Modeling the accumulation and dynamics of beaches on shore platforms

A mathematical model was used to study the development and dynamics of beaches on shore platforms. The model was based on assumptions that: the gradient of the beachface at its seaward edge must be greater than platform gradient; beach sediment tends to move landwards; and beach occurrence and extent depends upon the amount of sediment available. Equilibrium beachface gradient was related to breaker height, wave period, and sediment grain size. Three grain sizes were used, representing fine and coarse sand and pebble; two deep-water wave sets, representing storm wave and west coast swell wave environments; and two tidal ranges of 9.1 and 3.3 m. Runs were made using linear, concave, and convex platform profiles, with gradients ranging between about 0.6degrees and 8degrees. Because platform and beachface gradients are normally quite similar, shore platforms are unable to store much sediment. For a linear platform with a lower gradient than the beachface, sediment first accumulates at the cliff base and then, depending on the sediment available, extends increasingly seawards. Beach development first occurs on the gently sloping seaward portions of concave platforms and on the more landward portions of convex profiles. The amount of available material partly determines the landward and seaward extent of these beaches, respectively, although it is also controlled by beachface and platform gradients. Beaches on shore platforms become thinner and more gently sloping during storms, and they may disappear completely, depending on such factors as: sediment grain size, platform gradient, the amount of sediment that can be stored during calm conditions, wave characteristics, and the duration of the storm. The response of beaches on rocky coasts to rising sea level will be different from those on sedimentary coasts, and will depend, in part, on the morphology of the rocky foundation. (C) 2004 Elsevier B.V. All rights reserved.