Debris Flood

a sudden, temporary flow occurring in mountain rivers and characterized by a sharp rise in water level and a high content (10–15 to 75 percent) of sediment load (products of the breakup of rocks). Debris floods result from prolonged heavy rains and the rapid melting of glaciers or seasonal snow cover, as well as from the collapse, where the terrain has slopes of at least 0.08–0.10, of large quantities of loose rubble into the river. Such conditions are characteristic of most mountain regions; in the USSR they exist in the Carpathians and in the mountains of the Caucasus, Middle Asia, the Crimea, and Eastern Siberia.

Depending on the composition of the debris flood, a distinction is made between mud-rock, mud, water-rock, and water-gravel floods, and, depending on physical types, between consolidated and unconsolidated. In unconsolidated debris floods, the transporting medium for the solid inclusions is water; in consolidated floods, it is a water-soil mixture, in which most of the water is bound by finely dispersed particles. In contrast to ordinary floods, debris floods generally do not move continuously but rather in individual surges (waves). Such movement derives from the way in which the floods are formed and from the obstructed character of the floods’ motion—the buildup of solid material in the constrictions and bends of the channel and the subsequent breakthrough. Debris floods move with speeds up to 10 m/sec and more. The volumes of the floods reach hundreds of thousands, and sometimes millions, of cubic meters at a time. Separate pieces of transported debris can have cross sections of 3–4 m and weights of 100–200 tons. Having such weight and speed, debris floods destroy roads and structures and lay waste to arable land.

The chief measures for controlling debris floods include the establishment and stabilization of a soil and vegetative cover on mountain slopes, especially in areas where debris floods form, the protection of this cover against agricultural and industrial activities in mountain areas, and the preventive release of water from mountain reservoirs threatening breakthrough. Accumulations of loose debris are removed and the flow of mountain streams is regulated by systems of flood-control dams. The direct regulation of debris floods is accomplished by means of hydraulic engineering installations.