Geotechnical Aspects of Rock Erosion in Emergency Spillway Channels. Report 5 Summary of Results, Conclusions and Recommendations

Severe erosion of rock and soils flooring unlined emergency spillway channels may cause failure of spillway structures and catastrophic release of reservoirs waters. An observational data base was used to document cases of spillway erosion using data from site visits, periodic inspection reports, and videos of spillway flow. It was shown the data base showed that severe erosion occurred at discharges which were 10% of Project Maximum Floods (PMF), and at velocities which were greater than those currently recommended. Spillway channel erosion was driven by processes similar to knickpoint migration (headcutting) in natural stream channels. Stratigraphic and structural discontinuities in the spillway foundation were important factors in controlling the occurrence and extent of erosion. Flume studies showed that erosion and headcutting were maximized when waterfall became unvented. Model studies and computer simulations showed that erosion did not accompany peak discharge but occurred on the lower portions of rising and falling limbs of the hydrograph. Therefore, severe erosion may occur at discharges significantly lower than PMF or spillway design flood. Spillway channels that are susceptible to erosion require detailed engineering geologic maps and cross sections of rock distribution and rock mass properties. (mm)