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 lowlying 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 tidesādoes 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.
Management Box 1.1
Definitions of the coastal zone for planning and management
The definition of the coastal zone given in section 1.1.1 is very much for the use of physical scientists studying the coast. However, for planning and management purposes, where administration is involved, the coastal zone is much more variably defined. Kay and Alder (2005) give a range of definitions used by various organisations in international and national government. Some definitions are known as distance definitions, whether fixed or variable, where the coastal zone is defined as being so many kilometres landward, and so many nautical miles seaward, of the shoreline. Other definitions do attempt to recognise and incorporate aspects of the working complexity of the coastal zone. In abbreviated form, these include:
- āthe coastal waters and the adjacent shorelands strongly influenced by each other, and includes islands, transitional and intertidal areas, salt marshes, wetlands and beaches. The zone extends inland from the shorelines only to the extent necessary to control shorelands, the uses of which have a direct and significant impact on the coastal watersā (United States Federal Coastal Zone Management Act)
- āas far inland and as far seaward as necessary to achieve the Coastal Policy objectives, with a primary focus on the land-sea interfaceā (Australian Commonwealth Coastal Policy)
- ādefinitions may vary from area to area and from issue to issue, and that a pragmatic approach must therefore be takenā (United Kingdom Government Environment Committee)
- āthe special area, endowed with special characteristics, of which the boundaries are often determined by the special problems to be tackledā (World Bank Environment Department).
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 earthās 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 earthās interior to the surface is responsible for driving continental drift and is the energy source in the plate tectonics theory. Its influence on the earthās surface, and the coast is no exception, is to generally raise relief, which is to generally elevate the land.
- The second category of processes is known as exogenetic processes, which are those processes that operate at the earthās surface. These processes are driven by solar energy. Solar radiation heats the earthās 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).
Figure 1.1 Endogenetic and exogenetic energy and processes and their contribution to the development of coastal landscapes.
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 systemsā 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.
- Cascading systemsāthis 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 systemsāthis 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.
- Ecosystemsāthis 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.
Figure 1.2 Types of systems.
Source: Briggs et al. (1997: 5, fig. 1.6).
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 ...
