Remobilization of Sediments in Gully Pots During High Intensity Precipitation Events

The urban drainage system experiences increasing challenges due to limited capacity, increased precipitation amount, and intensity cause a higher risk of urban flooding and frequent combined sewer overflows (CSOs). This is a common problem in Norwegian cities, and around the world. The gully pots in the urban drainage system should trap sediments that is transported with stormwater and function as a pollutant trap for low flow events. However, this is dependent on regular maintenance for proper function. If poorly maintained the gully pots in the drainage systems can become a source of resuspension of accumulated sediments during high intensity rain events that. This can be a significant source of polluted sediment transported to receiving waters. This study investigates the pathways and occurrence of remobilization of sediments through the use of a SWMM model and a case study area in Damsgaard, Bergen. Steep slopes characterize the research catchment, with elevation ranges from sea level to 468 m above mean sea level. Built-up areas (i.e. buildings and roads) cover about 48.3% of the area while about 44.5% of the catchment is forested. The case study illustrates how poorly maintained gully pots can be a net source of pollutants to receiving waters, in this case the Puddefjord fjord, where the City of Bergen wishes to establish swimmable water quality in the inner harbor areas.