Revising a Combined Sewer System Design for Improved Results
A new combined sewer system was designed for an existing sewer district located in the city of Detroit to address chronic basement flooding. The existing sewer system generally consists of sewers located in alleys behind homes, picking up both sanitary flows from the homes and storm flows from catch basins located in the streets. The proposed plan called for locating new sewers down most of the streets and bringing the existing house leads from behind the houses around front to connect to the sewers in the streets. The new system proposed consisted of a complete replacement of the existing system; it did not consider potential use of portions of the existing system to economize on cost.
Upon further review by the Detroit Water and Sewerage Department (DWSD), this approach was not considered to be feasible due to the difficulty of being able to relocate the house leads. In many cases, there is not enough space to work between homes to reroute the house leads. In addition, such work would require construction on private property which would be quite disruptive to the neighborhood and community life, and furthermore, would not qualify for State funding.
The city revised the proposed new sewer system design to incorporate the existing system to the extent possible while providing the desired level of service for the district. Specifically, where possible, the revised design was configured to intercept a sufficient number of catch basins in the streets to prevent storm flows from surcharging the sewers collecting sanitary flows from the homes which are predominately 12 in. (300 mm) laterals located in alleys behind the houses. Approximately 50 to 75% of the storm flows, depending on the area, will be effectively intercepted with the revised design which proved to be sufficient to prevent surcharging within 8 ft (2.4 m) of ground level. Thus, the revised design will be able to provide the required level of service for the Oakwood District at a significant cost savings to the original proposed design.
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