Resilience Natural Disasters

Why are flash floods so hard to predict?

flash floods, flash flooding, rainfall drainage, predicting flash floods

Story at a glance

  • Flash floods happen when there is heavy rainfall.
  • There isn’t a global definition of what a flash flood is.
  • Predicting rainfall is harder than it seems, and there are other complicated factors that affect flash flooding.

You may have gotten one of these warnings on your phone: annoying beeps and a notification about flash flooding in your area. If you live in a city, it may not affect you. But if you depend on rural roads to in your daily life, it’s much more important that you pay attention. Nearly 100 people die each year in the U.S. by drowning during flood events, often by trying to drive through flooded roadways. Flash floods are serious weather emergencies and are becoming more frequent with extreme weather events.

On the surface, flash floods seem pretty self-explanatory. If where you live is prone to rainfall, the water can accumulate and lead to flooding. But on closer examination, it’s actually a much more difficult process to understand what areas are more at risk and how to predict the impact of flash flooding.

Not just any flood

A coastal flood or river flood is more easily defined because they are geographically restricted to coasts and rivers. Flash floods depend more on where excessive rainfall is occurring. Another reason flash floods can be dangerous and difficult to prepare for is that they can happen suddenly when there are heavy rains, sometimes within a few short hours.

“That’s where flash floods differ a little bit from big stem river floods, the latter tend to be…on the order of days and weeks,” says hydrometeorologist Jonathan Gourley of the National Oceanic and Atmospheric Administration (NOAA).

Turns out, there’s no one global definition of flash floods. “It’s very much context specific and place specific,” says climate researcher Andrew Kruczkiewicz of Columbia University’s International Research Institute for Climate and Society. “And that’s reflected in the definitions of flash floods that vary from country to country.”

For example, in Dhaka, Bangladesh, what we would call urban flooding is locally called water logging and doesn’t qualify as a flood by the government, says Kruczkiewicz. There could be standing water up to people’s knees, and it’s not defined as a flooding event. But because it’s manageable and no people are dying, it’s not viewed as having a big impact, says Kruczkiewicz.

In addition, flash floods can affect areas that are far away from bodies of water, so it’s not just coastal and riverine areas. It’s tied to where heavy rainfall will be, making it a meteorological problem. Hydrology of the land is also important, says Gourley, because the top layer of soil could be dry or wet. The type of soil matters, too; for example, water doesn’t move very well through clay soils, so less rainfall will be absorbed into the ground and more of it will runoff above ground. Even something like leaves on the ground can affect how much rainfall contributes to flooding. On top of that, drainage systems in towns and cities may not be adequately functional, adding another layer of complexity.

Predicting flash floods

For all these reasons, researchers work hard to better predict flash floods and their resulting impacts. To make a forecast, you need to look at historical data, which includes understanding what kind of data is available, what the quality of that data is, and how it was collected, says Kruczkiewicz. Depending on your location, you may be data rich or you may be data poor. Some cities may have data going back several decades, but some places only have a few years. And after researchers gather all that data, they put it into models that try to predict what may happen.

While there are models to help predict the behavior of rainstorms, they are not always accurate. Predicting where rainfall is the heaviest is not as straightforward as you may think; thunderstorms are complex systems and tricky — especially in warmer seasons — because convection of air that influences the storm can be unpredictable and can change pretty quickly. Trying to understand how heavy rainfall will impact people involves understanding the particulars on the ground.

One method researchers have been exploring is including land cover maps in their predictive models. They can take what they know about soil types and add that to the model to help predict what the surface water situation will be like during a big storm. Gourley says the models that they are currently working on are probabilistic, meaning that they can give a percent chance of impact depending the inputs put into the model. Kruczkiewicz and collaborators are using satellite data from NASA that provides highly detailed land cover data.

Although improving the models is important, that’s only the first step. “Is it sufficient for us to build a better model?” asks Shanna McClain, head of NASA’s Risk Reduction and Resilience program, which has funded Kruczkiewicz’s research. “That’s not enough and only part of the story. The other part is saying, ‘Okay, how do we then tell people that they need to be concerned? How do we get them to understand what action they need to take as a result?’”

They are also working with humanitarian organizations to better understand the various definitions of flash floods. For example, flash flooding in the forest where it wouldn’t affect people may not be a high priority to the humanitarian groups that would respond to natural disasters but flooding near temporary settlements would. In one case, they worked with UN groups in Bangladesh, focusing on refugee groups from Myanmar have settled in some areas that were vulnerable to flash flooding. They were in a mountainous landscape, in which the dangers of landslides were a real concern. Through this collaboration, researchers were able to determine areas that had high landslide risk and which specific settlements were likely to be impacted, says McClain.

“Then the next question is…if we know these things…how do we save lives? How do we make sure that we’re doing the right things before the risk happens?” says McClain. Gourley says that NOAA is working to improve their messaging so that people respond better. People should be aware when it can be unsafe to drive through areas affected by flash floods and should plan ahead, treating flash floods like they would other natural disasters like tornadoes.

A focus on impact

One key idea here is that the focus is on how flash floods impact people; it’s not solely an academic endeavor. “If we could move towards an impact-based definition of flash floods, at least the humanitarian community could have a better sense of risk of flash floods, and then get a sense of if the risk is increasing or decreasing,” says Kruczkiewicz. Then they would also be able to develop a global flash flood forecasting system or risk assessment process, which could in turn help determine where funding for preparedness is needed.

Part of being prepared is having those back-up plans set in place so that, if necessary, you can stay away from the flooding. The majority of flash flooding-related fatalities can be avoided by staying at home, Gourley tells Changing America. For example, if a road is flooded and you can’t drive to pick up your kids from school, don’t risk it and have an alternate plan for who could access the school in that part of town.

The National Weather Service is moving towards an impact-based warning format, Gourley says. That means only the more serious flash floods will lead to automatic warnings on your phone, hopefully meaning that people will take it more seriously when they do receive one.