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Innovative Design Increases Orwell Dam Spillway Capacity

Stanley Consultants’ design helps Orwell Dam safely release water with innovative tailwater control structure.

Orwell Dam's spillway could not safely release enough water during major floods, according to a Corps of Engineers' analysis.   As water flows through the dam's spillway the rapid drop in elevation causes it to amass kinetic energy.   The water's energy is dissipated in a concrete stilling basin in a phenomenon known as hydraulic jump.  The stilling basin protects the streambed from the destructive energy of the hydraulic jump.  The location of the hydraulic jump is determined by the water level in the stilling basin.  An increase in spillway discharge could force the hydraulic jump outside the stilling basin.  This would cause tremendous streambed damage and possible dam failure.

Stanley Consultants provided engineering services to help remedy the problem.  An innovative tailwater control structure, located 300 yards downstream of the dam, was the solution. (Tailwater is water immediately downstream of a spillway.)  Made of reinforced concrete and earth fill, the tailwater control structure consists of five 15-feet wide by 15-feet tall box culverts and an earthen dam.  Water usually flows through the culverts unrestricted. 

However, during flooding, the structure will restrict the flow of water, forcing the level of the stilling basin to rise.  The resulting increase in the tailwater level will keep the hydraulic jump within the stilling basin.  As long as the hydraulic jump remains within the stilling basin, additional floodwater can safely flow through the spillway.  A tailwater control structure of this size and scale is unique to the Upper Midwest.