LABORATORY INVESTIGATIONS INTO COHESIVE SEDIMENT TRANSPORT IN PIPES

In the UK many sewers are laid to shallow gradients and as a consequen ce, 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 i nduced ''foul flush'' of pollution which may subsequently be discharge d via a CSO to the receiving water course. This paper reports the resu lts of a series of controlled experiments in the laboratory in an atte mpt to better understand the factors which influence the erosion of co hesive sediment beds. Apparatus, formerly used for fixed bed work, was modified to assess the erosion characteristics of movable beds. Sampl es 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 wi th 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 ef fect 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 suspende d sediment transport rate of cohesive-like materials exposed to a vari able flowrate, is dependent both on the values of the final flowrate a nd on the rate of the initial temporal change of the flowrate, and is not solely dependent on the ''steady state'' now variables. Copyright (C) 1996 IAWQ.