ABSTRACT
Sediment is ubiquitously present in urban drainage systems. It is found deposited on catchment surfaces, in gully pots and in drains and sewers. Drainage engineers have long recognised its presence in stormwater and the problems it may cause. They have sought to exclude larger, heavier sizes from the piped system by the provision of gully pots and designed sewers to limit in-pipe deposition. The theory is that sediment that does enter the system is carried downstream, where it is eventually trapped and removed at the outlet of the system. This may be the case for newly-designed systems, but for older (especially combined) networks, sedimentation in sewers is commonplace. In fact, a review of sediment movement in sewers (Binnie and Partners and Hydraulics Research, 1987) concluded that 80% of UK urban drainage systems had at least some permanent sediment deposits.
