Three-Dimensional Hydrodynamic Modelling in Stormwater Wet Ponds and Wetlands in Calgary and Implications for Sediment Control

Optimized water quality management in stormwater wet ponds and wetlands requires a comprehensive understanding of in-pond physical processes. To address this need, an intensive field monitoring program was conducted in 2018 and 2019 at two stormwater wet ponds and two wetlands in Calgary, Alberta, Canada. The program captured flow and sediment dynamics, thermal and chemical stratification, and meteorological conditions. This data was used to calibrate and validate a 3D Environmental Fluid Dynamics Code (EFDC) model, enabling realistic simulation of in-pond physical processes.

The calibrated model showed that the hydraulic residence time (HRT) varied substantially across sites and events, with 5.3-48.6 days in a wet pond versus 3.0-53.2 days in a wetland, while the observed sediment removal efficiency (RE) ranged from 90% to 100% in a wet pond and 72% to 100% in a wetland. This variability was primarily driven by strong intra- and inter-event fluctuations in stormwater inflows. Sediment removal efficiency was strongly and negatively correlated with inflow duration (R² up to 0.71), as prolonged inflows inhibit effective settling, but showed little sensitivity to inflow rate because submerged forebay berms effectively dissipated inflow momentum before sediment could be transported to the outlet.

The calibrated model was subsequently used to evaluate design modifications proposed by the City of Calgary. Reducing wet pool depth from 3 m to 2 m to provide additional active storage to address climate change impacts increased the sediment outflow load by approximately one-third, as the shallower depth weakened inflow momentum dissipation and limited particle settling. While installing a second berm in a pond increased its minimum HRT from 5.3 to 9.3 days, and its minimum sediment RE from 90% to 94%. These results demonstrate the critical role of 3D hydrodynamic modelling in resolving complex in-pond physical processes and providing a robust tool for optimizing stormwater pond/wetland design