally the result of multiple processes and environmental factors acting together. Of importance are cli mate controls on coastal processes, a foremost factor being the global warming responsible for increased rates of sea-level rise. Beginning in the late nineteenth century and extending well into the twentieth century, global mean ocean levels rose between 0.15 and 0.20 m. With the continued addition of greenhouse gases into the atmosphere and the associated increases in temperature and rates of glacial melting, the pro spects are that sea levels during the present century will rise by significantly greater amounts, projections being on the order of 0.5–1 m. At the same time, global warming appears to be the primary cause of increased storm intensities, both of tropical hurricanes and extratropical storms at high latitudes, with increasing wave heights having been measured by ocean buoys. The expectation is that, compared with that experienced in the past, the erosion and flooding of ocean shores will be substantially enhanced during the twenty-first century. Coastal erosion is also the result of more local human interventions, particularly those leading to decreases in the volumes of sand and gravel that reach the ocean beaches. For example, the construction of dams on rivers can cut off what had been the

Figure 10 The erosion of Siletz Spit, Oregon, USA where an El Niño event resulted in the elevation of winter monthly-mean water levels by 10s of centimeters, raising all tides by that amount throughout the entire winter. In addition to elevated tides, erosion caused by high storm waves was at its greatest where rip currents had cut embayments in the fronting beach, exposing the shore-front properties to wave impact. Photograph by Paul Komar.

primary source of sediment to the beaches. Good examples include the Aswan High Dam on the Nile River and the dams constructed on nearly every river that flows to the coast of California. Another significant cause of erosion is the construc tion of jetties and breakwaters along the coast, which prevents the longshore movement of beach sand so it can no longer reach and nourish distant beaches. Reviews are presented of the models that are employed by coastal scientists and engineers to evaluate the erosion impacts that result from both ocean pro cesses and human interventions and can be applied to project the increasing future impacts expected in the course of sea-level rise. Management strategies reviewed include a range of poten tial responses to the problem of beach and property erosion, the most common being the construction of protective seawalls and/or the nourishment of beaches involving the importation of sand to replace the losses incurred by erosion (Figure 10).

References GESAMP, 1996. The contributions of science to integrated coastal management. IMO/ FAO/UNESCO-IOC/WMO/IAEA/UNEP, Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection. International Maritime Organization (IMO), London. GESAMP Reports and Studies 61, 66 pp. Intergovernmental Panel on Climate Change, 2007. Integrating analysis of regional climate change and response options. WG1 Meeting Report, 20-22 June 2007, Denarau Island, Nadi, Fiji. NOAA, Washington, 271 pp. Krishnamurthy, R., Glavovic, B.C., Kannu, A., Green, D.R., Ramanthau, A., Han, Z., Tinti, S., Agardy, T.S. (Eds.), 2008. Integrated Coastal Zone Management: The Global Challenge. Research Publications Services, Singapore, 780 pp. Pernetta, J., 1994. Atlas of the Oceans. Octopus Publications Group, London, 207 pp.

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