A modelling analysis of urban stormwater flow regimes and their implication for stream erosion

Stream erosion is a major issue in stormwater management. The objectives of this research are to investigate by means of computer modelling: (a) the flow-duration characteristics of a receiving stream before and after urbanization; (b) the implication of flow regime changes on stream erosion potential with and without control measures (e.g., extended detention basins and source control); and (c) the maximum degree of urbanization, for which stream erosion may be unavoidable regardless of the stormwater control measures applied. The U.S. EPA Stormwater Management Model was used in the analysis of flow-duration characteristics of a small northern Ontario watershed. It was found that flow-duration characteristics of the stream could change significantly after urbanization. Although no stream erosion was modelled in this investigation, the change in flow regimes after urbanization may indicate potential stream bed and/or channel erosion. Extended detention basins with short detention times (24-h) could control flow-duration characteristics at low flow rates better than those with long detention times (48-h). Both extended detention basins and source control measures should be applied in order to bring the flow duration curve after urbanization back to its original position. If the watershed were to be urbanized beyond 40% of imperviousness, it may not be possible to bring the flow duration curve back to its original position in spite of the various source control measures and detention basins used. The optimal location of detention ponds for erosion control depends upon the range of flows, which control the stability of the stream.