Increasing hydraulic residence time in constructed stormwater treatment wetlands with designed bottom topography.

The treatment efficiency of wetlands depends primarily on the residence time of the polluted storm water (Walker, 1998). Because of this, increasing hydraulic residence times (HRTs) at various flow levels will increase the treatment efficiency of constructed wetlands. In this research, the effects of characteristic bottom topographic features that increase HRT were explored through the use of a two-dimensional hydrodynamic model. Based on numerical simulations of rectangular test wetlands, relationships were made between topographic features and their effects on HRT. Results from the simulations showed that creating baffled wetlands with multiple vertical-scale topography can markedly increase HRT, as is illustrated in a design example. When compared (using the hydrodynamic model) with a wetland having no bottom topography, the design example wetland increased HRT by 113% for the low-flow (142 L/sec) peak flood, and 39% for the 2-year flood event (1700 L/sec).