Application of 2D Hydraulic Modeling to Diagnose and Manage Urban Spill Flooding

In older and dense urban areas, spill conditions can occur during formative storm events, whereby a portion of the flow within the main watercourse breaches the banks of the system and is conveyed through the urban area. The flow paths of the spill through these urban areas is defined by topography and municipal drainage infrastructure, and the proximity of the floodwaters to structures and transportation infrastructure requires particular consideration. Grading changes associated with development or improvements within these spill areas could alter the extent of flooding to adjacent properties, thus requiring more sophisticated analyses and considerations for mitigation measures.

This presentation offers a case study for analyzing flood extents and mitigation options associated with transportation infrastructure projects within a spill area. The study focuses on the hydraulic modelling and analyses completed as part of the Elgin Mills Grade Separation Class EA for York Region. The hydraulic analyses used the MIKE FLOOD 2D modelling methodology to analyze the flood extent through the urban area adjacent to the German Mills Creek for the Regional Storm event, and assessed the change in flood extent resulting from the proposed grade separation. The model was modified to evaluate various mitigation options, applying various techniques to account for the localize hydraulic influence while still achieving reasonable and representative results for the resulting impact on the overall system. Risk assessments were completed based on criteria applied by the Toronto Region Conservation Authority, by integrating the model output with GIS analyses. The application of the 2D modelling platform yielded a more detailed diagnosis of the spill condition and the influence of the Elgin Mills right-of-way within the overall extent of the spill, providing insight into the identification of candidate alternatives, and assessing the performance of the mitigation alternatives based upon spatially varied changes in flood depth and extent, and the constraints effectively which limit fully mitigating increased flood potential within the spill area.