MEASUREMENT AND PREDICTION OF SURFACE SHEAR-STRESS IN ANNULAR FLUME

Cohesive sediment beds occur in a variety of natural and man-made environments. A parameter that has a very significant influence on the sediment behavior is the critical shear stress. This is the shear stress at the sediment surface above which erosion will take place. The critical shear stress can be evaluated in the laboratory under conditions closely resembling those found in the field by use of an annular flume. The main purpose of the work described in this paper is to determine experimentally and numerically the velocity and surface shear stress fields generated by the flow of water in an annular flume. Very good agreement is found between measured circumferential velocity profiles, obtained by laser doppler anemometry, and profiles predicted by the computational fluid dynamics program HARWELL-FLOW3D. Measurements of the bed and wall shear stresses are made using hot-film probes and these also agree reasonably well with numerical predictions. Also of interest is the dependence of the sediment shear strength on sediment depth. By comparing measured eroded sediment contours with predicted shear stress profiles a simple method emerges for describing this dependence.