Coastal Systems offers a concise introduction to the processes, landforms, ecosystems and management of this important global environment. Each chapter is illustrated and furnished with topical case studies from around the world. Introductory chapters establish the importance of coasts, and explain how they are studied within a systems framework; subsequent chapters explore the role of waves, tides, rivers and sea-level change in coastal evolution.
Students will benefit from summary points, themed boxes, engaging discussion questions and graded annotated guides to further reading at the end of each chapter. Additionally, a comprehensive glossary of technical terms, a new list of associated videos made by the author, and an extensive bibliography are provided. The comprehensive balance of illustrations and academic thought provides a well balanced view between the role of coastal catastrophes and gradual processes, also examining the impact humans and society have and continue to have on the coastal environment.
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By Simon K. Haslett
University of Wales PressCopyright © 2016 Simon K. Haslett
All rights reserved.
COASTAL SYSTEMS: DEFINITIONS, ENERGY AND CLASSIFICATION
The space occupied by the coast is not easily defined. It is a complex environment that has attributes belonging to both terrestrial and marine environments, which defies a truly integrated classification. This chapter covers:
the definition of the coast from scientific, planning and management standpoints
the sources of energy that drive coastal processes
the architecture and working of coastal systems, introducing concepts of equilibrium and feedbacks
an introduction to coastal classifications, with an emphasis on broad-scale geological and tectonic controls
a discussion of the complexities of terminology used in studying coastal systems
1.1.1 DEFINING THE COAST
The coast is simply where the land meets the sea. However, applying this statement in the real world is not that straightforward. It is not always easy, for instance, to define exactly where the land finishes and the sea begins. This is particularly so for extensive low-lying coastal wetlands, which for most of the time may be exposed and apparently terrestrial, but a number of times a year become submerged below high tides – does this environment belong to the sea or to the land, and where should the boundary between the two be drawn? It is much more meaningful, therefore, not to talk of coastlines, but of coastal zones, a spatial zone between the sea and the land. Usefully, this has been defined as the area between the landward limit of marine influence and the seaward limit of terrestrial influence (Carter, 1988). If we accept this definition, then coasts often become wide spatial areas, for example, encompassing land receiving sea-spray and blown sand from beach sources, and out to sea as far as river water penetrates, issued from estuaries and deltas.
1.1.2 COASTAL ENERGY SOURCES
Coasts are not static environments and are in fact highly dynamic, with erosion, sediment transport and deposition all contributing to the continuous physical change that characterises the coast. Such dynamism requires energy to drive the coastal processes that bring about physical change, and all coasts are the product of a combination of two main categories of processes driven by different energy sources (Fig. 1.1):
1. The first category of processes is known as the endogenetic processes, so-called because their origin is from within the earth. Endogenetic processes are driven by geothermal energy which emanates from the earth's interior as a product of the general cooling of the earth from its originally hot state, and from radioactive material, which produces heat when it decays. The flux of geothermal energy from the earth's interior to the surface is responsible for driving continental drift and is the energy source in the plate tectonics theory. Its influence on the earth's surface, and the coast is no exception, is to generally raise relief, which is to generally elevate the land.
2. The second category of processes is known as exogenetic processes, which are those processes that operate at the earth's surface. These processes are driven by solar energy. Solar radiation heats the earth's surface which creates wind, which in turn creates waves. It also drives the hydrological cycle, which is a major cycle in the evolution of all landscapes, and describes the transfer of water between natural stores, such as the ocean. It is in the transfer of this water that rain falls and rivers flow, producing important coastal environments, such as estuaries and deltas. The general effect of exogenetic processes is to erode the land, such as erosion by wind, waves and running water, and so these processes generally reduce relief (however, sand dunes are an exception to this rule, being built up by exogenetic processes).
A third source of energy that is important for coasts is that produced by gravitational effects of the moon and sun. Principally such gravitational attraction creates the well-known ocean tides which work in association with exogenetic processes, but they also produce the lesser-known earth tides which operate in the molten interior of the earth and assist the endogenetic processes.
Ultimately, all coastal landscapes are the product of the interaction of these broad-scale process categories, so where endogenetic processes dominate, mountainous coasts are often produced, whereas many coastal lowlands are dominated by exogenetic processes. Commonly, however, there is a more subtle balance between the two, with features attributable to both process categories present.
1.2 COASTAL SYSTEMS
Natural environments have for some time been viewed as systems with identifiable inputs and outputs of energy (a closed system) or both energy and material (an open system), and where all components within the system are interrelated (Briggs et al., 1997). The boundaries of a system are not always easily defined, as we discovered in section 1.1.1 when trying to define the coast. Where we can identify a relationship between inputs and outputs, but do not really know how the system works, then we are dealing with a black box system (Fig. 1.2); the coast as a whole may be viewed as a black box system. A study of the system may reveal a number of subsystems within it, linked by flows of energy and matter, known as a grey box system; a coastal example of this may be a cliff system being eroded by wave-energy, which then supplies an adjacent beach system with sediment. Further investigation may reveal the working components of the system, with energy and material pathways and storages, known as a white box system; following on from our previous example, these components may include the rock type that the cliff is composed of, the type of erosion operating on the cliff, sediment transport from the cliff to the beach, beach deposition and its resulting morphology.
