S2o Designs Innovative Structure that Combines Diversion with Recreation and Fish Passage

S2o Design and Engineering created a diversion design for the Supply Ditch that diverts all required flows, provides for fish passage and is a recreational whitewater park.

During the September 2013 flooding every user of the St. Vrain Creek was impacted including agricultural, municipal, commercial, and recreational users. On top of impacts to the users of the St. Vrain the environmental impacts of the flood have been far reaching with the decimation of riparian habitat and severe impacts to the fishery. As the users reconstruct their critical infrastructure there is a new opportunity to create infrastructure that achieves the goals for the owner while also providing benefits for other users and the environment.

S2o is proud to be a part of this process and are implementing a design process that benefits all users and the environment in our projects. The diverse backgrounds of our team members bring expertise in whitewater park design, dam design, and river restoration design that shines in the design of multipurpose projects that benefit many interests. One example of a multipurpose project being implemented on the St. Vrain Creek is the Supply Irrigating Ditch Company diversion structure. Historically, the structure was an at grade concrete dam that was barely noticeable in the creek unless you looked close. During the flood significant erosion occurred downstream of the structure degrading the channel by up to three feet. S2o was hired by the ditch company to design a diversion dam that met their needs and benefited other users of the St. Varin Creek.

The ditch company used a temporary diversion structure for the 2014 irrigation season. The temporary structure was constructed of concrete barriers, concrete rubble, and anything else the ditch company could easily use to get water into their ditch. The temporary structure was not very effective at diverting water and was a significant hazard to any in stream user of the St. Vrain Creek. The hazards created by this temporary structure made the reconstruction of the diversion dam a high priority for safety as well as the ditch company’s need to divert water in 2015.
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With this significant amount of degradation an at grade structure was not going to be possible for the rebuilt diversion structure. The first step in the design process was to determine the crest elevation of the structure. The crest elevation of the structure was dictated by the design criteria of being able to divert the allocated water down the ditch inlet. The necessary crest elevation combined with the degradation of the channel lead to an elevation drop of two feet across the structure. To create a structure that would address environmental concerns of fish passage and create a recreational experience a grouted rock ramp structure with a notch for fish passage at low flow was designed. Also, to facilitate fish passage the grouting of the structure will not go to the tops of the rocks allowing paths for fish to swim through the interstitial spaces in the structure. The hydraulic conditions across the grouted rock ramp and in the tail water pool were designed in the same manner as a whitewater park structure. This will allow the water that is not diverted down the ditch inlet to be enjoyed by recreational users. The designed structure will meet all of the project requirements by diverting the allocated water down the ditch, allowing fish passage, and providing recreational opportunities.

Concept

This project is a great example of a multipurpose structure that achieves the goals of the owner while also providing for other users. In this case the owner’s agricultural, municipal, and commercial interests are maintained with a structure that benefits recreational users and the environment.

Construction of the structure is scheduled to begin in the near future and the structure will be completed by May 1, 2015. Upon completion a highly visible structure will be in the forefront for multipurpose projects as the St. Vrain and other nearby watersheds recover from the 2013 floods. S2o is leading the way with well-coordinated multipurpose projects.

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