Investigating the effect of storm events on the particle size distribution in a combined sewer simulator

The changes in particle size of sewer sediment particles rapidly eroded from a previously deposited sediment bed are described, using a rotating annular flume as a laboratory scale sewer simulator. This is the first time that particle size distributions of eroded sewer sediments from a previously deposited sediment bed have been monitored in such a controlled experimental environment. Sediments from Loenen, The Netherlands and Dundee, UK were used to form deposits in the base of the annular flume (WL Delft Netherlands) with varying conditions for consolidation in order to investigate the effect of changing consolidation time, temperature and sediment type on the amount and size of particles eroded from a bed under conditions of increasing shear. The median size of the eroded particles did not change significantly with temperature, although the eroded suspended solids concentration was greater for the higher temperature under the same shear stresses, indicating a weaker bed deposit. An increase in consolidation time caused an increase in median size of eroded solids at higher bed shear stresses, and this was accompanied by higher suspended solids concentrations. As the shear stress increased, the solids eroded from the bed developed under a longer consolidation time (56 hours) tended towards a broad unimodal distribution, whilst the size distribution of solids eroded from beds developed under shorter consolidation times (18 or 42 hours) retained a bi- or tri-modal distribution. Using different types of sediment in the flume had a marked effect on the size of particles eroded.