As the surrounding lands in the south-west of the City of Markham were developed in the 1960s, the Don Mills Channel was transformed from a natural watercourse through agricultural lands to a realigned and confined system of channels and culverts. The capacity of the channel system is less than the 2 year storm flow, and businesses near the channel have been impacted by flooding numerous times over the past several decades.
As part of the Municipal Class EA study, an integrated 1D-2D dual- drainage model of the Don Mills channel was prepared to simulate historical flooding events, assist in assessing the capacity of stormwater infrastructure, and evaluate the effectiveness of several scenarios for reducing flooding and flood damages. Model representation included the minor and major drainage systems leading to the channel, the channel itself, and the overland flow paths through the developed areas surrounding the channel. Calibration was based on observed high water marks from two historical flooding events.
The completed PCSWMM model consisted of more than 120 subcatchments covering the 7.3 km2 study area and over 500 conduits representing almost 38 km of drainage network including storm sewer pipes, major system flow paths, ditches and the channel itself.
The recommended solution involved construction of an on-line flood control facility, replacement of several undersized culverts under municipal roadways and implementation of a flood proofing and education program.
The study involved a rigorous process to estimate flood damages under current conditions, and estimate the net reduction in flood damages associated with the different flood reduction alternatives, including distributed and centralized measures. The initial capital cost and long term operation and maintenance costs were also estimated for each alternative. This information was used to calculate the payback period and return on investment associated with each alternative, which was a significant criterion in the evaluation of alternatives, and which supported the City’s target level-of-service. Benefit-cost analysis for the recommended solution was subsequently completed in support of Markham’s Infrastructure Canada Disaster Mitigation Adaptation Fund application, demonstrating a favourable benefit-cost ratio of 2.8:1. The approach to setting economically-supported levels of service can be used as a template for other municipalities to justify costs for infrastructure investments to reduce both urban and riverine flooding.