Floods from natural rock-material dams

Abstract

Summary Breached dams formed naturally of rock or rock debris have produced many of the largest floods of Earth history. Two broad classes of natural impoundments are (1) valley-blocking accumulations of mass movements and volcaniclastic debris, and (2) closed basins rimmed by moraines, tectonic depressions, and calderas and craters formed during volcanic eruptions. Each type is restricted to particular geological and geographical environments, making their incidence non-uniform in time and space. Floods from breached natural dams and basins result from rapid enlargement of outlets. Erosion commonly is triggered by overtopping but also by piping or mass movements within the natural dam or basin divide as the level of impounded water rises. Breaching also can be initiated by exogenous events, such as large waves caused by mass movements or ice avalanches, and upstream meteorological or dam-break floods. The peak discharge and hydrograph of breached rock-material dams depends mainly on the impounded volume, breach geometry and breach erosion rate. For impounded water bodies that are large with respect to final breach depth, including most tectonic and volcanic basins and many ice-dammed and volcanic-dammed lakes, the peak discharge is primarily a function of final breach geometry. These floods typically last longer and attenuate less rapidly than smaller impoundments. For impoundments of smaller volume relative to final breach depth, such as most moraine-rimmed lakes and landslide and constructed dams, peak discharge is a nearly linear function of vertical breach erosion rate.