CSO Siphon Design: Proper Planning Eliminates Constructability Issues and Reduces Construction Costs

Kokomo, like many communities in Indiana, is under an EPA order to reduce their combined sewer overflows (CSOs), which will require multiple projects and technologies in order to achieve compliance. Per their consent decree, Kokomo is to eliminate overflows into Wildcat Creek for storms less than or equal to a six-month recurrence interval. One of the larger projects designated during the development of their long-term control plan was the reduction of combined sewage overflow from CSO 041. The reduction of this CSO is significant in that it receives combined flow from an 84-inch (2,100-mm) combined sewer that serves a large area of northeast Kokomo, as well as flow from an existing 96-inch (2,400-mm) storm sewer. During the planning process, it was determined that Kokomo should construct a siphon at the intersection of the 84-inch (2,100-mm) and 96-inch (2,400-mm) pipes. The project provided several design challenges in regards to constructability: multiple existing utilities, a 40-foot (10.7 m) wide right-of-way, and homes bordering the project site. In addition, it was determined that a bypass system for the 96-inch (2,400-mm) storm sewer alone would need to handle as much as 250 mgd (946 ML/d) during construction. Due to the lack of available space around the site, a bypass pumping setup was neither feasible nor cost effective. In order to mitigate the construction issues due to the tight site and significant flows in the pipes, the design team developed a construction sequencing plan that required a phased construction of the siphon pipes. The phased construction allowed for continued operation of the 84-inch (2,100-mm) combined sewer and 96-inch (2,400-mm) storm sewer and utilized the siphon pipes to bypass storm flow in the latter stages of construction. The detailed sequence of construction was included in the bid documents for contractors’ use during the bidding and, ultimately, construction. Several installation technologies were considered during the preliminary design phase. An understanding of the site limitations allowed the design team to develop a constructible design while reducing the construction budget. Because of this attention to detail, this CSO reduction project in Kokomo finished construction at a budget of nearly $1 million under the original construction estimate.