Large-scale experimental observations of sheet flow on a sandbar under skewed-asymmetric waves

Abstract

A novel large wave flume experiment was conducted on a fixed, barred beach with a sediment pit on the sandbar, allowing for the isolation of small-scale bed response to large-scale forcing. Concurrent measurements of instantaneous sheet layer sediment concentration profiles and near-bed velocity profiles were obtained on a sandbar for the first time. Two sediment distributions were used with median grain diameters, d50, of 0.17 and 0.27 mm. Sheet flow occurred primarily under wave crests, where sheet thickness increased with increasing wave height. A proportionality constant, ᴧ, was used to relate maximum Shields parameter to maximum sheet thickness (normalized by d50), with bed shear stress computed using the quadratic drag law. An enhanced sheet layer thickness was apparent for the smaller sediment experiments (ᴧ = 18.7), when directly compared to closed-conduit oscillatory flow tunnel data (ᴧ = 10.6). However, ᴧ varied significantly (5 < ᴧ