Garrison Dam, located on the Missouri River in Central North Dakota, is the fifth largest earthen dam globally, and has been operational for flood control, hydropower, and water storage since 1953. Inaugural spillway use resulted from flooding in 2011, after which inspections identified six potential failure modes (PFMs), as well as increased the probable maximum flood (PMF) from 827,000 cfs to 1,150,000 cfs.

The Colorado State Hydraulics Laboratory was contracted by AECOM and the USACE to construct, test, and analyze a 1:24 scale model of a 96ft section of the 800ft wide spillway. The 4ft wide model represents 200ft of the upper chute (slope of 00.0075), the entire lower chute (slope of 0.20), stilling basin, and 1200ft of the downstream channel. Using Froude scaling, the 1.15 million cfs PMF is equivalent to 49 cfs in the CSU model. To address PFM 8 and mitigate the pressure differential creating a potential for uplift of the concrete slabs at the lower chute toe, the USACE proposed an additional overlay of 4ft over all sections, a new lower chute overlay design, and reconfiguration of the stilling basin and baffle blocks.

 Approximately 400 tests at PMF were performed on over 30 overlays to progressively optimize the lower chute overlay and stilling basin configuration. As of November 2025, testing of all proposed overlays has been completed, and the USACE is creating a final design to construct and perform extensive testing, data collection and analysis on.

Future testing includes using the final design to analyze the downstream channel sediment scour in the channel and potentially testing at other scales. The CSU sectional model is one of three total of Garrison Dam, along with a 1:32 sectional model of the spillway crest by the USBR, and a 1:55 comprehensive model at ERDC (USACE). Effective design elements identified at CSU are implemented in the other models to evaluate overall impact.