Prado Dam Spillway Risk Informed Structural Design

In June of 2024, the U.S. Army Corps of Engineers (USACE) completed the final design for the rehabilitation of the existing Prado Dam spillway. Construction of the project will mitigate three risk driving potential failure modes and increase flood storage at the facility by raising the existing spillway crest. The project consists of constructing a new spillway within the footprint of the existing spillway with an estimated construction duration of approximately six years. The spillway is founded on interbedded low strength, expansive mudstone and highly permeable sandstone that is near vertically bedded in a region of high seismicity, resulting in numerous structural design challenges. This paper will highlight risk informed design considerations used in conjunction with established design criteria for the structural design of the major project features including the labyrinth control structure, the spillway chute walls, and the spillway chute slabs. The purposes of the paper are to advance the application of the risk informed design process using a recent dam safety modification final design example and to describe constructability challenges in the context of life safety risks during construction highlighting the agency’s decision to consider early contractor involvement procurement strategies.