Relations of sand trapping efficiency and migration speed of transverse dunes to wind velocity

A self-consistent model which describes transverse dune migration in equili brium is introduced. It shows that an equilibrium expression for dune migra tion speed (c(d)) must take into account sand trapping efficiency (T-E), an d that T-E is strongly related to the wind speedup over the windward surfac e. An expression for sand trapping efficiency (T-E) is analytically derived from a microscale analysis of sand grain deposition on the slip face. Sand trapping efficiency (T-E) is mainly determined by shear velocity on a leve l surface (u(*)(-infinity)), and rapidly decreases as u(*)(-infinity) incre ases. For each dune height(H), dune migration speed (c(d)) first increases, and then decreases monotonically after reaching the maximum, as the shear velocity on a level surface (u(*)(-infinity)) increases. Dune migration spe ed (c(d)) is not inversely proportional to dune height(H). For low dunes, s mall sand trapping efficiency (T-E) suppresses c(d), whereas for high dunes , wind speedup and large T-E resist the decrease of c(d). Some field data s how the same tendency. The dune-to-plane-bed transition observed in subaque ous and venusian bedforms could be associated with the decrease of sand tra pping efficiency (T-E) Copyright (C) 2000 John Wiley & Sons, Ltd.