Riprap Design Criteria for the ARS Low Drop Structure

The ARS low drop grade control structure has proven to be an effective means to stabilize incised channels in Mississippi. However, field inspections and laboratory testing have indicated that current riprap sizing criteria may be inadequate for stabilizing the basin during design flood events. A model study was conducted to provide design criteria to enhance riprap stability in the ARS low drop structure. Forty tests were conducted with the discharge ranging from 3308 to 7794 cfs, median riprap sizes ranging from 23.9 to 36.5 in., and submergence ranging from 0.41 to 1.33. The structure was constructed with trapezoidal shaped inlet and exit channels, a six foot drop, and seven H-pile baffling devices. The results indicated the a stable riprap size can be predicted by relating the expected submergence condition to the ratio of the approach unit discharge to the median rock size. A rock movement prediction relation was presented.