Erosion of sediment beds by turbulent wall jets in combined-sewer-overflow reservoirs

Sediment management with the help of water jets in combined-sewer-overflow (CSOs) reservoirs in the Chicago area has motivated this work. Erosion caused by single and multiple submerged circular turbulent wall jets on a granular (non cohesive) sediment bed of finite thickness laying on a fixed boundary was studied with the help of laboratory experiments. Plane turbulent wall jets were also tested on sewer sediment in order to determine its critical shear stress. Though not strongly cohesive this sewer sediment presented some floculation. Regarding circular jets, different combinations of jet diameter, jet separation, and ratio between sediment thickness and jet diameter were tested. For granular sediments results show a relation between dimensionless parameters characterizing the steady state bed profile and the densimetric particle Froude number Fo given by the velocity at the nozzle, and the effective diameter and submerged specific density of the sediment. Sewer sediment is better characterized by the critical shear stress beyond which transport occurs. For both kinds of sediment, evolution of scour with time confirms previous studies where the erosion was found to initially grow with the logarithm of time up to a certain reference time. This time, made dimensionless with a time scale tc involving the volume of sediment scoured and the rate of erosion, was also related to the densimetric Froude number in the studies involving granular (non cohesive) sediment.