Going Under the Oklahoma River

Oklahoma City has completed the $6 million Oklahoma River Inverted Siphon System Improvements, Phase II project. This project included the potential rehabilitation of an existing three-barrel inverted siphon, new inverted siphon, a sewer flow distribution vault and a sewer line connecting the new inverted siphon to the collection system.

• By Erin Boudreaux
• Apr 29, 2014

Oklahoma City is the most highly populated city in the state and the 29th largest city in the U.S. At 621.2 square miles, it is one of the five largest cities in the country in geographic area. The city spreads into four counties: Oklahoma, Canadian, Cleveland and Pottawatomie. Consisting of over 1.2 million people, this region is Oklahoma's largest metropolitan area.

For many Oklahoma City residents, the waterway now known as the Oklahoma River was long thought of as little more than a drainage ditch. But all that was changed by the Metropolitan Area Projects (MAPS), a bundle of nine major Oklahoma City projects to be funded by a five year sales tax increase. In December of 1993, MAPS was enacted as it squeaked by voters.

Under the provision of MAPS, a seven-mile stretch of the North Canadian River that flows just south of downtown was transformed as part of the original initiatives. The $54 million development included dams to raise the water levels, recreational trails and more. These dams created river lakes for recreational activities. This seven-mile stretch was renamed "The Oklahoma River" in 2004. 

Oklahoma City has completed the $6 million Oklahoma River Inverted Siphon System Improvements, Phase II project. This project included the potential rehabilitation of an existing three-barrel inverted siphon, new inverted siphon, a sewer flow distribution vault and a sewer line connecting the new inverted siphon to the collection system. This project increased the city’s capacity to transport sewage from south Oklahoma City to the North Canadian Wastewater Treatment Plant. The existing siphon had inadequate capacity to handle flows during wet weather events. The project was developed to help reduce sanitary sewer overflows in the Brock Creek and Lightning Creek sewer sheds. 

Field Versatility

The new inverted siphon was installed under the Oklahoma River with 587 feet each of 24- and 48-inch fiberglass pipe. Inverted siphons allow storm water or wastewater sewers to pass under obstructions such as rivers.

In addition, 2,500 feet of 72-inch sanitary sewer main was installed by direct bury. Hobas Pipe USA was the pipe supplier for Phase II and custom manufactured centrifugally cast, fiberglass reinforced, polymer mortar (CCFRPM) pipe for this project.

Instead of continuing with the 72-inch pipe into the siphon, the dual line was preferred. “The smaller pipes generate flow velocities high enough to scour sludge buildup. The 72-inch pipe velocities are fine for the normal grades, but at the siphon the velocities would drop, causing solids to drop out and block the flow,” explained E. O'Neil Robinson, P.E., associate, Cardinal Engineering.

Ease of Installation

The schedule for the siphon installation was time-sensitive due to the activity on the Oklahoma River. This particular section of the Oklahoma River is the site of the Oklahoma City Boathouse District which includes rowing clubs. “We only had about 60 days to construct the new siphons. Since we had to lower the water level for construction, we had an agreement with the City that the water level would be raised to the appropriate level for the planned rowing events,” stated Robinson. It was built into the contract that the installation contractor would receive a bonus of $100,000 for completing this section by the requested due date. 

Lewis Contractors of Georgetown, Texas, was the installation contractor for this project and received the bonus for completing the project by the due date. Despite a four week rain delay, Lewis finished the project on time. With the water level lowered by the dams and controlled by a dewatering process, the siphons were installed.

Soil conditions presented a problem. “The biggest challenge of the siphon installation was the depth. In some areas we were 30 feet deep to rock; soil conditions were sand and water. It was a challenge to shore the area and keep the trench dry,” said Bill Geiser, north Texas area manager, Lewis Contractors, Inc.  

As the pipe was laid and joints connected, it was encased in reinforced concrete and covered with 18-inch rip-rap. The pipe was encased in concrete to prevent floatation. Also, this section of the river is periodically dredged, so the concrete would act as a barrier in case contact was made with the dredging equipment. 

Forty-five degree elbows were manufactured of the same material as the CCFRPM pipe. They were designed to take the flow from the 72-inch line into the smaller siphons and feed it into the 72-inch line on the other side of the river via inlet and outlet structures.

Construction Benefits

“We choose to specify CCFRPM in the design because we felt very comfortable using the product and it was appropriate for this project due to the construction technique chosen and the time constraints imposed,” said Robinson. The pipes light weight led to a faster installation due to the ease of handling using lighter equipment. 

“Access to the north side of the river with equipment and materials was difficult as it is confined by two major highways, Interstate 35 and Interstate 40. We worked with the Oklahoma Department of Transportation (ODOT) to develop a plan,” explained Robinson. Temporary access was proposed to coordinate entry and exit from Interstate 40 right-of-way with ODOT. 

In addition to the added capacity, the new siphon allows for easier and safer maintenance of either the existing or new siphon. The existing siphon, constructed of 24-, 30-, and 42-inch pipe, was cleaned and inspected to determine if it needed to be rehabilitated. 

The 72-inch direct bury portion was 25 to 30 feet deep. “Coarse sand and ground water were about ten feet above the pipe. We had to encase the CCFRPM pipe in flow fill to prevent floatation,” stated Geiser. The project was completed and is in service.