In the UK many sewers are laid to shallow gradients and as a consequence, suffer from recurrent sediment deposition. The loss in conveyance due to these deposits may contribute to hydraulic overloading problems and the wash-out of in-sewer sediment deposits may result in a rain induced “foul flush” of pollution which may subsequently be discharged via a CSO to the receiving water course.
This paper reports the results of a series of controlled experiments in the laboratory in an attempt to better understand the factors which influence the erosion of cohesive sediment beds. Apparatus, formerly used for fixed bed work, was modified to assess the erosion characteristics of movable beds. Samples were taken from the flow during computer generated hydrographs and these were later assessed for the concentration of suspended sediment using a spectrophotometer.
Whilst other studies have been concerned with the erodability of different sediments, this study has been limited to one cohesive sediment type and has concentrated on two aspects of the relationship between the properties of the flow hydrograph and the erosion and transport of cohesive sediment beds. Specifically, the effect of the rate of increase of the rising limb of the hydrograph and of the ultimate flowrate on the pollutograph have been examined.
This series of experiments has produced evidence that the measured suspended sediment transport rate of cohesive-like materials exposed to a variable flowrate, is dependent both on the values of the final flowrate and on the rate of the initial temporal change of the flowrate, and is not solely dependent on the “steady state” flow variables.