AEOLIAN DECOUPLING OF BEACH SEDIMENTS

Sediment population decoupling refers to the sorting of a source population into two or more subpopulations that have distinctly different size distributions. A field experiment along Monterey Bay, California investigated the nature of the decoupling process across a beach-dune complex and determined the implications for aeolian transport rate predictions. A series of instrument clusters were deployed along a shore-perpendicular array, beginning at the berm and extending to the dune back-slope. A total of 14 anemometers, 4 wind vanes, and more than a dozen sediment traps were deployed. Grab samples were taken from the beach surface before and during the wind storm. During shore-perpendicular onshore winds, an internal boundary layer developed in the downwind direction, and surface shear stress and flow competency decreased with distance from the foreshore. Fine-grained sediments were entrained from the back beach and transported inland by suspension and saltation. But the rate of sediment transport was always less than the wind's potential because of the absence of an upwind sediment source- an equilibrium saltation layer never developed fully. The coarse sediments, supplied to the foreshore and back beach by swash processes, were mobilized only intermittently and were transported by surface creep. Eventually they became organized into discontinuous ripples that migrated over the cohesive, finegrained matrix underneath. Since surface shear stress decreased in the landward direction, the distance of transport of coarse grains was limited to the zone fronting the foredune, whereas the finer-grained sediments were not similarly restricted and were transported to the dune back-slope. © 1991 Taylor & Francis Group All right reserved.