Stormwater Pollution Control at a Major Highway Interchange.
A main concern of the Ontario Ministry of Transportation (MTO) has been for the control of the quantity of stormwater runoff in the design of drainage systems for highways. This trend is now being revised to include the control of the quality of the stormwater runoff. This policy is presently being applied in the design of sections of Highway 407. Highway 407 will ultimately be a ten lane freeway extending from the western edge of Mississauga, Ontario, at Highway 403, and continuing easterly to Highway 35/115, east of Oshawa, Ontario. Sections of Highway 407 have partially been constructed with the installation of bridge structures and road base grading. Construction is proceeding from the west to the east. The proposed Highway will be in excess of 100 km (60 miles) in length. This study focused on one of the major proposed Highway 407 interchanges located at the crossing of Highway 404 in Markham, Ontario. This interchange will be a four level interchange which will increase the area percent imperviousness to 44%.
The study drainage area is approximately 380 ha (940 ac). The receiving channel for the stormwater runoff is located to the south of the interchange and is a small, unnamed tributary which flows to the west to German Mills Creek which, in rum, joins the Don River system and discharges to Lake Ontario. A tunnel will be constructed to convey the flow from the interchange to the creek. The area surrounding the interchange is fully developed or developing. A utility corridor (Ontario Hydro) runs south and parallel to the proposed Highway 407 in the study area.
The objective of the study was to develop a strategy for both quantity and quality control for the interchange. Quantity control includes providing protection against flooding for the 1: 100 year storm. Quality control implies providing treatment for a relatively frequent storm. The goal of quality control for stormwater runoff for the interchange is to meet the guidelines provided by the publication Interim Stormwater Quality Control Guidelines for New Development, issued jointly by the Ontario Ministry of the Environment (MOE) and the Ontario Ministry of Natural Resources (MNR) in May of 1991.
The provision of a formalized quality control scheme in a principal interchange such as the 404/407 interchange represents a concerted effort by the Ontario Ministry of Transportation to incorporate the new Guidelines into their work. A number of agencies were consulted during the course of the study including:
Ontario Ministry of Transportation (MTO)
Ontario Ministry of Environment (MOE)
Ontario Ministry of Natural Resources (MNR)
Metropolitan Toronto and Region Conservation
Authority (MTRCA), Town of Markham
Regional Municipality of York
Ontario Ministry of Municipal Affairs (MMA), Ontario Hydro
As a result of these contacts, a number of objectives were developed for the quality control scheme for the interchange. The objectives include:
Provision of quality control for suspended solids and associated pollutants for runoff from a 25 mm (1 inch) rainfall event.
Protection of regional groundwater quality.
Maximization of the use of overland flow networks, natural channels, infiltration trenches and basins, and wetlands.
In addition, a number of constraints affecting the provision of quality control measures were identified for the study area. Constraints included:
Limited land area available in the vicinity of the interchange.
The proposed Highway 407 pavement is to be a minimum of 1.0 m (3 ft) above the high water levels observed from the 1:100 year storm.
High groundwater table at the northern section of the interchange.
Operation of treatment facility under variable weather conditions.
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