coasts and coastal processes

coasts and coastal processes In his book on coastal environments, published in 1988, R. W. G. Carter defined the coastal zone as ‘that space in which terrestrial environments influence marine (or lacustrine) environments and vice versa’. The coastal zone is of variable width and may also change in time. Delimitation of zonal boundaries is not normally possible, more often such limits are marked by an environmental gradient or transition. At any one locality, the coastal zone may be characterized according to physical, biological or cultural criteria. These need not, and in fact rarely do, coincide. In more simple terms, the coastal zone is often taken to be that area where land, water, and air meet or the zone in which wave processes have an active affect upon the Earth's crust. It is thus important to realize that the coastal zone includes areas both above and below sea level.

The coastal zone has always been important to society, for it provides food and was once a focus for transport and industrial development. In recent years, the emphasis has shifted towards leisure. The importance of this zone is highlighted by the fact that the United Nations estimate that some 66 per cent of the world's population live within a few kilometres of the coast. The coastal zone is thus under pressure, and a good understanding of the processes associated with it is essential to society.

Classification

Various attempts have been made to classify coastal landscapes, but it has been recognized that none of them is ideal. On the one hand there are the genetic classifications based on the origin of the coastal landforms (e.g. shorelines of emergence or submergence); on the other there are descriptive classifications based on the form of the landscape (e.g. fjord, delta, cliffed, coral, barrier island, mangrove). A widely used classification was developed in the 1950s by Valentin, who made a fundamental distinction between advancing and retreating coasts (Fig. 1). His classification recognized that advance may be due to coastal emergence and/or progradation by deposition, whereas retreat is due to coastal submergence and/or retrogradation by erosion (Table 1).

Table 1. Classification of world coastlines (after Valentin (1952))

(A) Coasts that have advanced

(1) as a result of emergence

(a) emerged sea-floor coasts (e.g. Hudson Bay)

(2) as a result of organic deposition

(b) phytogenic (formed by vegetation): mangrove coasts (e.g. north coast of Western Australia)

(c) zoogenic (formed by fauna): coral coasts (e.g. Tahiti)

(3) as a result of morganic deposition

(d) marine deposition where tides are weak: lagoon-barrier and dune-ridge coasts (e.g. Gulf coastof USA)

(e) marine deposition where tides are strong: tide-flat and barrier-island coasts (e.g. Holland)

(f) fluvial deposition: delta coasts (e.g. Rhône, Niger, Mississippi)

(B) Coasts that have retreated

(1) as a result of submergence of glaciated landforms

(g) confined glacial erosion: flord-skerry coasts (e.g. Norway)

(h) unconfined glacial erosion flard-skerry coasts (e.g. Baltic Findland)

(i) glacial deposition: morainic coasts (e.g. Baltic Denmark)

(2) as a result of submergence of fluvially eroded landforms

(j) on young fold structures: embayed upland coasts (e.g. Greece)

(k) on old fold structures: ria coasts (e.g. south-west England)

(l) on horizontal structures: embayed plateau coasts (e.g. Red Sea)

(3) as a result of marine erosion

(m) cliffed coasts (e.g. the English Channel)

A more recent classification based on a tectonic framework was produced by Inman and Nordstrom in 1971. This scheme (Fig. 2) recognizes broad distinctions which are influenced by tectonics (mountain coast, narrow shelf, broad shelf) and morphologic form (headlands and bays, coastal plain, deltas, reefs, glaciated regions) and it accepts that more one than factor may influence a region.

Coastal processes

The geomorphic form and evolution of coasts is largely controlled by six major factors: waves, tides, offshore topography, bedrock geology, sediment supply, and sea-level changes. However, in the recent past society has been having an increasing impact on the system.

