It’s been two weeks since the catastrophic flash floods in the Texas Hill Country that killed at least 134 people, including 27 campers and counselors from Camp Mystic. It’s a good time to take stock of what we have learned from this event and try to put it into the larger context of the evolving flood risk landscape in the US.

When trying to understand a particular flood event, or flood risk more broadly, it is helpful to break risk into its three components: hazard (the physical event itself), exposure (where people and property are positioned relative to the event), and vulnerability (how societal responses before, during, and after the event increase or decrease its impact). I will discuss each of these aspects of the Texas flooding and end with some thoughts on how this event fits into long-term trends.

Flash floods are driven by rapid runoff in response to intense precipitation. As this article from Grist explains nicely, the Texas Hill Country is particularly prone to flash floods because it receives lots of warm, moist air and because its shallow soils and hilly landscape promote rapid runoff.

 One widely-used measure of extreme precipitation is the 100-year, 6-hour rainfall: the amount of rain over a 6 hour period that is expected to be reached or exceeded only every 100 years or so (more accurately: the event with an annual exceedance probability of 1%). As shown in Figure 1, areas near the Gulf of Mexico have the highest values of this metric in the US: over 250 mm (10 inches) of rain in many cases.

A key factor in the damage done by a flood is the extent to which people and property are exposed to that flood. As Gilbert White, the so-called father of floodplain management, said in 1945, “Floods are acts of God, but flood losses are largely acts of man.” In other words, it is when we build in floodplains that we create the conditions for catastrophic flood events, by putting ourselves in harm’s way. Or, as Tim Palmer wrote in the New York Times after the Texas floods, “The best option to save American lives is . . . preventing unnecessary new development on floodplains, and amping up efforts to help people move their homes from high-risk areas to safe territory.” Of course, this is hard to do, because Americans – including those along the Guadalupe River – are irresistibly drawn to rivers for their natural beauty and the recreational, spiritual, and economic possibilities that they offer. Still, we need to learn how to appreciate our rivers while also respecting them; how to spend time near them while also giving them room to flood; how to think of them not as static lines on a map but as beings that change and move over time.

Given the extreme rainfall and runoff in the Texas event, the area flooded was not limited to the 100-year floodplain of the Guadalupe River, but extended up to 200 feet beyond it. Still, it seems that the majority of the damage and loss of life took place near the river, in areas that were known to flood (with or without climate change). Particularly tragic is the news that some low-lying Camp Mystic cabins may have been artificially re-mapped as being out of the floodplain in order to allow them to be kept in use and even renovated in place. This is just one small example in the long-running battle over flood mapping – but it illustrates that reality, in the end, does matter. We have to hope that this tragedy helps people take this issue more seriously: we should not be living in flood-prone areas, and we need to use the best available information to assess where those are (more on that in another post).

When people are exposed to a flood event, the impact of that event depends on the coping ability of individuals, families, and societies. For the Texas flooding, there has been extensive media coverage of the warning systems that were – or weren’t – in place to get people to higher ground. This situation – a rapidly-developing middle-of-the-night disaster in an area with poor cell coverage – is, of course, vastly different from a hurricane hitting a coastal city, where the hazard is generally known several days in advance and the challenge is evacuating large numbers of people over significant distances. But what they have in common is the importance of pre-planning: of thinking through communication and evacuation plans before the disaster happens. I suspect that in the aftermath of the Texas event, many communities around the country will be having this conversation – but the key is to keep talking about it even when flooding is not in the news.

The Federal Emergency Management Agency (FEMA) plays a critical role in saving lives and property by reducing vulnerability and supporting post-storm recovery. Yale Climate Connections has a great piece on how FEMA cuts under the Trump administration have hampered the response to the flooding in Texas. Even more worrisome for the long term is that the elimination of FEMA, as floated by Trump, would also jeopardize the critical work it does pre-storm to reduce community exposure and vulnerability (not to mention that FEMA administers the National Flood Insurance Program).

