Quantifying natural sediment erodibility using a mobile oscillatory flow channel

Abstract

Erosion of tidal flats is to a large degree determined by the erosion threshold of their cohesive sediments, i.e., the critical bottom shear stress identifying the onset of erosion. Given that the erodibility of tidal flats can vary strongly over both space and time, rapid in situ measuring methods for quantifying the critical bottom shear stress are needed. As the largest changes in tidal flat elevation are generally observed during storms when shear stress is wave dominated, we developed a mobile Oscillatory-Channel Resuspension flume, OsCaR, as a rapid method to measure the critical wave-generated shear stress on sediment cores directly taken from the field. We assessed the performance of the OsCaR-flume by conducting erodibility measurements on a range of artificial sand – mud mixtures, and along a natural longshore estuarine sediment type and benthic assemblage gradient. Measured erodibility of the artificial sediments matched well with sediment transport theory, indicating that the OsCaR-flume is able to reproduce wave-generated shear stress accurately. Moreover, the patterns observed in the influence of total benthic metabolic rate divided over functional groups on sediment erodibility show that the OsCaR-flume is able to capture the main benthic processes, as they corresponded well to previous laboratory based studies.