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.
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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. Flooding can lead to secondary consequences in addition to damage to property, such as long-term displacement of residents and creating increased spread of waterborne diseases and vector-bourne disesases transmitted by mosquitos. While riverine flood damage can be eliminated by moving away from rivers and other bodies of water, people have traditionally lived and worked by rivers because the land is usually flat and fertile and because rivers provide easy travel and access to commerce and industry. Floods often cause damage to homes and businesses if they are in the natural flood plains of rivers. While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, these changes in size are unlikely to be considered significant unless they flood property or drown domestic animals.įloods can also occur in rivers when the flow rate exceeds the capacity of the river channel, particularly at bends or meanders in the waterway. įlooding may occur as an overflow of water from water bodies, such as a river, lake, or ocean, in which the water overtops or breaks levees, resulting in some of that water escaping its usual boundaries, or it may occur due to an accumulation of rainwater on saturated ground in an areal flood. In particular climate change's increased rainfall and extreme weather events increases the severity of other causes for flooding, resulting in more intense floods and increased flood risk. Human changes to the environment often increase the intensity and frequency of flooding, for example land use changes such as deforestation and removal of wetlands, changes in waterway course or flood controls such as with levees, and larger environmental issues such as climate change and sea level rise. Floods are an area of study of the discipline hydrology and are of significant concern in agriculture, civil engineering and public health. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Cities and towns built on waterbodies or with infrastructure designed around historical rainfall patterns are increasingly susceptible to flooding.Ī flood is an overflow of water that submerges land that is usually dry. Flooding is increasing with extreme weather events caused by climate change are creating rainfall events with much more rain than in the past. Flooding in a street in Morpeth, England.
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