Concentrations in oscillatory sheet flow for well sorted and graded sands

In wave-generated sheet flow conditions, oscillatory near-bed flow is large, ripples are washed out and sand is mainly transported in a thin layer, a few centimetres thick, close to and below the no-flow bed level. New experiments in the Aberdeen Oscillatory Flow Tunnel have produced a comprehensive dataset of transport processes for well-sorted and graded sands in large-scale sinusoidal and asymmetric oscillatory sheet flow conditions. In this paper, detailed time- and height-varying concentration measurements are analysed. The range and level of detail in the measurements make it possible to quantitatively analyse concentration behaviour more rigorously than before. A new empirical equation is presented which characterises the time-dependent concentration profile in the sheet flow layer. The equation involves two time-dependent parameters: erosion depth and reference concentration. The dependence of both parameters on flow and bed conditions is analysed. New results are presented which make it possible to estimate time-dependent erosion depth, reference concentration and, therefore, concentration profile. The effects of grading on sheet flow concentrations are analysed by comparing results from the large range of sand beds tested. (C) 2003 Elsevier B.V. All rights reserved.