What is a flash flood?

The following article has been reprinted with permission from The Conversation, an online publication covering the latest research.

Flash floods are a specific type of flood that occurs in a short time frame after a rainfall event – generally less than six hours. It is most often caused by heavy or excessive rainfall and occurs in areas near rivers or lakes, but it can also occur in places where there are no nearby bodies of water.

Flash floods occur in rural and urban areas, such as in late July 2022 in St. Louis and eastern Kentucky. When rain falls in an area than the ground can absorb, or falls in areas with a lot of impermeable surfaces like concrete and asphalt that prevents the ground from absorbing precipitation, the water has few places to go and can rise very quickly.

If an area has experienced rainfall recently, the soil may be saturated and may not be able to absorb more water. Floods can also occur after a drought, when the soil is too dry and hardened to absorb rainfall. Flash floods are common in desert landscapes after heavy rainfall and in areas with shallow soil over hard rock that limits the soil’s ability to absorb rain.

As the water flows downhill, precipitation will look for the lowest point in a possible path. In urban areas, streets, parking lots, and basements are often in low-lying areas. In rural areas with steep terrain, such as the Appalachian region, flash floods can turn streams and rivers into raging torrents.

Flash floods often catch people off guard, although weather forecasters and emergency personnel try to warn and prepare communities. These events can sweep away cars and move buildings off their foundations.

The best way to stay safe in flash floods is to be aware of the danger and be prepared to respond. Low-lying areas are at risk of flooding, whether it occurs slowly or quickly and whether it is in an urban or rural environment.

It is crucial to know where to get updated weather information for your area. And if you’re outside and encounter waterlogged spots, such as waterlogged roads, it’s always a good idea to wait for the water to recede or turn back and find a safer route. Don’t try to cross it. Floodwaters can be faster and stronger than they seem – and therefore more dangerous.

Building for a wetter future

Engineers design rainwater control systems to reduce the damage that rain can do. Sewers carry water and help control where it flows, often directing it down roads and railroads so that people and goods can continue to move safely. Rainwater containment ponds and retention basins retain water for later release after the floods stop.

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

The design criteria and rules we use to engineer these features are often based on historical rainfall data for the site we are working on. Engineers use this information to calculate the volume a microscope, pond, or other structure would need. We’re always building some extra capacity to handle unusually large floods.

Now, however, many parts of the United States are experiencing more severe storm events that drop large amounts of rain over an area in a very short period of time. The recent St. Louis and Kentucky floods were on a scale that is statistically expected to occur in these areas once every 1,000 years.

With climate change, we expect this trend to continue, which means planners and engineers will need to reconsider how they will design and manage infrastructure in the future. But it is difficult to predict how frequent or severe future storm events will be at a particular location. And while it is highly likely that there will be more severe storm events based on climate forecasts, designing and building for the worst case is not cost-effective when there are other competing requests for funding.

Currently, engineers, hydroscientists, and others are working to understand how best to plan for the future, including modeling flood events and development trends, so that we can help communities make themselves more resilient. This will require more updated data and design criteria that are better adapted to expected future conditions.

This article was originally published on The Conversation. Read the original article.

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