Dam Failures and Engineered Flood Risk

Dams are built and operated for flood control, water supply, irrigation, and hydroelectric power, but their failure converts stored water into a sudden, often catastrophic downstream flood wave with far less warning time than a naturally occurring river flood, a hazard category engineers term "dam-break flooding." Failure mechanisms are conventionally classified into several categories: overtopping, in which floodwater exceeds the dam crest and erodes the structure from above, historically the most common cause of embankment dam failure worldwide according to statistics compiled by the International Commission on Large Dams; structural or foundation failure, including internal erosion or "piping," in which seepage progressively erodes fine material within an earthen dam or its foundation until a breach forms; and, more rarely, failures triggered by earthquake-induced ground shaking or liquefaction. The Banqiao Dam disaster of 1975 in China, discussed in the historic-floods module, exemplifies catastrophic cascading overtopping failure under extreme rainfall, while the collapse of Italy's Vajont Dam in 1963 illustrates a distinct mechanism: the dam structure itself did not fail, but a massive landslide into the reservoir on the night of October 9, 1963, displaced a wave of water that overtopped the dam by roughly 250 meters, according to subsequent geological reconstructions, and killed an estimated 1,900 to 2,000 people in the town of Longarone and surrounding villages downstream.