Erosion in 33 unlined spillways in rock has been studied for dams in Australia, South Africa and the USA. Geological factors which influence the amount of erosion have been identified using published and project data and spillway site inspections by the authors. These are the orientation, persistence, spacing and nature of rock defects including bedding partings, joints, foliation and shears. The presence of kinematically viable blocks which can be detached and the persistence of the basal defect for these blocks are the most important factors.
Where spillways discharge on to a natural slope the presence of valley stress relief features, such as sheet joints parallel to the slope, or kinematically viable blocks, often with open sub-vertical defects, can lead to significant erosion even with small spillway discharges. The mechanism can be one of slope instability rather than erosion as water pressure destabilizes the slope.
A rock mass characterization index, the ‘Rock Mass Erodibility Index’ (RMEI), which considers spillway flow conditions and erosion mechanisms, has been developed. It can be used as a guide to spillway erosion and, when coupled with stream power for spillway flows, provides a method for preliminary assessments of likely amounts of spillway erosion.