Waves

Waves are undulations of the water surface produced by winds blowing over it. The turbulent flow of air across the water surface sets up pressure variations which initiate the waves. Once in place the waves help to disturb the air flow, and as a consequence continue to grow while the air flow continues. The final size of the wave depends upon the wind speed, how long the wind continued to flow, and the distance over which the wind blew (fetch). In enclosed basins (Irish Sea, North Sea) fetch is usually of vital importance, since it will determine the magnitude of the wave and hence the energy it can expend in sediment movement and erosion.

Waves provide a means of transmitting energy through water with relatively small displacement of the water particles in the direction of energy flow. The water particles move in an orbital fashion, and the magnitude of movement decreases with depth. In deep water the waves move unimpeded through the liquid, but as they move into shallow coastal waters (where the water depth is less than half the wavelength) they begin to interact with the sea floor. In general, water movement at depth becomes retarded and as a result the wave starts to slow down. As the wave slows down there is a decrease in wavelength (the distance from one wave crest to the next) and an increase in wave height (the distance between the wave crest and the trough). During this process waves approaching at an angle to the shoreline tend to become realigned (refracted) so that their angle to the shoreline is reduced. The orbital movements within the wave become more elliptical and the shoreward movement of the water increases until it is greater than the wave velocity. At this point, the orbital motion can no longer be completed and so the wave front collapses (breaks) sending a rush of water (swash) on to the shore. The breakers are said to plunge (Fig. 3) when the wave crest curves over and collapses with a crash, or to spill (Fig. 4) when the crest of the wave flows more gently down the wave front to produce a rush of water on to the shoreline. The water then withdraws (the backwash) either as undertow (sheetflow near the sea bed) or in localized currents known as rip currents. In simple terms, spilling (constructive) breakers tend to be associated with the transport of sediments onshore, whereas plunging (destructive/ storm) breakers are associated with the movement of material offshore. Along the open coast waves are generally the principal source of energy and so they do most of the work. The waves arriving at the coast will thus determine its form and how it evolves. What must be remembered is that the waves are controlled by wind direction and air speed and, as a result, they can vary from day to day. The direction of wave approach will also vary and, consequently, sediment movement in the coastal zone can be complex, moving onshore, offshore, and alongshore. Often most of the work is achieved during storm (high-energy) events in which more sediment is transported in a few hours than during the preceding months of low-energy conditions.

Tides

Tides are movements of water bodies (oceans or seas) set up by the gravitational effects of the Sun and Moon in relation to the Earth. They are important in the coastal zone since they lead to regular changes in water level along the coast. In general, as the Earth rotates the gravitational attraction of the moon results in a highstand of water on the Earth's crust directly beneath the moon and on the opposite side of the earth. When the Sun and Moon are in alignment, the gravitational forces are greater and a higher (spring) tide is produced, but when they are aligned at right angles to each other the gravitational forces are reduced and a low tide (neap) results. The form of the tidal wave is also dependant upon the size and shape of the ocean basin. As a consequence, some water bodies (for example, Mediterranean Sea, the Black Sea; many Atlantic and Pacific shorelines) experience only very small tidal variations (less than 2 m). In other areas, in particular estuaries and embayments, the tidal variations become accentuated, resulting in spring tidal ranges in excess of 4 m (for example, the Bristol Channel (UK), 12.3 m; the Bay of Fundy, Canada, 15.2 m).

Contrasts in tidal range have implications in coastal geomorphology. A large tidal range produces a broad intertidal zone, and waves breaking against the high-tide line thus have much reduced energy, having crossed the shallow shore zone. In such circumstances, the waves will break along the high-tide line for only a short period of time. Wave action then has a limited opportunity to modify the shoreline. Such areas are often characterized by extensive salt marshes and mud flats. In contrast, where the tidal range is limited, waves constantly break along the same section of the shoreline and thus the potential to develop coastal wetlands is limited.

