We always build in some excess capacity to handle unusually large floods. Engineers use that information to calculate how large a culvert, pond or other structure might need to be. Often the design standards and rules that we use to engineer these features are based on historic rainfall data for the location where we’re working. Restoring wetlands along rivers and streams helps mitigate flooding as well. Many cities also are using green infrastructure systems, such as rain gardens, green roofs and permeable pavement, to reduce flash flooding. Stormwater containment ponds and detention basins hold water for release at a later time after flooding has ceased. Culverts transfer water and help control where it flows, often directing it underneath roads and railways so that people and goods can continue to move safely. In rural areas with steep terrain, such as Appalachia, flash flooding can turn creeks and rivers into raging torrents.Įngineers design stormwater control systems to limit the damage that rainfall can do. In urban areas, that’s often streets, parking lots and basements in low-lying zones. Since water runs downhill, rainfall will seek the lowest point in a potential pathway. Flash floods are common in desert landscapes after heavy rainfalls and in areas with shallow soil depths above solid bedrock that limits the soil’s ability to absorb rain. Flooding can also occur after a drought, when soil is too dry and hardened to absorb the precipitation. If an area has had recent rainfall, the soil may be saturated to capacity and unable to absorb any more water. When more rainfall lands in an area than the ground can absorb, or it falls in areas with a lot of impervious surfaces like concrete and asphalt that prevent the ground from absorbing the precipitation, the water has few places to go and can rise very quickly. It often is caused by heavy or excessive rainfall and happens in areas near rivers or lakes, but it also can happen in places with no water bodies nearby.įlash floods happen in rural and urban areas, as in late July 2022 in St. Flash flooding is a specific type of flooding that occurs in a short time frame after a precipitation event – generally less than six hours.
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