Intelligent Sizing of Detention Basins Using a Dynamic Hydraulic Model

Stormwater detention is the temporary storage of stormwater runoff in ponds, depressions, basins or underground containers. (For the purposes of this paper we will refer to these structures collectively as stormwater detention basins though the discussion is equally applicable to any stormwater detention structure). These detention basins are an important part of Water Sensitive Urban Design (WSUD) because they typically detain the runoff for some time to reduce the peak runoff rate, while allowing some (or all) of the runoff to infiltrate slowly into the ground to reduce the total runoff volume. This is a useful stormwater management technique at the site of new urban developments, because the increased imperviousness of the developed area (after construction of streets, driveways, roofs, etc.) increases the runoff rate, which, if left uncontrolled, typically increases the severity and frequency of flooding downstream. By controlling this excess runoff with a detention basin the effects of the increased runoff can be mitigated, the natural hydrologic behaviour of the surrounding waterways can be maintained, and the basin can also then be utilized to improve runoff quality by allowing sediments to settle out, etc. However one of the main difficulties with utilizing a detention basin is in the design, because calculating the appropriate size for the basin (and connected appurtenances) so that they can handle the flows directed to them is a very demanding computational task. Yet it is critical to size these basins correctly: if they are too small there is a chance that they could overtop in a severe storm – potentially damaging the basin structure or causing flooding in nearby urban areas; but if they are too big, they may be unsightly, expensive to construct, use too much potentially developable lane, and may also retard natural environmental flows. 

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