Direct bed shear measurements under loose bed swash flows

Abstract

Shear plate measurements of bed shear stress under fixed and loose bed swash uprush flows are presented and contrasted. A shear plate previously developed for fixed bed flows was modified and tested for coarse and fine grain loose beds. Transient flows were generated by a dam-break flow over initially dry horizontal and sloping beds, with flows upslope analogous to swash uprush flows on beaches. Performance of the shear plate varied with grain size. For the coarse grained loose beds only the peak shear stress at the leading edge of the swash flow was consistently obtained; for fine grained loose beds the shear stress was reliably obtained over the whole uprush swash cycle. Data were obtained for varying thickness of loose bed material, commencing with a single grain layer, which correspond to partially starved bed conditions. The direct measurements clearly indicate that the bed shear stress comprises of a fluid shear stress component and a grain shear stress component. The grain shear stress increases linearly with sediment load and is significantly larger for the coarse loose grain bed tests due to the much larger normal stress compared to that for fine grain loose bed tests with the same number of grain layers. The data show a high degree of consistency with the classical theory of Bagnold (1956) and the derived coefficient of dynamic internal friction is similar to that observed by Hanes and Inman (1985).