The erosion of cohesive mixed deposits: implications for sewer flow quality modelling

Abstract

The movement of sediments in sewers has been widely recognised as being strongly linked with poor sewer flow quality. Currently, empirically calibrated transport capacity relationships are used to estimate the movement of in-sewer sediments. The majority of these relationships were developed and calibrated with laboratory data sets that used uniformly sized granular sediment. In many combined sewers, there are both organic and inorganic sediments. The fine-grained organic sediments are thought to be able to develop considerable cohesive strength under certain conditions. This paper describes the results from laboratory experiments in which the erosion of non-homogeneous in-pipe sediment deposits was investigated. The types of sediment mixtures used ensured that a range of cohesive deposit strengths was developed. The measured bed load and suspended load were compared with transport capacity relationships currently used for sewer design. Significant divergence between the transport capacity predictions and experimental observations was discovered. It was seen that it is unwise to model in-sewer deposits as homogeneous and granular and that it is important to take into account the impact of deposit cohesion.