Time and space scales of bedform evolution in sandy nearshore and inner shelf environments

Abstract

Recent experiments in wave-forced nearshore and inner shelf environments have demonstrated through acoustic and optical imaging techniques a remarkable range of bedform patterns in mobile sandy sediments at sub-10 m horizontal scales. The observed bedstates differ both in the level of complexity of their spatial patterns, and in the time scales of their response to changes in hydrodynamic forcing conditions. Yet, the (re)occurrence of these different bed states at a given location can be quite repeatable and, once formed, the migration of the bedforms characterizing a particular state can in some instances be predicted reasonably well from the local hydrodynamics. These results provide a basis against which the present capability for predicting bedstate evolution in wave-forced mobile bed environments can be assessed. It is argued that, with the possible exception of a few simple cases, current models are able to predict neither the level of complexity in the observed characteristic patterns, nor the time evolution of the bed given the prior history of the forcing. Nevertheless, the repeatability of bedstate occurrence, and the reasonably good predictions of migration velocity and shear stress at the bed, together suggest that such a predictive capability may be achievable in the not-too-distant future.