Stormwater Pond Sediment Loading and Accumulation Analysis Using Continuous Simulation

The permanent pool in stormwater detention facilities is designed for water quality treatment by providing an appropriate storage volume for the settling of suspended solids and particulate matter that is 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 if 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/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 methodology was developed that improves the current state-of-the-practice design methods. It begins by representing the catchment hydrology based on source area delineations, to better characterize the generation of TSS loads across a range of urban land uses and using continuous simulation to better represent long-term average annual rainfall conditions. The unique capabilities of the EPA SWMM5 model were used to represent the dynamic control structure hydraulics as well as the settling characteristics of TSS and particulate pollutants in stormwater runoff. It is intended that the methodology could apply to similar models with the same capabilities.

In this paper, a modeling and design methodology using SWMM5 will be presented for estimating the sediment loading and accumulation in stormwater detention facilities, including lessons learned from two recently permitted and constructed ponds in Ontario, Canada. For both ponds, the water quality module of SWMM5 was used to simulate the generation of TSS loadings from all source areas within each subcatchment tributary to the pond, including pollutant buildup during dry weather periods and washoff during rainfall events. The pollutographs were subsequently routed through the collection system and the deposition of particulate solids in the pond was simulated.

In SWMM5, particulate settling is represented by a characteristic settling velocity distribution. No local stormwater settling velocity measurements were available and so empirical data from other regions were used (Final Report of the Nationwide Urban Runoff Program, U.S. EPA, 1983). Six mass fractions were used to characterize the particle size distribution of the stormwater washoff load.

The pond removal efficiency and accumulation rates for the non-winter period of April through October were then estimated using local rainfall data. Snowmelt and hydrologic conditions during winter months were not included in this analysis. For comparative purposes, sediment accumulation rates within the pond were normalized by total tributary area and by impervious area.