Velocity profiles over a rippled bed and the threshold of movement of sand

Abstract

Measurements of velocity profiles have been obtained within 2 m of the sea bed over a rippled sand in Start Bay, Devon, U.K. Simultaneous observation by underwater television has enabled the threshold of movement of the sand to be determined. The threshold criteria are considerably higher than laboratory measurements for a flat bed. As the sand moves the ripples change their shape. This causes a change in the bottom roughness length, the drag coefficient and the apparent threshold of sand movement during the tidal cycle. The bottom roughness length and the threshold of movement also increase with decreasing tidal amplitude. The calculated suspended sediment concentration at the height of bed roughness length was proportional to the normalized excess shear stress with a constant of proportionality of 0·78×10−4. The concentration at a height of 10cm was proportional to u*5.