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1 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 INTRODUCTION
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):
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.
Figure 1.1 Endogenetic and exogenetic energy and processes and their contribution to the development of coastal landscapes.
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.
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.
Figure 1.2 Types of systems. Source: Briggs et al. (1997: 5, fig. 1.6).
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.
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 ...
