2.2.6. Hard Rock Coasts

Contrary to what one is led to believe, erosion of high coasts generally provides a very low sedimentary input (less than 5% according to Inman [104]), and those models of coastal evolution, which see the retreat of promontories and the filling in of the gulfs to the complete rectification of the coast, appear to be linked to a Davisian concept of coastal geomorphology by now largely outdated. The Cycle of Normal Erosion by Davies held sway for many years and hindered acceptance of the Gilbertian viewpoint that came to the fore again in the evolution of quantitative geography in the 1950s.

Nevertheless, rock coast erosion can be locally significant and constitute the main sediment source not only for pocket beaches, but also for significant segments of open coasts. Erosion is very dependent upon lithological setting and can be very high in favorable conditions, e.g., soft rocks strata. On some sectors of the Algarve coast, a 10 to 50 m high soft rock cliff is retreating for 1–2 m/year [105], but the highest cliff retreat rate was measured on pyroclastic deposits on Nishinoshima Island (Japan), with 80 m/year [106]. In East Anglia, UK, at Dunwich, coastal retreat has been 2 km since the Roman time, with 13 parishes disappeared after XII century, the last one in 1919 [107]; this sediment input feeds the coast down to Orford Ness, more than 30 km south. Obviously, sediment input from a rocky coast is extremely discontinuous [108] and beaches taking advantage of

this follow an uneven evolution [109]. As protection of Fairlight Cove, Sussex, UK was being undertaken, French [103,110] noted that "cliff stabilization represents a net loss of sediments to the coastal budget", as an input of 9750 m3/year was interrupted.
