ESTIMATING LOCAL BED SHEAR-STRESS FROM VELOCITY OBSERVATIONS

Replicate velocity observations using conventional equipment under typ ical field conditions are used to evaluate the precision of different methods for estimating local boundary shear stress from velocity measu rements. The bed shear velocity u can be estimated within 3% using th e depth-averaged velocity in the vertically averaged logarithmic veloc ity profile. To be accurate, this method is limited to relatively simp le flow geometries which may be expected to have the appropriate veloc ity structure. Estimates of u made using a single near-bed velocity o bservation are less precise by a factor of 3 because of the larger unc ertainty associated with a single observation. Accuracy of this method requires appropriate flow conditions only near the bed, so it may be applied in a wider range of flow conditions, including spatially varia ble flow. Estimates of u from the slope of the near-bed velocity prof ile are the least precise and require the most restrictive flow condit ions for accuracy but offer the advantage that they may be made withou t independent knowledge of the bed roughness.