What is a flash flood? A civil engineer explains

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(The Conversation) – A flash flood is a specific type of flood that occurs within a short period of time after a rainfall event – ​​usually less than six hours. It is often caused by heavy or excessive rainfall and occurs in areas near rivers or lakes, but it can also occur in places without nearby bodies of water.

Flash floods occur in rural and urban areas, such as late July 2022 in St. Louis and eastern Kentucky. When more rain falls in an area that the ground cannot absorb, or it falls in areas with lots of impermeable surfaces like concrete and asphalt that prevent the ground from absorbing precipitation, the water has little of places to go and can go up very quickly.

If an area has had recent rainfall, the soil may be saturated to capacity and unable to absorb more water. Floods can also occur after a drought, when the ground is too dry and hardened to absorb precipitation. Flash flooding is common in desert landscapes after heavy rains and in areas with shallow soil depth above solid bedrock, which limits the soil’s ability to absorb rain.

Since water is flowing downward, precipitation will seek the lowest point in a potential trajectory. In urban areas, these are often streets, car parks and basements in low areas. In rural areas with steep terrain, such as Appalachia, flash floods can turn streams and rivers into raging torrents.

Flash floods often take people by surprise, even as meteorologists and emergency personnel try to warn and prepare communities. These events can wash away cars and even move buildings from their foundations.

The best way to stay safe during a flash flood is to be aware of the danger and be prepared to respond. Low-lying areas are at risk of flooding whether it happens slowly or quickly and whether it is urban or rural.

Knowing where to get up-to-date weather information for your area is essential. And if you’re outdoors and encounter flooded areas, such as roads covered in water, it’s always safer to wait for the water to recede or turn around and find a route. safer. Don’t try to cross it. Floodwaters can be much faster and stronger than they appear – and therefore more dangerous.

Building for a wetter future

Engineers design stormwater control systems to limit the damage caused by precipitation. Culverts transfer water and help control where it flows, often directing it under roads and railroads so people and goods can continue to move safely. Stormwater retention ponds and retention ponds hold water for later release after flooding has subsided.

Many cities also use green infrastructure systems, such as rain gardens, green roofs, and permeable pavements, to reduce flash flooding. Restoring wetlands along rivers and streams also helps to mitigate flooding.

Often the design standards and rules we use to design these features are based on historical rainfall data for the location where we are working. Engineers use this information to calculate the size of a culvert, pond or other structure. We are always building excess capacity to deal with exceptionally large floods.

Now, however, many parts of the United States are experiencing more intense storms that drop significant amounts of precipitation over an area in a very short time. The recent floods in St. Louis and Kentucky were both on a scale that, statistically, would occur in those regions once every 1,000 years.

With climate change, we expect this trend to continue, which means that planners and engineers will have to reconsider how to design and manage infrastructure in the future. But it’s hard to predict the frequency or intensity of future storms at any given location. And while it’s extremely likely that there will be more intense storms based on climate projections, designing and building for the worst-case scenario isn’t cost-effective when there are other competing funding demands.

Right now, engineers, hydrologists and others are working to understand how best to plan for the future, including modeling flood events and development trends, so we can help communities become more resilient. This will require more up-to-date data and design standards that better adapt to anticipated future conditions.


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