CSO Management Study: Inline Storage vs. Basement Flood Protection in Combined Sewer Systems
Combined sewers service the older, central portion of the City of Winnipeg. This area comprises approximately 40% of the City's developed area, and covers approximately 10,500 hectares. This service area is partitioned into 42 individual combined sewer districts. Typically, each district has a trunk sewer to convey flows to a diversion facility where dry weather flows are diverted to the interceptor sewers, and wet weather flows in excess of the diversion capacity are discharged to the river system. The relatively flat topography and the shallow rivers provide limited hydraulic gradient for the sewer system. This has resulted in the installation of large trunk sewers at relatively mild slopes.
The City of Winnipeg is conducting two major programs associated with the combined sewer system. The combined sewer relief program continues to upgrade the level of basement flood protection in the individual sewer districts. The ongoing combined sewer overflow (CSO) management study is a planning study on the effects of combined sewer overflows on river water quality, and will result in a control strategy to mitigate these impacts. These programs have conflicting objectives: the relief program enhances and upgrades the conveyance capacity of the combined sewer system, while combined sewer overflow control seeks to reduce the number and frequency of overflows.
Basement flood relief works have been implemented in seventeen of the combined sewer districts. This typically has been accomplished through the installation of relief sewers. The combined sewer overflow study has determined that, in may districts, the trunk sewers represent significant potential for developing inline storage. This was particularly the case in those districts in which a large diameter relief trunk sewer had been constructed. The question was how much, if any of the available storage could e utilized without affecting the current level of basement flood protection.
An analysis was conducted to assess the inline storage potential in the Clifton combined seer district. The analysis was carried out with the XPSWMM model. The model was developed, calibrated and used to test various inline storage alternatives under a number of rainfall ad operational scenarios. The results indicated that a substantial volume of inline storage could be accessed without compromising basement flood protection in the district. In turn, this would result in a significant reduction in the number and volume of CSOs to the river.
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