Analysis of ARS Low‐Drop Grade‐Control Structure

The Agricultural Research Service's (ARS) low‐drop structure was developed to provide grade control for streams in northern Mississippi. Since development, the U.S. Army Corps of Engineers has broadened application of the structure to include energy dissipation. Although the ARS structure is an effective means of stabilizing a stream channel, it is vulnerable during construction, requires refinement to be an effective energy dissipator, and is hydraulically submerged at moderate discharges. A model study was conducted to evaluate the performance of the ARS structure for high submergence conditions, to determine if the baffle plate could be replaced by H‐piles, to evaluate the basin effectiveness by reducing the depth, and to evaluate the structure during flow frequency events greater than 10 years. Test parameters included the discharges ranging from 2,200 cu ft/sec $(62.3m^3/s)$ to 6,800 cu ft/sec $(192.6m^3/s)$ and 0.5, 1.0, 1.25, 1.50, and 1.75 submergence levels. The study results indicated that the basin baffle can be replaced by two rows of H‐piles (seven H‐piles) without a reduction in basin efficiency while improving overall basin flow conditions. It was determined that the riprap becomes unstable using current design procedures. Also, a coefficient‐of‐discharge relationship was developed. Further, the stilling‐basin depth can be reduced by approximately 3.0 ft (0.91 m) without loss of basin effectiveness.