Resilient Diversion Design in a Dynamic Braided Channel: Integrating a Coanda Screen and Infiltration Gallery for Sediment Continuity and Municipal Supply
Crystal Young
RESPEC
Rapid City, SD
A debris-flow event in 2025 caused catastrophic failure of the City’s primary municipal diversion structure, burying the facility beneath more than five feet of boulder- and cobble-sized material. The diversion supplies approximately 90 percent of the community’s water supply. The affected reach is a steep, braided cobble-bed channel with an average slope of 8 percent, located in an active alluvial channel where baseflow pathways migrate frequently across the valley bottom. These highly dynamic geomorphic conditions, combined with continued debris loading, rapid aggradation and degradation processes, and significant downstream headcutting following the debris-flow deposition, created substantial technical challenges for restoring a reliable diversion.
To re-establish the diversion within the City’s water lease window and meet strict water rights administration requirements, the project team developed an innovative diversion system combining a Coanda screen with an infiltration gallery. This hybrid approach was selected to maintain sediment continuity and provide operational flexibility for seasonal diversion needs. The design also provides aquatic organism passage at all flows. The design relocated the diversion to a more stable location capable of accommodating future channel migration while continuing to safely pass high sediment and debris loads inherent to this watershed.
The project was funded through the NRCS Emergency Watershed Protection (EWP) Program due to the urgent need to restore municipal water supply within ten months. The resulting design balances hydraulic, geomorphic, and ecological objectives by ensuring sustained sediment transport, avoiding structural burial during future debris-flow events, and maintaining reliable diversion capacity across a wide range of seasonal operational scenarios.
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