The Influence of Gully Pot Performance on the Entry of Runoff Sediment into Sewers

The aim of this research project has been to study the sediment retention process of the gully pot in an attempt to evaluate its efficiency in preventing sediment entry into sewers. An initial field study of the sewers in the London Borough of Lambeth has shown Significant sediment deposits in many sewers. Analysis of gully pot basal sediment characteristics has indicated that they contain proportionately more finer material than sewer sediments.
A laboratory rig was constructed to investigate the settlement of sediment, sediment bed erosion, the influence of non-particulate materials on erosion, the blockage mechanism and the removal of dissolved solids using a tracer study. A model based on modified reactor theory was successfully developed to describe the sedimentation process. The trap efficiency of the pot varied between 15 to 35%, 36 to 73% and 74 to 95% for sediment size gradings 63-100, 100-150 and 150-300 micron respectively depending on the flow rate and the accumulated sediment bed height. A trap efficiency of greater than 95% was always observed for sediments of size greater than 300 micron. Erosion of a particulate sediment bed occurred only at the beginning of water flowing into the pot although the presence of non-particu- late materials in the bed sediment has shown a positive influence on erosion.
A hypothesis has been developed to explain the mechanism of erosion. Investigation of blockage has led to the conclusion that blockage cannot occur without the existence of non-particulate materials and secondary sediment sources. The tracer study confirmed that the gully pot is nearly a complete-mix reactor.