Stormwater Pond Sediment Loading and Accumulation Analysis

The permanent pool in stormwater detention facilities is designed for water quality treatment by providing an appropriate storage volume for the settling of the suspended solids and particulate matter that are carried in stormwater runoff. The facility may include additional storage components designed to provide erosion and flood control, but it is the layout and configuration of the permanent pool volume that typically dictates the overall pond footprint size. Sizing of the permanent pool volume is often based on local regulatory requirements that dictate a prescribed total suspended solids (TSS) removal rate, expressed as a percentage removal on an average annual basis. The volume requirement is typically given as a function of the imperviousness and area of the tributary catchment. It is presumed that a pond that meets or exceeds the required volume will achieve its TSS removal target. The actual treatment efficiency of the pond is a function of catchment hydrology (of which imperviousness is just one of many variables), control structure hydraulics, and the particle size distribution of TSS in stormwater runoff. After the pond has been constructed it is also important for the owner or operator of the pond to understand the sediment loading and accumulation characteristics, given the high cost of sediment removal. Presumptive design criteria used to size the permanent pool of stormwater ponds do not account for these site specific characteristics. To overcome the limitations inherent in presumptive design criteria, a modeling and design methodology using SWMM5 was developed to estimate the sediment loading and accumulation rate for a facility in Ontario, Canada.