Field measurements of the flow pattern in a stormwater retention pond

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Carrie Gillis, Kerry Mazurek, Gordon Putz, Cory Albers

Tuesday 30 june 2015

8:30 - 8:45h at South America (level 0)

Themes: (T) Hydro-environment, (ST) Impacts of pollutants on the water environment

Parallel session: 4J. Floodrisk – Flooding

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In the Canadian Prairies stormwater retention ponds are often used to store urban runoff, attenuate peak flows during rain events, and improve water quality entering receiving surface waters such as rivers, lakes, and wetlands. In many cases, stormwater retention ponds are located within residential neighborhoods and serve an additional recreational purpose as public park space. Water quality is therefore a major concern for residents not only due to negative environmental impacts, but also for recreational activities and property value. A number of Canadian case studies have assessed the treatment efficiency of stormwater retention ponds for specific contaminants, often citing negative treatment efficiencies during large rain events or during certain times of the year. However, few studies have measured the internal flow patterns of the pond which directly affect the treatment efficiency. This research aimed at assessing the internal flow patterns of a typical stormwater retention basin of the Canadian Prairies located in Saskatoon, Saskatchewan during the summer of 2014. To assess the flow pattern drogues were constructed, which consisted of fins at varying depths, a weight, and a float. Two methods were then used to determine the movement of the drogues. The first method involved the use of a camera with an appropriate resolution and known surveyed coordinates, drawing on concepts from particle tracking velocimetry. The second method involved securing a prism to one drogue float and tracking the drogue by surveying with a robotic total station. Unlike in smaller retention ponds, where general circulation patterns are found within the pond depending on the locations of the inlet(s) and outlet(s), wind direction was the dominant factor for internal pond flow patterns. This research suggests that wind influence on stormwater retention basins may contribute to flow short-circuiting depending on the wind direction, and should be a consideration in design.