Innovating Stormwater Retention Pond Design

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Stormwater runoff from urban areas has a significant impact on rivers. One issue is an increase in downstream flooding; urban watersheds are more impervious, which speeds transmission of runoff to a river. This runoff also negatively impacts river water quality. Of the many pollutants it carries, sediment is a significant concern because of its impacts on fish and other aquatic life. Fine sediments in urban runoff are toxic and carry heavy metals, as well as nutrients that impact eutrophication of lakes. Many cities use stormwater retention ponds to attempt to mitigate these flooding and water quality issues, as aesthetic features, and, in some cities, for recreation. However, current design procedures have produced many ponds that do not provide reliable retention of sediment and other pollutants. The flow in ponds often short-circuits, and this lessens the time for treatment and sediment settling. The long-term objectives of this research program are to develop the fundamental understanding and a set of tools to allow for the design of retention ponds that provide reliable improvements in water quality. The current focus is to ensure sediment removal by ponds. The first short-term objective is to evaluate whether counter-rotating vortices can be used to eliminate short-circuiting in ponds and provide levels of sediment retention that meet standards. The second objective is to understand pond bed sediment erodibility better. Computational fluid dynamics and physical modelling will be conducted to assess the behavior of a pond with counter-rotating vortices. First, using a single-vortex, an M.Sc. student will test means to eliminate the secondary flow that moves the portion of the flow near the bed to the pond outlet more quickly than desired. Then, a Ph.D. student, supported by one summer student, will study the flow dynamics in a pond with two vortices set in parallel and evaluate the residence times and sediment retentions. This work will be done for various pond diameter to depth ratios and under a range of potential flow conditions. For the assessment of the pond sediment erodibility, a Ph.D. student, supported by four undergraduate summer students, will sample bed material from ponds under different water chemistries, wind, and times from rain events. The variation of erodibility with depth into the sample will be determined in laboratory testing along with the sample composition, geotechnical properties, and structure. The understanding of erodibility is needed to improve computational modelling capabilities for sediment resuspension in ponds, which are necessary if pond performance at sediment retention is to be enhanced to meet standards consistently.  The counter-rotating vortex pond offers an innovative approach for pond designers to minimize short-circuiting while providing reliable sediment retention. It also offers Canadian communities a way to retrofit their poorly operating ponds inexpensively.