James H. May’s scientific contributions

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)

REMR research at the US Army Engineer Waterways Experiment Station has established that remediation of unlined emergency spillway erosion damage is a relatively new, but major, concern to the US Army Corps of Engineers (CE) Districts and to other dam owners and operators. The REMR work unit conducting the current investigation has identified numerous CE and other Federal, institutional, and private-sector dams that have experienced erosion damage in their unlined spillway channels. However, only a few projects have implemented or planned remedial and/or preventive measures. Remediation design is highly site-specific and must be cost-effective, address public safety, and provide continued reservoir operations. Selection of remedial technique(s) must be established by site-specific characterization of the rocks forming an unlined spillway channel in terms of rock composition(s), hardness, structural and stratigraphic discontinuities, and precursor erosion elements, all of which determine rock erodibility and its rate.