West Thornhill Flood Control on Three Streets with Structural Sewer Replacement in the City of Markham

completed

To improve extreme weather resiliency the project will upgrade storm sewer capacity consisting of structural drainage system works in the West Thornhill Phase 2B implementation area. Involves sewer construction to upgrade the capacity of the existing storm sewers in the area from a one-in-5-year storm event capacity to a one-in-100 year storm event capacity. Storm sewers to be upgraded include: 1.Proctor Avenue - upgrading existing 1350 mm diameter storm sewer to 1800 mm diameter, 180 m long storm sewer. 2.Henderson Avenue - upgrading existing 1350 mm diameter storm sewer to 1650 mm diameter, 180m long storm sewer. 3.Doncaster Avenue / Grandview Park - A new 1350 mm diameter storm sewer to provide parallel relief capacity to an existing 1200 mm diameter storm sewer. This 250 m section is being proposed to run through a City owned park. An Oil and Grit separator to treat urban runoff discharge to the receiving Don River watercourse will also be incorporated to enhance water quality and improve aquatic habitat. The objective is to reduce the frequency and severity of flood damage in an existing development area (60 years old) with limited resiliency to accommodate extreme rainfall conditions. The scope, defined in the approved Municipal Class Environmental Assessment Study, applies to identified "flood cluster" vulnerable areas in West Thornhill that were adversely affected by an extreme rainfall event on Aug 19, 2005. The proposed Phase 2B project area is within one of the identified priority flood vulnerable areas. Project output will be a higher level of service and enhanced flood conveyance capacity compared to the existing storm drainage system performance - this will be achieved through sewer size increases and inlet capture improvements.