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 round 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 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 needed to work between homes to reroute the house leads. In addition, such work would require work on private property and would be quite disruptive to the neighborhood and community life.

The city drastically 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. Approximately 50 to 75 percent 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 eight feet of ground level. Thus, the proposed 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, as well as avoid work on private property and reduce the amount of disruption to the residents of this community.