Depth, bed slope and wave climate dependence of long term average sand transport across the lower shoreface

Abstract

We analyse measured 46year changes in nearshore bathymetry to quantify rates of net shore-normal sand transport (qx) in 10–20m depths at northern Gold Coast, Australia. These are significant both in understanding local disequilibrium shoreface evolution and as prototype data that may be used to assess theoretical predictive methods. This analysis is feasible because the migration history of a nearby river mouth has formed a disequilibrium shoreface lobe that is evolving towards an equilibrium profile shape, which is identified in adjacent non-lobe profiles. The data include various profile transect surveys from 1966 to 2012, sediment characteristics and locally recorded wave conditions.A generally applicable form of qx that is a function of both a depth-dependent horizontal bed transport forcing component and bed slope qx=qx(h,S) is developed and quantified in terms of long term average behavior for this site. Dependence of the forcing component on wave climate is shown from higher than average qx derived from the lobe response during a recent five year period with abnormally high waves. Our qx(h,S) quantifies the balance between generally shoreward wave boundary layer sand transport and the opposing effect of the seabed slope, as incorporated in the predictive methods.