5.2 New Hydrologic and Hydraulic Analysis
A new hydrologic model of the Marentette-Mangin Drain watershed was developed and analyzed by Landmark for the purpose of independently confirming flood flows and assessing the capability of the recommended drainage improvements to safely convey these flood flows. This was deemed to be the best course of action for the following reasons:
There are remaining uncertainties with regard to the reliability of the previously completed hydrologic modelling due to discovered issues with calibration data; and,
The analysis of PIE only extended to the upstream limit of the Huron Estates Subdivision sewer system, and was not extended through the subdivision to a suitable outlet.
The complete modeling approach and results are presented in Section 6.3 of this report.
6.0 Findings and Recommendations
6.1 Outlet Considerations
The report authored by PIE under Section 77 of the Drainage Act represented the downstream drain boundary to be the limit of the existing open channel, where flows enter the municipal sewer system. Section 15 of the Drainage Act prescribes that every drainage works shall be continued to a sufficient outlet. Drains shall be discharged at a point where they can do no damage to other lands or roads.
As noted previously, the upstream open-channel segment of the drain discharges to a 1,200 mm sewer, thence to a 1,350 mm sewer, thence to the Grand Marais Drain. During the on-site meeting, a resident expressed concern that the sewer system through Huron Estates may not have sufficient capacity to receive the flows that are being delivered by upstream lands.
Based on these considerations, we elected to include the trunk sewer system through Huron Estates as part of the drain. The Marentette-Mangin Drain outlets to the Grand Marais Drain, which we consider to be a sufficient outlet for the subject drainage project for the following reasons:
A report prepared by the undersigned in 2019, under Section 28 of the Drainage Act for the portion of the Grand Marais Drain lying with the City limits, was recently adopted by City Council; and,
An engineer has been appointed under the provisions of the Drainage Act, to examine and report on what improvements need to be undertaken to the Grand Marais Drain / Turkey Creek, through the downstream municipality to provide a secure outlet for its tributary area.