FLOODS FROM NATURAL DAM FAILURES

Breached dams of geologically emplaced rock debris have produced many of the largest floods in Earth's history. Two broad classes of natural impoundments are (1) valley-blocking accumulations of mass movements, glacier ice, 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 geologic and geographic environments, making their incidence non-uniform in time and space. Floods from breached natural dams and basins result from rapid enlargement of outlets. Erosion is commonly triggered not only by overtopping, but also by piping or mass movements within the natural dam or basin divide as impounded water level rises. Exogenous events, such as large waves caused by mass movements or ice avalanches, and upstream meteorologic or dam-break floods, can trigger outlet erosion.

The peak discharge and hydrograph of a flood from breached rock material dams depends mainly on the impounded volume, breach geometry, and breach erosion rate. For impounded waterbodies that are large with respect to final breach depth, like most tectonic and volcanic basins as well as ice- 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 those from 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 breach erosion rate. In steep environments, dam breaching may lead to downstream sediment entrainment, thus increasing peak discharge and flow volume depending on flow interactions with channels and valleys.

Floods from natural dam failures are geomorphically important because their high flows achieve shear stresses and stream powers orders of magnitude greater than meteorologic floods. Such flows can exceed critical thresholds for eroding bedrock and can transport clasts with diameters of many meters, thus forming some of the most spectacular landscapes on Earth.