How has flooding changed in the US over the last few decades, and what is driving these changes? The first place we might go to to answer this question is NOAA’s Billion Dollar Disaster Database, which tracks the damages from weather-related disasters (going back to 1980) that individually caused at least a billion dollars in damage (inflation-adjusted). Combining the data for flooding (mostly inland floods) and tropical cyclones (coastal floods) shows a clear trend of increasing damages over time (Figure 3).

Figure 3. Damages from floods and tropical cyclones in the US, 1980-2024. Data from NOAA’s Billion Dollar Disaster Database.

This is consistent with expectations from climate change, but is the trend caused by climate change? Not necessarily! As development extends into floodplains, there is more and more value for floods to damage. (For more on the drivers of increased exposure, and strategies for combatting it, see Chapter 7 of my book.) The US Fifth National Climate Assessment (NCA) puts it well (caption to Figure 4.1): “Across the US, the number of water-related disasters with damages exceeding $1 billion (adjusted for inflation) during 1980-2022 rose due to increases in exposure, or assets at risk; vulnerability, or how much damage a hazard of a given intensity causes; and climate change-driven increases in the frequency of extremes.” (One could quibble that the caption doesn’t mention other possible drivers of increasing extremes, such as higher imperviousness, but those are likely minor.)

Despite the careful wording in the NCA, the billion-dollar disaster database has been used less carefully by others and has become a target of the climate denial crowd. In another example of ignoring inconvenient realities, the Trump administration has terminated both the database and the NCA, as I discussed in a previous post.

If damage trends don’t distinguish between increasing exposure (development in floodplains) and increasing hazard (e.g., climate change), how can we figure out the role of climate change? One approach is to look at trends in the hazard itself. Figure 4 (from the NCA) shows that extreme precipitation is indeed increasing over time, especially (but not exclusively) in the eastern US, consistent with the basic physics of a warming atmosphere.

Figure 4. Image from the Fifth National Climate Assessment illustrating changes in extreme precipitation. This report is no longer available on government websites.

Once the changes in the hazard are understood, we can go one step further and link those changes to increasing damages. A 2021 study did exactly that, using a data-driven model of the relationship between extreme precipitation and flood damages, to show that about 37% of flood damages over the period 1988-2017 were caused by increases in precipitation (Figure 5).

Figure 5. Estimate of the extent to which precipitation changes are driving increased flood damages. Image from Davenport et al. (2021).

Of course, economic impact is only part of the picture. How have flood fatalities changed over time? The National Weather Service maintains a database of weather-related fatalities going back to 1940. Over that time period, floods and hurricanes have caused 12,356 deaths, or some 30% of total weather-related fatalities. Lightning strikes are the leading cause of death over the entire time period, but they have dropped dramatically over time, while flood deaths have not (Figure 6).

Figure 6. Annual fatalities from flooding (including tropical cyclones) and lightning strikes. Data from National Weather Service.

There are no doubt multiple factors driving the decline in lightning fatalities, but improved forecasts, better education, and changes in behavior – in short, adapting to the hazard rather than ignoring it – are an important part of the picture. While the flood problem is very different in many ways, I am convinced that we could reduce flood fatalities by improved adaptation and preparation: better land-use decisions, better pre-storm planning, better communication during disasters, and better response and recovery efforts at all scales. The constantly-evolving risk landscape produced by climate change certainly complicates the picture, but only makes the task more urgent. As David Wallace-Wells argues in a powerful piece in the Times: “Every weather disaster now has both human and climate causes, but we often argue about which side of the ledger should get the blame when, either way, the headline message is that we were not ready. . . . We may want to focus on the risks of warming to come, but as the eminent climate scientist Michael Oppenheimer likes to emphasize, we’re not all that well adapted to the climate we have now. That is what it means to be overwhelmed, again and again, by weather horror; the standards for safety and security we set for ourselves, in the wealthy modern world, are breached pretty regularly by weather events we should be able to manage much better. In theory, at least.”