![]() The leading edge of the flood thus advances more slowly than later and higher flows. In that setting, the first flood water to arrive is depleted as it wets the sandy stream bed. The cause may be localized convective precipitation (intense thunderstorms) or sudden release from an upstream impoundment created behind a dam, landslide, or glacier.įlash floods are the most common flood type in normally-dry channels in arid zones, known as arroyos in the southwest United States and many other names elsewhere. Rapid flooding events, including flash floods, more often occur on smaller rivers, rivers with steep valleys, rivers that flow for much of their length over impermeable terrain, or normally-dry channels. However, large rivers may have rapid flooding events in areas with dry climate, since they may have large basins but small river channels and rainfall can be very intense in smaller areas of those basins. The increase in flow may be the result of sustained rainfall, rapid snow melt, monsoons, or tropical cyclones. Slow-rising floods most commonly occur in large rivers with large catchment areas. Localized flooding may be caused or exacerbated by drainage obstructions such as landslides, ice, debris, or beaver dams. When overland flow occurs on tilled fields, it can result in a muddy flood where sediments are picked up by run off and carried as suspended matter or bed load. Riverine (Channel)įloods occur in all types of river and stream channels, from the smallest ephemeral streams in humid zones to normally-dry channels in arid climates to the world's largest rivers. Endorheic basins may experience areal flooding during periods when precipitation exceeds evaporation. Areal flooding begins in flat areas like floodplains and in local depressions not connected to a stream channel, because the velocity of overland flow depends on the surface slope. Infiltration also is slow to negligible through frozen ground, rock, concrete, paving, or roofs. Surface soil can become saturated, which effectively stops infiltration, where the water table is shallow, such as a floodplain, or from intense rain from one or a series of storms. The excess accumulates in place, sometimes to hazardous depths. ![]() A flood-surrounded house in Ilmajoki, South Ostrobothnia.įloods can happen on flat or low-lying areas when water is supplied by rainfall or snowmelt more rapidly than it can either infiltrate or run off. In spring time, the floods are quite typical in Ostrobothnia, a flat-lying area in Finland.
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