The existing drop structure in Sand Creek just downstream of the Las Vegas Street Bridge (Las Vegas Drop Structure) was completed around 1998. The drop has failed, which has allowed channel grades upstream and downstream of the drop to roughly equilibrate. The purpose of the project was to protect the channel grade in Sand Creek from experiencing additional degradation by replacing the failed drop structure and restoring channel grades.

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Channel Stabilization Challenges

Sand Creek at Las Vegas Street serves 53 square miles with a total 100-year discharge of over 25,000 cfs. The sandy soils in and around Sand Creek posed a challenge to curtailing the high-velocity flow of groundwater under the drop structure and maintaining channel grades around the drop structure in a highly erosive environment. The replaced drop structure needed to remain stable in the 100-year flood and protect against a high degree of seepage and uplift.

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Drop Structure Design & Permitting

The proposed design utilizes a combination of 36” and 48” grouted rock in conjunction with a sheet pile cutoff wall and a large number of weep drains. The 12 feet of elevation drop was broken into three sloping faces to provide hydraulic jumps at two locations for maximum energy dissipation. A low-flow channel within the drop guides the perennial base flows through the center of the drop structure. The last sloping face was designed to run below natural grade to accommodate expected degradation in Sand Creek downstream of the drop structure, and a staff gage was integrated into the drop structure to assist the County in monitoring the creek’s erosion.

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The project is located in the Waters of the U.S. and requires a Corps of Engineers permit.  The project was permitted under Regional General Permit 37 (Stream Stabilization Projects in Colorado).