High‐resolution comparison of sediment dynamics under different forcing conditions in the bottom boundary layer of a shallow, micro‐tidal estuary

Abstract

Data for high‐resolution profiles of current velocity and suspended sediment concentration (SSC) were collected in bottom boundary layer (BBL) of Mobile Bay, Alabama. The data were used to study the vertical and temporal variability in SSC under various forcing conditions of tide, wind and freshwater discharge. During the winter stormy season, the background SSC was low (0.015–0.03 g l−1). An episodic storm‐induced erosion/resuspension was responsible for the short‐lasting high SSC in BBL. During the spring flooding period, the background SSC was relatively high (0.04–0.07 g l−1) likely due to the large amount of suspended sediment from the fluvial input and bed softening, and the contribution of wind forcing to sediment resuspension was somewhat enhanced by the destratification in BBL. When the freshwater discharge was extremely high (>5000 m3 s−1), the entire water column in shallow areas of the Bay was influenced by freshwater input. Therefore, the thermohaline anomaly's contribution to the stratification considerably weakened, while the SSC's contribution strengthened. When the freshwater discharge was relatively low (<5000 m3 s−1), a critical wind stress for sediment erosion (0.08–0.1 Pa) was observed to abruptly increase the SSC. Despite a micro‐tidal regime, Mobile Bay exhibited the cyclic erosion and deposition pattern induced by the tidal acceleration and deceleration.