Verdantas Flow Labs (formerly Alden Research Laboratory) was engaged by Stantec, on behalf of the U.S. Army Corps of Engineers (USACE), to evaluate riprap stability for a major rock weir being constructed between the Arkansas and White Rivers. The purpose of the weir  is to allow flood flow between the two rivers without causing excessive scour. The primary challenge was to ensure that the specified riprap gradations would remain stable under a wide range of flood conditions, as displacement during high flows would be difficult to detect and repair in the field.

To address this, Verdantas Flow Labs designed and constructed a 1:20 Froude scale physical model representing a cross-section of the proposed 1,000-foot-long, curved rock weir. The model allowed for controlled testing of riprap stability under varying hydraulic conditions. Thirteen test scenarios were conducted, with variations in headwater and tailwater elevations, and unit discharges up to 191 cfs/ft. The testing program included both short-term and a 24-hour long-duration test to simulate sustained flood events.

Results showed that the larger R7400 riprap remained stable up to velocities of approximately 20 ft/s, with only minor movement at higher velocities. The smaller R2200 riprap exhibited initial movement at velocities above 7 ft/s, with more significant displacement occurring between 12.5 and 21 ft/s. The long-duration test demonstrated that most riprap movement occurred early in the event and did not progress with time, indicating a tendency for the riprap layer to stabilize rather than fail progressively. Additionally, the end treatment sand at the downstream end of the structure showed no significant erosion in any test scenario.

The physical modeling provided clear, data-driven confirmation that the specified riprap gradations were appropriate for the expected hydraulic conditions at the Three Rivers site. This work gave Stantec and USACE the confidence to proceed with construction, knowing the design would perform as intended during major flood events.