Stormwater ponds: Improved efficiency, characterisations and risks of accumulated sediment

Stormwater solids are generally recognised as one of the most important water quality parameters in stormwater runoff, as pollutants are transported on the surfaces of the solids from the pollution source to the receiving water bodies. The most common method of treating solids in stormwater runoff are stormwater ponds and this is how it has been for decades.

Some of these ponds were poorly designed from the beginning and others have through the years with increasing urbanisation under-dimensioned capacity. Furthermore, the sediment, retained in a well-functioning stormwater pond, will at some point need to be properly removed which will result in handling of potentially harmful substances.

Limiting factors when designing the stormwater pond is the area required for the solids to settle out as sediment, and the cost of the removal of sediment from the pond. To address this, a device for improving the sedimentation process, and reducing resuspension of already settled sediment, was proposed – bottom-grid sedimentation structure. The device would act as a sediment trap placed immediately downstream of the inlet to a stormwater pond, or other sedimentation facilities.

In collaboration with the Czech Technical University in Prague, Czech Republic, impact of different geometries of the bottom-grid structure on sedimentation efficiency was investigated. Parameters analyzed included cell footprint, cell depth and inclination of the walls of the bottom-grid structure. A scaled physical model of the bottom-grid structure was constructed, and experimental runs were conducted at the hydraulic laboratory in Prague.

Heléne Österlund, Annelie Hedström and Kelsey Flanagan