Urban Regulatory Floodplain Mapping: Update for the City of Greater Sudbury

Floodplain mapping is an important tool for urban planning and assessing public safety risks. Updating the 20-year-old floodplain mapping for the Algonquin Road watershed introduced several challenging conditions for regulatory mapping and hydraulic modelling. The watershed includes both urban and rural features, the main channel transitions between open-channel flow and enclosed storm sewer systems, and multiple culverts function as hydraulic bottlenecks. This presentation focuses on discussing technical approaches which were applied to map the regulatory storm extents in an urban watershed with these complications, to provide a comprehensive flood mapping product that balanced realistic flood risk against local development pressures.

The Ministry of Natural Resources guideline for floodplain mapping instructs modellers to apply unattenuated flows at hydraulic control structures; however, directly applying unattenuated flow at a bottleneck culvert can result in unrealistic backwater depths upstream of the crossing. In this location, a hybrid flow-assignment method was introduced. Attenuated flows are applied at the upstream face of culverts to represent the highest physically achievable backwater assuming the presence of bottlenecks, while unattenuated flows are applied at the downstream to represent the maximum discharge if the culverts were removed or upgraded.

Additionally, the presentation will address the difference between the SWMM and HYMO calculation engines in modeling large urban sub-catchments, and will also explain the approach developed to model the open-channel and storm-sewer integration in HEC-RAS for this urban environment. The storm sewer inlet capacity for each design storm was calculated separately, and the excess flow beyond the pipe capacity was reassigned to the open channel. This allowed the model to more closely represent how the real system behaves during large events, with controlled flow entering the sewer and excess flows being conveyed along the overland route.

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