Des Moines Sewer Project Underscores Commitment To Infrastructure, Flood Control

November 2012, Vol. 67, No. 11

“In the downtown business district, there are storm sewer systems, sanitary sewer systems and combined sewer systems,” Kamp says. “During large rain events the combined sewers become very full, very quickly and usually become pressurized. These can cause backups to properties which have their sanitary sewers connected to the storm sewers. Also, during these deluges the combined sewers cannot handle all the flow and the only alternative is for the combined sewer to overflow directly into the river. Therefore, the plan specifies separating the storm flow from the sanitary sewage flow; an approach with a number of benefits.”

According to Kamp, by reducing the demand on the combined system, there are fewer backups into properties, which allow more capacity for sewage in the systems. Having separate storm sewers provides the storm water runoff a direct outlet to the river. The greatest challenge, however, is when these large rain events occur and the river level is already high. The solution -- provided by the new sewer system design -- includes a gatewell which is a large structure with a cast-iron gate that can close and not allow river water to backflow into the storm sewer.

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Project award
Coined the Court Avenue and Water Street Sewer Separation Project, city officials unanimously approved nearly $10 million in funding to address the inadequacies of the existing sewer system. The project includes the construction of storm sewer along Court Avenue from Water Street west to 2nd Avenue; then north three blocks to Grand Avenue; and west again to 3rd Street -- a distance stretching approximately six blocks.

The second component includes the construction of a storm-water pump station positioned directly across the earthen levee from the river at the corner of Water Street and Court Avenue. The pump station incorporates an existing gatewell structure that was built in response to the 2008 floods. The gatewell was built in advance of the pump station and was constructed in the winter during low river levels. Since the bottom of the pump station is situated well below the river, construction would not be delayed if river elevations would have risen the following spring.

The larger storm sewer was specified as RCPP; a reinforced concrete pressure pipe with a gasketed joint. The deeper sewers are below the river level and will always have infiltration into the pipe. Also, during high river levels, the pipe will become pressurized. Therefore, the gasketed pipe will reduce the infiltration and material being conveyed into the pipe through the joints.