Physical Processes in Stormwater Wet Ponds and Wetlands and Implications for Water Quality Control

Abstract The desire for optimized water quality control in stormwater wet ponds and wetlands necessitates a profound comprehension of the physical processes in these water bodies. Therefore, a detailed field investigation was undertaken in an urban stormwater wet pond and an engineered natural stormwater wetland in Calgary, Alberta, Canada, in 2018 and 2019 to monitor the flow, thermal and chemical stratification, and local meteorological conditions. The field data were used to calibrate and validate the environmental fluid dynamics code (EFDC) hydrodynamic model to represent the physical processes in the study pond and wetland. The calibrated albedo was four times higher in the wetland than in the wet pond, as a significant fraction of the wetland is shaded by dense riparian vegetation and trees. Additionally, the calibrated light extinction coefficient was two times larger in the sediment forebay than in the main cell of the pond and wetland. The hydraulic residence time was highly variable and ranged between 4 and 53 days at both sites due to the high intra- and inter-event variability of stormwater inflows, which may also result in large variations in observed pollutant removal efficiencies. Both wind and stratification resulted in short-circuiting in the wet pond when up to 18% of the inflow volume exited faster than in the ideal plug flow case. In the wetland, the outflow rate was controlled by a small 10 cm orifice, which increased the resident time above that of the ideal plug flow. The study also investigated a few potential design and operation improvement strategies, including altering the normal water level, separating the sediment forebay and the main cell, adjusting the positioning of inlets, and constructing an additional berm.