Flood Risk Mapping for the City of Toronto

Abstract

The city of Toronto has experienced many major floods over the past century: the flood following hurricane Hazel in October 15, 1954, the August 27, 1976 floods, the August 19, 2005, and the flooding of July 8, 2013. During the latest flooding, some parts of the City of Toronto received over 120mm of rain, while the monthly average for Toronto is 74.4mm. The impact was felt as 300,000 residents were affected by power outages. Other serious disruptions included flight cancellations, subway and other transportation closures. It was the most expensive disaster for the province of Ontario. According to the Insurance Bureau of Canada, the damage of the insured properties exceeded $850 million. This event renewed a debate on a number of issues, such as decaying infrastructure, insufficient flood management, and inadequate standards. Don River, the main river crossing the city, is wide but not deep enough, which together with sedimentation contributes to frequent flooding of surrounding areas. In addition, natural creeks have been buried in sewer pipes, thus losing the natural waterways towards the lake Ontario and forcing existing rivers and creeks to overflow their banks. While floodplain maps are generally available, the estimation of flood risk maps based on population, economic development, and critical infrastructure will enhance city's flood mitigation and preparedness planning. In this paper, we present an approach for determining spatial flood risk index map based on population vulnerabilities and terrain morphological characteristics using a geographic information system.