1.2.1 SYSTEM APPROACHES
At the finest scale then, a system comprises components that are linked by energy and material flows. However, there are four different ways in which we can look at physical systems.
1. Morphological systems – this approach describes systems not in terms of the dynamic relationships between the components, but simply refers to the morphological expression of the relationships. For example, the slope angle of a coastal cliff may be related to rock type, rock structure, cliff height, and so on.
2. Cascading systems – this type of system explicitly refers to the flow or cascade of energy and matter. This is well exemplified by the movement of sediment through the coastal system, perhaps sourced from an eroding cliff, supplied to a beach, and then subsequently blown into coastal sand dunes.
3. Process-response systems – this combines both morphological and cascading systems approaches, stating that morphology is a product of the processes operating in the system. These processes are themselves driven by energy and matter, and this is perhaps the most meaningful way to deal with coastal systems. A good example is the retreat of coastal cliffs through erosion by waves. Very simply, if wave- energy increases, erosion processes will often be more effective and the cliff retreats faster. It is very clear from this example, that the operation of a process stimulates a morphological response.
4. Ecosystems – this approach refers to the interaction of plants and animals with the physical environment, and is very important in coastal studies. For example, grasses growing on sand dunes enhance the deposition of wind-blown sand, which in turn builds up the dunes, creating further favourable habitats for the dune biological community, and indeed may lead to habitat succession.
1.2.2 THE CONCEPT OF EQUILIBRIUM
Coasts are dynamic and they change frequently. These changes are principally caused by changes in energy conditions, such as wave-energy for example, which may increase during storms. The morphology of the coast responds to changes in energy because it aims to exist in a state of equilibrium with the reigning processes (Smithson et al., 2008). However, there are three types of equilibrium (Fig. 1.3):
1. Steady-state equilibrium – this refers to a situation where variations in energy and the morphological response do not deviate too far from the long-term average. For example, along a coast that experiences relatively consistent wave-energy conditions, the gradient of a beach may be steeper at certain times of the year, and shallower at others, but the average annual gradient is similar from year to year.
2. Meta-stable equilibrium – this exists where an environment switches between two or more states of equilibrium, with the switch stimulated by some sort of trigger. An example of this includes the action of high-energy events, such as storms or tsunami, which can very rapidly switch a coastal system from one state of equilibrium to another, by removing or supplying large volumes of beach sediment for example. Also, human activity often has this effect on coastal environments.
3. Dynamic equilibrium – like meta-stable equilibrium, this too involves a change in equilibrium conditions, but in a much more gradual manner. A good example is the response of coasts to the gradual rise in sea levels that we have experienced through the twentieth century as a result of climate change.
1.2.3 SYSTEM FEEDBACKS
Understanding states of equilibrium in a system requires some knowledge of feedbacks within a system. Feedbacks occur as the result of change in a system and they may be either positive or negative, respectively switching the system to a new state of equilibrium or attempting to recover to the system's original state of equilibrium. Positive feedbacks therefore tend to amplify the initial change in the system so that, for example, the ridge of a coastal sand dune breached by storm wave erosion may be subsequently laterally undercut by wind erosion, so fragmenting the dune ridge and leaving it more susceptible to further wave erosion (Fig. 1.4a). Ultimately, the entire dune ridge may be relocated further inland and a new state of equilibrium reached. Negative feedbacks, however, tend to dampen the effect of the change. For example, sand eroded during a storm from the front of sand dunes at the back of a beach may be redeposited as offshore sand bars, which help to protect the beach-dune system from further storm waves, by reducing the amount of wave-energy reaching the dune front (Fig. 1.4b). Managing coastal systems requires a detailed knowledge of feedbacks, as all too frequently, as we shall see throughout this book, human intervention in one part of a coastal system often leads to a number of apparently unforeseen and undesirable feedbacks.
1.3 THE CLASSIFICATION OF COASTS
Because there is such a wide variety of factors that affect coasts, it has been very difficult to actually create an integrated classification scheme (Finkl, 2004). As a result there have been a number of attempts to classify coasts according to single parameters, such as wave or tidal environment, geology, and tectonic setting. Waves and tides are covered individually in other chapters in this book (see Figs 2.1, 3.4 and 3.6), and furthermore are usually only applicable on the local to regional scale. Here we will concentrate on the broad-scale geological and tectonic settings of coasts, which often are applicable to coasts along entire continental margins.
1.3.1 GEOLOGICAL CLASSIFICATION
E. Suess in 1888 put forward a coastal classification based on geological structure and its orientation as regards the general trend of the coastline. On this premise he recognised two types of coasts:
1. Pacific type – the orientation of the rock structure lies parallel to the coastline. This type of coast is also known as accordant or concordant, and often forms rather straight coastlines interrupted by relatively small embayments. The Dalmatian coast of the former Yugoslavia, in the Mediterranean, is an excellent example of a Pacific type coast.
