Clarence Canyon Flood Damage Safety Investigation (MO)

The Clarence Cannon Dam and Reservoir is a multi-purpose project located on the Salt River in northeast Missouri.  Major components of the project include Mark Twain Lake, Clarence Cannon Dam, and a Re-regulation Structure.  Closure of the dam occurred in late summer 1983.  The Re-regulation Structure is located on the Salt River, 9.5 miles downstream of Clarence Cannon Dam.  The purpose of the Re-regulation Structure is to provide storage capacity for hydropower pumpback operations and dampen surges from power releases.  The Re-regulation Structure is a compacted earth embankment with a concrete navigation-type, low-head overflow spillway.  Flow through the spillway is controlled by two 30-ft wide by 31 ft high tainter gates.

Record rainfall occurred in the year 2008.  In late July and early August, Mark Twain Lake experienced record high pool elevations due to the extensive rainfall.  This resulted in large sustained releases of water from the main Clarence Cannon Dam and required both tainter gates at the Re-regulation Structure to be fully opened, leading to complete inundation of the stilling basin and downstream exit channel reach.    Significant erosion damage occurred to the overtopped embankment section and the banks of the exit channel in the reach downstream of the Re-regulation Structure.  A large circular-shaped basin was formed, presumably scoured out by the reverse flow of a highly erosive recirculation flow pattern affecting both sides of the downstream reach

WEST Consultants was contracted to evaluate the hydraulic design basis of the Re-regulation Structure, conduct computer modeling of the stilling basin and downstream exit channel, investigate involved failure mechanisms, and evaluate alternative concepts for repair or improvement of the stilling basin and the exit channel.