In March 2011, workers began an erosion control project at Deadman’s Basin, a water storage reservoir located on a rocky, rolling prairie in Wheatland County, Montana.
This project was part of an erosion protection for the outlet of the service spillway of the dam. There is a box culvert running through the dam, entering a stilling basin and exiting the basin onto 48-inch A-Jacks® units provided by Contech Engineered Solutions. Additionally, the channel is protected with ArmorFlex® class 50-s on the side slopes and downstream section.
“Riprap had been in place on the old outlet,” explained Shawn Higley, P.E., Branch Manager for Western Water Consultants – the Helena, Montana, company in charge of designing the $800,000 project. “When the outlet was lengthened to address high uplift pressures within the dam, a new energy dissipation system was needed.”
“The problem we had was that large diameter riprap was located a long distance away and the cost to transport it would be astronomical,” said Higley. “The riprap design would have required excavation of bedrock to achieve the required depth. The large diameter riprap was evaluated prior to the decision to select A-Jacks and the A-Jacks came out on top. We didn’t have to excavate much bedrock to dissipate energy and that is what put this product ahead of everything else. The depth of large diameter riprap would have been significant when compared with the installation of A-Jacks.”
Approximately 1,000 square feet of ArmorFlex and 180 A-Jacks units were used at the site. The roughness of the A-Jacks was utilized to create a very short hydraulic jump discharging into the irrigation canal. The primary use of the water is irrigation. They needed to have the system in place to deal with a high velocity hydraulic jump. The erosion control system needed to be in place prior to the main irrigation season, which would begin in the area in early May.
“This is an off channel reservoir system” added Higley. “They divert water from the Musselshell River into Deadman’s Basin. We were able to push a lot of water through the outlet at very consistent flows. They could not store any more water and had to let it go. It was tested under normal circumstances and also under high flow circumstances and tested very well”
The river in the basin hit record flood stages this year and the discharge test was a success at 345 cfs.
“High velocities came out of the outlet structure” concluded Higley. “The A-Jacks performed as well or better than we expected them to. We were talking velocities...coming out of the outlet basin into the A-Jacks that were around 20 feet per second or so. We used the A-Jacks product at the end of the hydraulic jump in the higher velocity zone. The ArmorFlex was used to provide a good transition into the downstream channel without significant erosion, to ease the transition to avoid erosion”
The project was completed in April 2011 and a full test of the site was conducted in August. It passed with flying colors.