The geology of dam spillways

Over the past 25 years, embankment dam design has shown trends towards increased use of poorer quality rockfill and omission of concrete lining and energy dissipation structures in spillways. The most economic design may be an unlined rockcut spillway, supplying the total rockfill requirement for the embankment construction. However, such a design does impose significant restrictions upon construction program flexibility, and the apparent cost advantage can quickly disappear if geological problems are encountered during construction. Geological criteria for the acceptability of rockfill and for the omission of concrete lining in spillways have, therefore, become increasingly important in recent years. The decision to omit concrete lining and energy dissipation structures from the spillway should only be taken after detailed consideration of the site geology, the probable erosion mechanism and the operational characteristics of the spillway. It should be recognised that frequently operating spillways with severe discharge conditions require special consideration and a certain degree of conservatism over and above that required for spillways where discharge conditions are only moderate.