Evaluation of surface flooding vulnerability for large scale models

When intense rainfall generates a runoff volume that exceeds the capacity of existing drainage systems, overland flow can occur and water can pond in low elevation areas and even flood nearby buildings. With the anticipated impacts of climate change, concern for urban flooding has grown amongst city managers and infrastructure owners.

Numerical models have been used in many different ways to predict overland flow during high-intensity rainfall, but the modeling task can become overwhelming when the vulnerable infrastructures are distributed on a very large area where there are thousands of kilometres of underground network to model.

In order to assess the risk of surface flooding in urban areas, a global methodology has been developed in which the modeling effort can be reduced with basic assumptions and a simplified approach. This methodology aims to provide an evaluation of the vulnerability and exposure of buildings and critical infrastructures during high intensity rainfall that causes significant overland flow.

The methodology uses PCSWMM and high-resolution DEM to map the major overland basins. Then, using multiple GIS layers, a 2D mesh is constructed in HEC-RAS and carefully adjusted to make sure ponding areas and preferred pathways are correctly modeled. The Rain-on-Grid tool is then used to model high-intensity rainfall across the basin. Since there is no underground network in this methodology, the rainfall event is modified to take into consideration the capacity of the existing drainage network. The results provide a detailed, step-by-step progression of the overland flow. Water levels, velocity and depths can easily be extracted at multiple locations.

The methodology was applied to evaluate the vulnerability of over 300 buildings and critical infrastructures distributed in over 240 km² in Montreal. The results were then compared to real events that occurred between 2012 and 2022.

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