beaches

beaches Beaches consist of accumulations of unconsolidated sand or shingle deposits, or both, along the shoreline. The form of a beach is largely controlled by wave energy and particle size, and its shape can change rapidly in response to short-term variations in wave climate. The form of the beach is thus able to maintain a dynamic equilibrium with the prevailing environmental conditions.

Although many people consider the beach to extend from low water of spring tides to the limit of wave action, this is in error since this definition does not contain the sediments in the offshore zone, which are still moved by wave processes. Perhaps a better definition is provided by Komar, who describes the beach (littoral zone) as the zone in which coastal sediments are affected by wave processes. The zone thus extends from water depths of 10–20 m below low-tide level to the landward limit of storm waves.

In general, two major beach profiles tend to be recognized: steep/storm profiles and shallow/swell profiles. These are, however, really end-members within a continuum. The gradients associated with these profiles also vary between sand and shingle beaches, the latter being generally steeper. The profile usually consists of a series of ridges and troughs that occur both above and below the water level (Fig. 1). These features tend to be mobile, moving offshore and onshore. Steep profiles usually consist of a marked landward ridge, usually referred to as the berm, which forms the landward limit of wave swash. In contrast, shallow beaches rarely have a berm developed; rather, a bar forms just below the low-tide level. This bar is referred to as the long-shore bar. The shape of the profile is controlled by two factors: waves and sediment type. In terms of waves, spilling breakers tend to be associated with shallow gradients and the landward movement of material, whereas plunging/surging breakers are commonly associated with steeper gradients and the seaward movement of material. In terms of sediment type, the coarser the material, the steeper the gradient. This relationship has often been linked to the associated percolation rate. With shingle, the water tends to sink into the beach and hence there is little backswash. The material is consequently not combed down and the gradient is steep. In contrast, on sandy beaches percolation is limited. Backwash is thus more pronounced and the process maintains a lower gradient. In many instances a combination of sediments is present. Hence along the shoreline there is a well-developed, steep shingle ridge, whilst seaward of this is a low-gradient sandy beach (Fig. 2). In such circumstances the day-to-day evolution of the profile may vary with tidal movements. At low tide, the low sandy beach promotes spilling breakers and thus landward movement of beach material. In contrast, at high tide, the steep shingle beach produces plunging breakers against the berm causing the seaward movement of material. Clearly, contrasting processes will also occur during storms, when the characteristics of the waves change.

Viewed from the air, most beaches are curved in outline and many of them contain a series of regularly spaced secondary curved features. In general, the smaller the feature the shorter its residence time within the environment. For example, cusps (small crescentic forms on the upper beach) may last only for a matter of hours, whereas accumulations developed between headlands may remain for several hundred years.

A wide variety of beach features have been described. More detail is provided in the books by Pethick and Carter. The various forms include:

Bay-head/pocket beach: an accumulation of sand and/or shingle located along the shoreline between two headlands (Figs. 2 and 3).

Zeta-form or fish-hook beach: the width of the beach increases down-drift to terminate against a headland. The feature is then usually repeated downdrift of this headland (Fig. 3).

Spits: beach, which is attached to the coastline at only one point (Fig. 3; see spits).

Cuspate foreland, where the beach leaves the coastline and runs seaward at an angle before returning to meet the coast once more (Fig. 3); e.g. Dungerness, Sussex.

Tombolo: beach, which joins an island to the mainland (Fig. 3).

Barrier beach: a sand or shingle accumulation completely detached from the main shoreline (Fig. 3); e.g. The Bar, Nairn, Scotland.

The form of the beach is largely dependent upon the balance between wave energy and availability of sediment. In general, the waves will transport material along the shore until an equilibrium form is developed (sediment arriving in the area is balanced by material being eroded). Clearly, however, the nature of the feature varies on a day-to-day basis with changing wave energy.

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.
Komar, P. D. (1976) Beach processes and sedimentation Prentice-Hall, Englewood Cliffs, New Jersey.
Pethick, J. (1984) An introduction to coastal geomorphology. Edward Arnold, London.
Viles, H. and and Spencer, T. (1995) Coastal problems: geomorphology, ecology and society at the coast. Edward Arnold, London.