Halt the Headcut A Discussion on Earthen Spillway Erosion Cutoff Walls

Thousands of dams across the United States have been constructed with auxiliary earthen spillways, routing excess flood water around a dam site to prevent overtopping. In the last two decades, mathematical models and computer programs such as the Water Resources Site Analysis Program (SITES) program were developed, which can predict spillway erosion and the propagation of the erosion headcut through the spillway towards the reservoir. When the evaluation of spillway erosion for existing dams determines that the earthen spillway breaches during the required design storm, designers, owners, and operators can get stuck trying to find a solution to provide spillway capacity.
Simple geometric solutions such as widening the spillway or raising/lengthening the control section will reduce unit discharge and shear stresses and solve the breach dilemma. However, modifying the earthen spillway to prevent a breach is often a significant alteration, and site constraints may not permit widening the spillway or raising the top of dam. Designers have solved this problem by utilizing structural concrete spillways that overtop the dam while abandoning the earthen spillway. But when significant hydraulic capacity is required and the dam site is not favorable to a large structural spillway, the earthen spillway may still be needed. Erosion cutoff walls can be added in the auxiliary spillway to stop the progression of the headcut and prevent the earthen spillway breach.
The SITES program allows users to add a cutoff wall to the erosion analysis thereby demonstrating that the spillway will not breach. The developers of the program added this as an option but provided no guidance about the design of the cutoff wall. This paper focuses on cutoff wall design options and discusses one project where a cutoff wall is being utilized to keep the earthen auxiliary spillway in service.