Emergency Redesign of Silver Jack Spillway

A massive landslide destroyed the nearly completed spillway stilling basin at Silver Jack Dam in Colorado. Geologic conditions at the site, the extent of the slide, subsequent damage, and the emergency redesign of the spillway are described. Also described are the hydraulic model studies performed to assure satisfactory flow conditions and with unsymmetrical approach flow resulting from a circular curve in the approach conduit. A deflector vane was installed in the crown of the tunnel downstream from the curve to prevent the flow crossing over the top and sealing the portal. Vanes were developed for the stilling basin approach chute to improve flow distribution in the basin. Unique baffle blocks were developed to provide good energy distribution in a basin that had insufficient tailwater depth to form a conventional hydraulic jump. Pressure measurements were made on the conduit bend, conduit vane, chute vanes, and baffle blocks.