2019 Harlan County Dam - Stilling Basin Dewatering

Harlan County Dam, located near Republican City, Nebraska, and was completed in 1952. This article focuses on dewatering of the stilling basin, which is critical to evaluation of the gated, concrete spillway. The spillway structure is founded on a shaly Niobrara Chalk formation which is interlayered with near horizontal bentonite seams. The bottom of the spillway has inclined steps from heel to toe. Original features included a clay plug to seal the upstream spillway/bedrock interface, foundation drain holes to decrease uplift pressures, and anchor rods in the stilling basin slab to increase passive resistance. The deepest anchors were tipped at approximately elevation 1824 feet to minimize sliding along the critical bentonite seams. Inspection of the spillway and other features of the dam were critical in informing an update to the most recent risk assessment, completed in February 2020. The observed performance of the stilling basin informed understanding the potential response of the spillway structure to a range of hydrologic loading conditions.