length, the drain has a bottom width ranging from 1.5 to 3m, an approximate depth in the range of 1.5 to 2 m, and an average longitudinal slope in the order of 0.2%.
In general, the open-channel segment of the drain is well defined, and has sufficient size and adequate conveyance capacity to provide outlet for the upstream tributary lands. However, the channel traverses a mature woodlot and therefore exhibits a heavy growth of large trees over its length – along its channel banks and within the drain itself. There is an excessive amount of blowdown (large trees that have been blown over by the wind) throughout the drain segment. In some instances, the blowdown has dislodged large root balls that have disturbed the drain bottom. Some of the blowdown is crossing the drain and has the potential to induce clogging and impede flow as the trees decompose over time.
Portions of the open-channel segment of the drain border on, and in some locations, encroach within the east boundary of a provincially-designated Area of Natural and Scientific Interest (ANSI) known as the Spring Garden Natural Area. In addition, portion of the open-channel traverse areas that have been previously mapped as Provincially Significant Wetland (PSW) and Environmental Significant Area (ESA). Any works along the open channel will require approval of governing agencies and ministries.
6.3 Hydrologic and Hydraulic Assessment of Drain Capacity
In order to assess the capabilities of the drain to safely convey the anticipated flows, we constructed a PCSWMM model to represent the existing site condition within the Marentette-Mangin Drain watershed. Simulations were then undertaken to estimate the runoff rates that would be produced during a range of statistical runoff events.
A few notable aspects of the modelling are summarized below:
i. The hydrologic and hydraulic analysis was performed using the PCSWMM 2019 Professional 2D software version 7.2.2780. PCSWMM provides a modern, easy-to-use graphical user interface for the U.S. EPA SWMM5 program. The EPA Storm Water Management Model (SWMM) is a dynamic rainfall-runoff simulation model used for single event or long-term (continuous) simulation of runoff quantity and quality from primarily urban areas.
ii. The open channel segment of the drain was represented based on surveyed cross sections with a channel roughness of 0.035 and an overbank roughness of 0.05. The enclosed segment of the drain was constructed from as-built sewer information and assumed a pipe roughness of 0.013.
iii. The major storm events selected for the analysis included the Chicago 5-year and 100-year 4-hour storms. A 20-minute time step was used for the simulations.
iv. The estimated pond discharge rates that were presented in Table 9.4 of the SWM report authored by PIE were applied to the model at constant rates. The discharge rates for the 1:5 and 1:100 year events were 0.29 and 0.45 m3/s respectively.