Effects of silt loading on turbulence and sand transport

The transport of bed material and fluid turbulence are affected by many factors, including the fine sediment load being carried in a channel. Current research has focused on sand-sized particles introduced to gravel beds, while the effect of silt load on sand transport has received less attention. Experiments on the effects of silt load, in concentrations 0-26,900 mg l−1, on sand transport were performed with a recirculating laboratory flume using three different sand bed configurations: ripples (Fr=0.24), dunes (Fr=0.34), and dunes (Fr=0.48). Three Acoustic Doppler Velocimeters were arranged to measure flow and turbulence quantities simultaneously in one vertical. Sand transport did not change in a consistent manner with increasing silt load, increasing up to 4,000 mg l−1 for dunes (Fr=0.48) and up to 2,000 mg l−1 for dunes (Fr=0.34) and then declining to near the clear water case with increasing silt concentrations. Silt addition for the ripple case caused a relatively small change in sand transport, decreasing with added silt up to approximately 2,000 mg l−1 and then increasing as silt went up to approximately 10,000 mg l−1. Dunes (Fr=0.48) decreased in length and height as silt increased, while dunes (Fr=0.34) did not show a consistent trend. A clear trend of decreasing Reynolds stress with increasing silt concentration was observed in the ripple case, with a 33% reduction in near-bed Reynolds stress caused by an 8,900 mg l−1 concentration of silt.