Stormwater Quality Control – Enhanced (i.e., long term average removal of 80 percent of suspended solids) water quality control will be provided to treat storm runoff from Highway 401 and Highway 3.

8.2   Hydrology

Consistent with the methods adopted by the URS in the Preliminary Design, the StormNET model with the EPA SWMM runoff procedure was used for the watershed hydrology to define design flows for drainage infrastructure. StormNET was also used to evaluate the proposed conditions and demonstrate that the design complies with the Executed Project Agreement and Regulatory Authority design criteria. Figure 8.1 illustrates the proposed conditions sub-watershed catchment delineation.

8.2.1 Updated MTO IDF Standards

The MTO has developed new Intensity-Duration-Frequency (IDF) curves which are to be used in the design of Ministry highway infrastructure. The new IDF curves replace those in the MTO Drainage Management Manual, 1997. The implementation date for these new curves was 28 February 2011. As per Highway Design Bulletin 2011- 001, projects that have passed the 30 percent completion stage are to assess the impacts of using the new IDF curve values on the design. If the new rainfall intensities are greater than 10 percent of the values used in the design, the new values should be used.

The design of the drainage elements proposed for the Windsor-Essex Parkway was developed to the 60 percent completion stage and is based on two IDF curves, as follows:

The Windsor Airport IDF curves were selected and deemed more appropriate for the sub-watershed hydrology and drain realignments as the station is within 15 km of The Parkway study area and provides rainfall statistics that are more representative of the area compared to the MTO District 1 curves.

The new IDF curves were reviewed and a comparison of the rainfall intensities for the 100-year return period event, which the design of the drainage infrastructure is based on, is provided in the following tables.