CS14D - Concurrent Session 14D: Cart Path Only: Spillway Breaching at Black Creek Site 53 Dam

Golfers are familiar with the term “Cart Path Only” which instructs golfers the keep motorized golf carts on the designated paths to avoid damaging the grass surfaces on the course. A similar term should be applied for grass-lined spillways which also require uniform, well-maintained vegetated surfaces. Unlike golf courses, the consequences of discontinuities and surface disturbances of grass-lined spillways are much greater than the bumps felt riding in the golf cart. Poor maintenance of the vegetative cover and post-construction modifications such as road cuts, fences, drainage ditches, woody vegetation, etc. can concentrate the flow in an earth cut spillway and accelerate erosion. These were the circumstances surrounding the 1983 Black Creek Site 53 Dam incident in Mississippi. In May of 1983, two significant storm events occurred within one week of each other causing the spillway to activate and eventually breach. It was determined that the breach was initiated and accelerated by a farm road that was added after construction of the spillway that crossed the spillway exit channel perpendicular to the flow. This discontinuity created a steep cut across the spillway exit channel. The final breach through the spillway was approximately 40 feet deep, 160 feet wide, and over 500 feet long. The breach effectively drained the flood control pool, and approximately 190,000 cubic yards of spillway material was eroded from the spillway. This paper will review the history of Black Creek Site 53 Dam and discuss the root causes for the development of this issue. The primary focus of the paper will be to review how the discontinuity and poor maintenance of the spillway led to the spillway failure. The paper will highlight the importance of earth-cut spillway inspections, maintenance, and owner education to prevent catastrophic failures.