Measurement of local bed shear stress in combined sewers

Bed shear stress is a relevant factor to determine erosion and deposition of sewer sediments. Experiments have been undertaken in a laboratory flume and in two sewer trunks of "Le Marais", in Paris, to compare different methods to calculate local bed shear stress (based either on the log-law of the wall or on the Reynolds shear stress distribution) and to calculate mean bed shear stress (with τ_m = ρgR_hJ). Measurements were performed with a micro-propeller and an Acoustic Doppler Velocimeter during dry weather flow. Values of local bed shear stress calculated with the different methods were comparable but measurements with ADV revealed a great dispersion of shear stress close to the bed. It was found that in sewers were b/h < 5 the spatial variability of local shear stress was very low, and so local bed shear stress in mid-flow was equivalent to mean bed shear stress. The relation τ_m = ρgR_hJ_c was found to be applied with the energy slope because data plotted on maps are very imprecise and lead to big errors.