Beach processes during storm events.
(U.S. Army Corps of Engineers, 1984)
Berm and swale on the foreshore
Definition: a strip of land between high and low water mark and made up of deposits of mud, sand or gravel which are frequently set in motion by the waves
A beach of sand and boulders is at first thought an unlikely landform to be found facing the high-energy environment of the open sea. Yet beaches take a twice-daily pounding from the waves as easily as bamboo in a breeze. The beach reacts - it adjusts its shape continuously in response to changing wave energy and so maintains a dynamic equilibrium with its environment. The beach is an energy sink and act as a buffer between waves and the coast. Wave energy is dissipated by wide flat beach profiles that spread the oncoming wave energy. Low energy flat waves are easily dissipated by a narrow steep beach. Shingle or boulder beaches can also dissipate wave energy in inter-particle friction and jostling.
A feature of many beaches is that low, flat swell waves, typical of summer, bring in 1m to 2m of offshore sand to fill the beach area and form a steepened profile. During winter, the high, steep storm waves erode this beach face and transport the sand seaward where it forms as a long-shore bar near the base of the beach profile, reducing the gradient.
Beaches can be divided into several units:
Both backshore and foreshore may include berms and swales.
Major storms bring major changes in beach profiles and may lead to long-term changes in the backing dune systems.
In the early stages of the storm the foreshore and backshore surface is drawn down as sand is transferred to the shoreface.
During extreme storms erosion extends to the dune face. Sand may be deposited in deep water, below wave base, and so be permanently lost to the beach system.
After the storm the beach profile readjusts as sand is returned to the beach surface from the shoreface
As well as the onshore and offshore components of sand movement there are also important longshore movements. In East Lothian the dominant winds are from the southwest and so blow offshore. The dominant waves however are from the north and northeast, giving an easterly component tolongshore drift. The main spits are found at Tyne Mouth and Aberlady Bay and are backed by dune systems.