Wave-cut Platforms and Coastal Erosion
A The mechanical action of the waves, whipped up in the surface of the sea by the wind, eats away at the coast. Even in storms, most of the power of the waves is at or close to the surface – deeper down, the water moves only relatively slowly. In most places, the power of tides or other currents at the surface or at any depth is small by comparison with the average power of wind-driven waves at the surface.
B The result is the development of cliffs along the coast, with a wave-cut platform at their foot, and an accumulation of debris on the seaward side of that. As the sea advances, the cliff gets taller, and the shallow area gets wider. The waves break more and more on the shallow area, dissipating much of their energy before they reach the cliffs, breaking the rocks that have fallen from the cliffs into smaller and smaller pieces, and finally smashing them to sand. With most of the waves’ energy expended on the platform, the actual coastline is attacked less and less – only the wildest storms really affect it, and even they lose much of their energy just moving sand, stones and boulders around.
C If the climate cools a lot for long enough, snow and ice accumulate on land, lowering sea level, but also depressing the land under the ice. The local depression of the land may be considerably more than the global lowering of sea level. (Land distant from the ice is pushed upwards.) When the ice melts, the land that had been depressed rebounds – rapidly at first, but then more slowly. Scotland and Scandinavia (amongst other places) are still rebounding after the last Ice Age. The rate of rebound is down to a few millimetres a year now, but was much more in the period just after the Ice Age. The old wave-cut platforms are left high above sea level, as raised beaches.
(At the same time, old wave-cut platforms in other places are submerged, and form offshore shelves.)
D If sea level remains fairly stable at the new level, eventually a new line of sea cliffs and a new platform form.
E If sea level rises relative to the land (as it is at the moment, due to global warming melting land ice, mainly in Greenland and Antarctica), the shallow offshore area becomes deeper – less of the waves’ energy is expended on the platform, and wave action on the coast itself is reinvigorated. Coastal erosion speeds up. (Where there are no cliffs, as sea level rises the sea simply advances over the land unimpeded.)
I wrote this particularly thinking of the effect on nuclear power stations, which are often on the coast, because they need a ready supply of cooling water. See When Sea Level Rises.
Note also that this effect is likely to be exacerbated by an increase in the strength of storms which is an expected consequence of global warming.