Background

On August 19, 2005, the West Thornhill community was impacted by a severe rain storm during which stormwater runoff exceeded the capacity of drainage systems, causing extensive flooding of private property and roadways.  The storm was characterized by high rainfall intensities and volumes which resulted in the flooding of many basements in homes and institutions as well as watercourse erosion, and damage to the City’s infrastructure such as roads, culverts, sewers and watercourses

Environmental Assessment

A Class Environmental Assessment (EA) study for the West Thornhill Stormwater Flood Remediation Study was completed and approved in August 2011. The purpose of the Class EA study was to:

• Identify and understand the causes of flooding in the study area;
• Develop a range of alternative solutions to increase capacity and reduce flood risks in West Thornhill;
• Identification and prioritization of the causes of basement flooding and surface flooding;
• Evaluation of various flood remediation measures and develop a comprehensive remediation plan for the areas identified as highest priority and most likely to flood again in the future.

Preferred Alternative

The following Preferred Alternative was proposed from the findings in the Class EA Study for the West Thornhill Stormwater Flood Remediation Study:

• Roof downspout disconnections;
• New inlet control devices in catchbasins that limits the inflow to sewers;
• Increased sewer inlet capacity at key locations where sewer capacity is available; and,
• Storm sewer upsizing and inlets within and downstream of the high flood potential areas as well as ‘diversion’ pipes that reduce flows at critical locations relieving overloaded sewers.