What is a flash flood? A civil engineer explains

Flash flooding is a specific type of flooding that occurs in a short time frame after a precipitation event – generally less than six hours. 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.

Flash floods happen in rural and urban areas, as in late July 2022 in St. Louis and eastern Kentucky. 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.

If an area has had recent rainfall, the soil may be saturated to capacity and unable to absorb any more water. Flooding can also occur after a drought, when soil is too dry and hardened to absorb the precipitation. 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.

Since water runs downhill, rainfall will seek the lowest point in a potential pathway. In urban areas, that’s often streets, parking lots and basements in low-lying zones. In rural areas with steep terrain, such as Appalachia, flash flooding can turn creeks and rivers into raging torrents.

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