2. Atlantic type – the orientation of the rock structure is at right angles (perpendicular) to the coastline. This coastal type is often known as discordant, and is characterised by prominent headlands and embayments. The southwest coast of Ireland, including Bantry Bay, is a good example of this coastal type.
Horsfall (1993) explores the application of this classification to the classic coastal scenery of Dorset, England. Although it is useful, this scheme is limited in that it gives no indication as to the dynamics of a coastline: is it submerging/emerging, or is deposition/erosion dominant? It is just a statement of the relationship between rock structure and general coastline orientation, which is a random relationship. Small-scale structure can also influence the character of a coast, such as the attitude of joints in granite (Plate 1.1).
1.3.2 TECTONIC CLASSIFICATION
The theory of plate tectonics describes the creation and destruction of crustal material, and in doing so it explains the movement of the continents around the globe. The earth's surface comprises a number of continental and oceanic crustal plates (Fig. 1.5). Each one of these plates is bounded by zones where either new crust is created (constructive boundary) or where old crust is destroyed (destructive boundary). Plate movement is usually away from constructive boundaries (divergence), and towards destructive boundaries (convergence). At the broad scale, there is often a consistent relationship between the characteristics of a coastline and the type of plate boundary that it is nearest to. Inman and Nordstrom (1971) recognise a number of coastal types based on the plate boundary they are associated with. Davis (1997) reviews leading edge coasts, trailing edge coasts and marginal sea coasts, all of which are briefly described below.
18.104.22.168 Leading edge coasts
These occur where a continental plate converges with an oceanic plate at a destructive boundary. Because of this, these types of coasts are also known as convergent margin coasts. Continental crust is less dense and, therefore, more buoyant than oceanic crust, resulting in the oceanic plate going under or subducting beneath the continental plate. The compressional forces created by convergence cause the rocks along the coast to buckle, fold and fault, uplifting them to create chains of coastal mountains. Earthquakes are commonly associated with this coastal type (see Box 1.3). The continental shelf in front of the mountain chain is usually narrow or even absent, and therefore the gradient from the top of the coastal mountains to the sea floor is usually very steep. Therefore, although much sediment is eroded from the uplifting coastal mountains by fast-running streams, it is usually lost into deep water via submarine canyons when it is introduced into the sea. Commonly then, leading edge coasts are characteristically mountainous, dominated by erosional processes, and so often rocky. The longest leading edge coast occurs more or less along the entire western seaboard of the Americas (Plate 1.2), both northern and southern continents, with the exception of southern California.
22.214.171.124 Trailing edge coasts
These are coasts that form as plates rift apart due to divergence, allowing the ocean to inundate the rift to create a new sea. Once formed, these coasts are carried away from the diverging boundary as passive continental margins (Plate 1.3). Shortly after rifting, the coasts comprise relatively high relief and possess a fairly steep gradient, and in many respects their topography is very similar to leading edge coasts. These are known as neo-trailing edge coasts and the present-day coasts of the Red Sea belong to this subdivision. As divergence progresses the sea expands and erosion of the coast increases, both by wave activity at the shoreline and through the action of high-energy streams flowing down steep hills. In this way, the continental shelf begins to widen, the relief is lowered, and the afro-trailing edge coast is created. As the name implies, most of the African coastline is of this type, with the exception of course of some Mediterranean and Red Sea coasts. Africa has been tectonically stable for many millions of years, and although the continental shelf is relatively wide now, large sedimentary features such as deltas are limited in comparison with the most mature trailing edge coastal subtype, the amero-trailing edge coast. This mature coastal type is characteristic of the eastern seaboard of the Americas, with an extensive coastal plain existing along the North American section, and vast deltas, such as the Amazon Delta, characterising the South American section.
126.96.36.199 Marginal sea coasts
This is a relatively uncommon coastal type and occurs where plate convergence takes place offshore, with a relatively wide continental shelf separating the plate boundary from the coast. In many ways, this coastal type is similar to trailing edge coasts, especially in the sedimentary features that may develop, but it suffers earthquakes more regularly and may experience limited tectonic movement. The Gulf of Mexico coasts are an example, with the volcanic islands of the Caribbean marking the zone of plate convergence.
Excerpted from Coastal Systems by Simon K. Haslett. Copyright © 2016 Simon K. Haslett. Excerpted by permission of University of Wales Press.
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Table of Contents
ContentsList of plates,
List of figures,
List of tables,
List of boxes,
Author's preface to the first edition,
Author's preface to the second edition,
Author's preface to the third edition,
1 Coastal systems: definitions, energy and classification,
2 Wave-dominated coastal systems,
3 Tidally-dominated coastal systems,
4 River-dominated coastal systems,
5 Sea level and the changing land-sea interface,
6 Coastal management issues,
Videos by the author,