Internal hydraulics of a sediment-stratified channel flow

Abstract

Mudflats along the macrotidal Yellow Sea coast of South Korea are incised by numerous channels that act as drainage and sediment dispersal routes, primarily during ebb flows. Observations in two channels at maximum ebb revealed stable vertical distributions of suspended sediment (density) capped by a lutocline at mid-depth, across which suspended-sediment-related density varied rapidly. Internal responses, as revealed by turbidity and velocity data, varied with flow, suspended sediment, and morphology. At one location, upstream-propagating lowest mode internal waves with period T = 140 s were observed. Superimposed on the interfacial waves were higher frequency Kelvin-Helmholtz billows, apparently related to mean flow shear. Visual observations indicated that billows filled the channel from surface to bottom. At the second location, an undulatory jump consisting of a series of surges was observed. Each surge resembled the head of a turbidity current. Kelvin-Helmholtz billows were present, but only at surge crests and troughs. Internal hydraulic jumps, sediment spilling from feeder channels, and collapse of instabilities may contribute to wave generation. A range of bottom sediment sizes permits the development of a stable stratification within the bottom boundary layer beneath a lutocline. A simple numerical model shows that the lutocline is the upper limit of a boundary layer with a thickness proportional to mean flow velocity and to the intensity of suspended-sediment stratification within the layer. Billows appear to enhance resuspension of sediment, thereby increasing the sediment load and thickness of the boundary layer. Locations of hydraulic jumps appear to be sites of deposition.