Evaluation of the ARS Low Drop Grade Control Structure

The ARS low drop grade control structure was developed by Little and Murphy to provide efficient energy dissipation and effective grade control for streams in northern Mississippi. The ARS structure was developed for low submergence for storm occurrence intervals of 10 years or less. Since development, the U.S. Army Corps of Engineers has successfully installed several ARS structures. Although the ARS structure is an effective means of stabilizing a stream channel, it is vulnerable during construction, and is hydraulically submerged at moderate discharges. A model study was conducted to evaluate the performance of the ARS structure for high submergence conditions and flow frequency events greater than 10 years. A 1:12 (model:prototype) Froude scale model was constructed and 93 tests were conducted evaluating the original basin and six basin modifications. Test parameters included the 2, 10, and 100 years flow frequencies and 0.5, 1.0, 1.25, 1.50, and 1.75 submergence levels. Basin modifications included the installation of H-Piles, raising the basin bed, and shortening the basin. The study results indicated that the basin baffle can be replaced by two rows of H-Piles (7 H-Piles) without a reduction in energy dissipation while improving overall basin flow conditions. It was determined that the riprap becomes unstable using current design procedures. Also, coefficient of discharge relationships were developed. Finally, consideration should be given to developing an alternative grade control structure for high submergence conditions.