Using acoustic backscatter to measure sediment flux in the surf zone

Abstract

Transport of sediment in coastal regions directly impacts mine countermeasure operations and naval construction efforts. Wave induced shear stress in the surf zone is responsible for entraining sediment particles into suspension within the combined wave and current boundary layer, where momentum is imparted through highly nonlinear processes. Therefore, a detailed understanding of sediment flux processes in the surf zone is essential to accurately model net sediment transport. This study examines the use of acoustic backscatter inversion as a means of measuring sediment concentration profiles. Measurements of sediment concentration and velocity profiles were made by a high frequency Doppler velocity profiler deployed on Blackâ s Beach during the Nearshore Canyon Experiment, NCEX. Profiles of sediment flux were compared with hourly mean current measurements from a cross-shore/long-shore array of PUV sensors and two-dimensional planner images of the morphological evolution provided by a three camera Argus video suite. Observations from a seven day period containing the development and evolution of a weak rip channel demonstrated that acoustic backscatter inversion techniques, when calibrated with in situ sediment samples, provide high spatial and temporal resolution estimates of sediment concentration and fluxes into the thin wave boundary layer. These sediment transport measurements were correlated with observed mean currents and rip channel evolution, and show a strong morphological response to the sediment flux.