The Beach: Where Is the "River of Sand"?
Keywords:Beach, beach ridge, beach ridge plain, dq/dx model, offshore sand, parallel beach ridges, ridge tips, "river of sand, " shore-parallel transport, wide inlet
The "river of sand" concept is basically a model of sediment transport; sand is carried lengthwise along a beach from "input" end to "output" end, like a slurry in a pipe. Such transport can be seen locally, in nature, but the primary sand path is at right angles to the river of sand line. Thousands of beach ridges have tips terminating in marshland or swamp, with no attachment to any possible source of sand, commonly both ends. The termination is generally by tapering, not truncation, Others are attached at both ends, but in a pattern such that neither end was the source. Wide inlets on both sides of many beach ridge plains show that the only source of sand was offshore. Grain size studies do not indicate shore parallel motion, but show sine-like variations in size parameters: patches of coarser sand must have been derived from offshore, here and there, The dq/dx model (shore parallel movement) fails over long distances: it may hold for short segments. The dq/dy model (shore-normal) is more successful. Parallelism of ridges also requires an offshore source. The "river of sand" idea is a gross oversimplification. In the areas studied it is generally wrong.