The modelling revealed that the open-segment of the drain has sufficient capacity to safely convey both the 5-year and 100-year events, without overtopping the drain banks. The existing enclosed segment of the drain has capacity to convey the 5-year event without surcharging significantly, but cannot convey the 100-year event without causing a backing up into the open channel segment.
Model simulations were undertaken to assess the impact of the orifice that exists within the outfall chamber near the Grand Marais Drain. Simulations were run with the orifice in place, and with it removed, to determine its potential effect on outflows to the Grand Marais Drain. Our evaluations revealed that removal of the orifice would only slightly increase flows at the outlet, which would have negligible impact on downstream flows and water levels. However, removal of the orifice would significantly lower the internal hydraulic grade line along the Huron Estates trunk sewer, which would significantly reduce the flood risk within the subdivision.
The timing of the subdivision construction in relation to improvement of the Grand Marais Drain / Turkey Creek through the Town of LaSalle (hereafter LaSalle) is relevant to justifying its removal. When the subdivision was planned and constructed in the late 1980s, the lower Grand Marais Drain through the LaSalle was unimproved and known to have significant capacity limitations. Design of the subject sewer outlet structure and orifice was presumably intended to mitigate potential outflow increases so that the downstream flood risk was not increased. In 1994, the lower Grand Marais Drain / Turkey Creek was substantially enlarged and improved through LaSalle, to increase its capacity to accommodate a 100-year flood flow. The 1989 hydrologic modeling that established the channel improvement requirements would have accounted for the land use at that existed in 1989, which would have included the Huron Estates residential lands.
6.4 Recommended Drain Improvements
Based on our consideration of the drain history, the information obtained during the site meeting, our examination and analysis of the survey data, and our hydrologic and hydraulic analyses, we recommend that the following improvements to the Marentette-Mangin Drain be undertaken:
Given that the open drain has sufficient size and gradient to convey up to the estimated 100-year storm event, no excavation works are recommended. The existing growth of vegetation on the channel banks and within the drain is not critically limiting drain capacity at this time. However, some of the blowdown is lying within the channel cross section and has the potential to create a flow obstruction or deflect flows towards channel banks causing erosion. We recommend that all blowdown that exists along the drain bottom and drain banks be removed, in accordance with the drawings and specifications that form part of this report. Work should entail the following:
o Complete removal of all trees that have blown over within the limits of the channel; and,