The movement of water associated with tides can also result in the formation of tidal currents. Along the open shoreline the currents associated with the rising (flow) tide and the falling (ebb) tide often counteract each other. In enclosed settings, such as estuaries, the tidal movements often set up currents which can transport sediment. For example, in some estuaries the ebb tide is often strongest within the central channel, whereas the flood tide is stronger along the shoreline. In this way, currents are set up which tend to move sediment up the estuary close to the shore and towards the open sea near the centre of the embayment.

Offshore topography

As a wave nears the coast in starts to interact with the sea floor. The topography of the offshore zone can thus influence how the wave energy is dissipated along the shoreline. When a wave moves into an embayment, the wave at the centre is in deeper water than those at the edge. As a consequence, the wave in the centre moves more rapidly and the wave front becomes refracted, tending to become aligned parallel to the shoreline (Fig. 5). The result is that wave energy tends to become dispersed in bays and concentrated on headlands. The offshore topography will thus determine the distribution of wave energy along the coast for a given direction of wave arrival. If the offshore topography is shallow, there is greater opportunity for the waves to interact with the sea bed and hence to become fully refracted. If the offshore zone is instead steep and narrow, the waves have little time to interact with the sea bed and thus refraction is minimal.

Bedrock geology and sediment supply

The evolution of the coastal zone is clearly controlled by the bedrock geology, both in terms of rock strength and structure. In general terms, ‘hard-rock’ coasts (granite, quartzite) evolve more slowly than ‘soft-rock’ coastlines (clays, tills). It is noteworthy, however, that processes of coastal erosion are also controlled by the climatic setting of the region (see rock platforms). Within erosional coastal settings wave processes tend to exploit zones of weakness, and the structural form of the bedrock geology can thus influence the form of the shoreline. Lines of weakness (e.g. faults) tend to undergo accelerated erosion, and as a consequence bays and headlands develop.

The nature of coastal sediments and their rate of supply will also influence coastal evolution. On open coasts where wave processes dominate, clastic sediments (sands to cobbles) will characterize the coastal zone. It is only in sheltered locations (embayments, estuaries) that energy levels are low enough for fine sediments (silts and clays) to be deposited. Where the rate of sediment delivery is high, the coastline will tend to prograde (build seaward). Where sediment supply is limited in relation to available wave energy, erosion will dominate.

Relative sea-level changes

The level of the water surface varies over time as a result of changes in ocean volume (see Quaternary sea-level changes) or movements of the Earth's crust. Such changes clearly affect the zone over which coastal processes operate, and thus influence the long-term evolution of the coastal zone. The impact that changes in sea level have upon a coastline are complex and are dependent upon other factors, such as sediment supply, coastal topography, and wave climate. Changes in sea level can certainly result in changes in the tidal range since they will alter the shape of an ocean basin. A detailed account of the impact that sea-level changes can have upon coastal systems is provided by R. W. G. Carter and C. D. Woodroffe in their book on coastal evolution.

Impact of society

In the recent past, society has increasingly had an impact upon coastal processes. Many of the structures (jetties, harbours, sea walls, breakwaters) built by society disrupt the natural coastal processes and consequently result in erosion and deposition. A classic example is provided by the breakwater at Santa Barbara, California, built in 1930 to protect the harbour. This structure produced a barrier to sediment transport which resulted in the accumulation of material updrift of the barrier, within 7 years a spit began to build across the harbour mouth. In contrast, downdrift of the structure the lack of sediment being moved along the coast resulted in erosion of the shoreline over a distance of some 40 km. Additional examples of the impact that society can have upon the shoreline are given in Carter's book on coastal environments.

Callum R. Firth

Bibliography

Carter, R. W. G. (1988) Coastal environments: an introduction to the physical, ecological and cultural systems of coastlines. Academic Press, London.
Carter, R. W. G. and and Woodroffe C. D. (1994) Coastal evolution: Late Quaternary shoreline morphodynamics. Cambridge University Press.
Inman, D. and and Nordstrom, C. (1971) On the tectonic and morphological classification of coasts. Journal of Geology, 79, 1–21.