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Waverly Heights Land Drainage Study Using
the Stormwater Management Model (SWMM)

Douglas D. McNeil, 1996

Four stormwater retention basins (SRBs) in the Waverly heights residential subdivision in Winnipeg, Manitoba have extended periods of above normal water levels. The SRBs are connected in series, all with the same normal water level. They can take up to 10 days or more to return to normal water level after a rainstorm event, which exceeds the City of Winnipeg’s design criterion of seven days. This study was undertaken to determine the most cost-effective way to eliminate this problem.

The discharge from the SRBs is directed to the Lot 16 Drain, a regional land drainage channel, which in turn discharges to the Red River. Existing flows and water levels in the Lot 16 Drain heavily influence the discharge from the SRBs and it is not uncommon to have zero outflow from the SRBs for several hours. Residents, especially those living adjacent to the SRBs with walk-out basements, are quite concerned about flooding because the storage available to accommodate another rainfall is reduced when water levels remain high.

An urban hydrologic analysis was undertaken to simulate the existing conditions and to evaluate alternative solutions, utilizing the U.S. Environmental Protection Agency Storm Water Management Model (SWMM), Version 4.3. Input data for the model was primarily obtained from the City of Winnipeg. Typical values for various parameters were obtained from Winnipeg engineering consultants with SWMM modeling experience under local conditions.

The construction of the model was completed in steps to avoid errors greater than two percent (+ or -) in continuity of volume in the modeled land drainage system. The actual rainfall event chosen for model construction and calibration was complicated by several factors, which were introduced into the model on at a time as it was determined that the error was within the acceptable range. The factors were: elevated SRB water levels at the start of the rainstorm; elevated water levels in the Lot 16 Drain, and; a closed flap gate on the SRBs’ outfall piping.

Model calibration was completed with the adjustment of percent imperviousness, which was determined, from a sensitivity analysis of selected parameters, to be the most effective parameter to adjust the total flow. The results of the verification run supported the conclusion that the SWMM model was adequately calibrated.

The model was used to test the performance of the modeled land drainage system with the 25-year return frequency design rainstorm under existing conditions. Subsequently the effectiveness of alternative solutions were verified. Two upgrading alternatives were analyzed: modifications to the Lot 6 Drain, and ; installation of a pump station. Water levels in the SRBs return to normal water level in six days with the pump station alternative, and in seven days with the Lot 16 Drain modifications alternative. The estimated project cost of the pump station alternative is $199,000, which is approximately $300,000 less than the Lot 16 Drain modifications alternative. The recommended alternative is the pump station.

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