What To Know About Flash Floods
- The Phenomenon: Flash floods can be caused not only by extreme rainfall but also the collapse of infrastructure or major landslides. They can occur in just minutes.
- The Climate Change Factor: A warmer atmosphere holds more moisture, which can lead to greater rainfall that can overwhelm natural and artificial drainage systems. Other natural disasters induced by climate change can also make flash floods more devastating.
- Infrastructure Gaps: Outdated infrastructure can’t keep up with more intense rainfall and a changing climate. Increased concrete and asphalt development prevents soil from absorbing water, which can contribute to flooding.
- How to Stay Safe: Understanding the local geography, identifying high ground to evacuate to and taking quick, decisive action can help keep you safe in a flash flood.
In late August, two flash floods on opposite ends of the planet drew major headlines.
On Aug. 26, a collapsed glacier and landslide flushed a torrent of water and debris through a valley in Tibet and Nepal that killed over 1,300 people, with several thousand people still missing.
Days later, a flash flood in the Grand Canyon killed two people and left some 80 people in need of search and rescue, with one other person still missing. A year earlier, a flash flood killed 28 people at a children’s summer camp in Texas.
Are flash floods happening more often, and what environmental and human-made factors can make them more dangerous?
We asked two professors about how climate change influences flash floods and how to better prepare communities to deal with these natural disasters.
Megan Lickley is an assistant professor in the Earth Commons who studies the drivers and impacts of climate change and uses atmospheric science, math and policy to determine the risk of climate disasters. Roshi (Roshanak) Nateghi is an associate professor in the Earth Commons and leads the Sustainability & Resilience in Global Environments Lab, where she combines disaster resilience research, risk science and engineering to support policymakers and urban planners in creating communities that mitigate natural disasters.
Learn about why flash floods can happen without warning, what infrastructure upgrades could improve climate resiliency and how to stay safe if you are in a region prone to flash flooding.
Ask a Professor: The Science of Flash Floods and Effective Policymaking to Build Climate Resilience
The Physics and Atmospheric Drivers of Flash Floods
What causes flash floods, and what makes them different from typical floods?
Megan: While a flood describes water rising over what is typically dry land over days or weeks, a flash flood occurs over minutes to hours after the triggering event. They are characterized by fast-moving water that can bring down infrastructure or anything in its path. Flash floods can be caused by extreme rainfall events, sudden snowmelt or the collapse of a dam or levee. Predicting a flash flood requires high accuracy in rainfall prediction. A shift of a few miles could be the difference between rainfall being divided into multiple basins or concentrated into a single catchment, which can lead to drastically different flooding outcomes.
Predicting the timing of flooding from snowmelt can be even more difficult, as you now have to predict a historical buildup of water and interactions between the atmosphere, snow and stored water. Tension in a system like that can build over time without adequate observing or modeling systems in place to predict their collapse.
What environmental and geographical factors influence the severity and impact of a flash flood?
Megan: The severity and impact of flash floods are highly influenced by the magnitude of rainfall, soil saturation, land cover and the shape of the topography. In a steep mountainous region, narrow valleys act like funnels, allowing fast-moving water and debris to build up quickly. In Nepal, the combination of moving water and debris began having a snowball effect, bringing in more mass flow as the flood progressed. In an urban area, with hard surfaces, water can build up quickly as it has nothing to absorb it.
How does climate change shape the intensity or frequency of flash floods?
Megan: Climate change is contributing to more intense rainfall events, as a warmer atmosphere can hold more water (about 7% more moisture per degree Celsius). Therefore, when it does rain, more water is released, overwhelming the natural system or infrastructure.
Warming is also occurring faster in higher elevation areas, thawing glaciers and snowpacks. High-elevation glacial regions that used to have consistently cold temperatures throughout the year are no longer remaining frozen. This is leading to a growing number of unstable glacial lakes that risk glacial lake outburst floods when their natural dams fail.
Cascading Climate Risks and the Risk Factors of Flash Floods
How do other climate hazards like heatwaves, droughts and wildfires influence the behavior of flash floods?
Megan: Soil can act like a sponge when moist. Heat waves and drought can lead to harder soils that are hydrophobic (repelling water). This leads to water building up above ground more rapidly when it does rain. Wildfires destroy the vegetation and root systems that absorb water and hold soils in place. Immediately following a wildfire event, the spongy vegetation is lacking, so water can more easily build up and dislodge soil, contributing to more mass flow during severe rainfall events.
Do flash floods happen more often in certain times of the year or in certain climates?
Megan: On the U.S. East Coast, we typically see more flash flooding during warmer months in the late spring to early fall, as warm, moist air leads to more extreme rainfall events. In mountainous regions, you would be more likely to see flash floods during spring snowmelt. Geographically, we tend to see flash flooding in arid or urban locations, where the ground is hard and can’t absorb as much water, and in mountainous regions, where the funneling of water can accelerate runoff.
Infrastructure Vulnerabilities and Policy Responses
What are common vulnerabilities in our infrastructure that magnify the risk flash floods pose to communities?
Roshi: Much of the infrastructure we depend on — including roads, bridges, stormwater systems and flood defenses — was designed using historical weather records that no longer reliably represent current and future climate conditions. Engineers sized storm drains, culverts and flood barriers according to assumptions about rainfall intensity and frequency, but increasingly intense downpours can deliver far more water in a shorter window than those older designs can handle.
Covering natural land with asphalt and concrete prevents heavy rain from soaking into the ground, forcing massive volumes of water to rush across paved surfaces toward low points. Undersized drainpipes and bridge openings easily become clogged with mud, trees and trash, which act like a temporary dam until the water forces a sudden, destructive surge downstream.
Additionally, critical infrastructure such as electric grids and water networks are often interdependent, causing cascading failures during disasters. For example, if a power station floods, local pumps fail, leaving entire towns without clean drinking water or working sewage systems. In rapidly developing mountain or rural areas, roads and buildings are frequently developed close to riverbeds without extra reinforcement, making them prone to collapsing entirely when rivers overflow.
How can communities better prepare for flooding? What urban policies would make communities more resilient to these natural disasters?
Roshi: Adapting to the realities of climate change requires blending nature-based solutions with smarter land planning and reliable emergency warnings. Instead of relying solely on concrete walls, communities can restore wetlands, protect natural riverbanks and design public parks that safely absorb and store excess rainwater. These green spaces act as natural sponges, filtering water into the ground and slowing its flow before it reaches neighborhoods.
On the policy side, governments must update flood-risk maps to reflect changing rainfall patterns, expanding development and future climate conditions. Those maps should be used to restrict new construction in the highest-risk floodplains and require flood-resilient design elsewhere. For properties that experience repeated, severe flooding, government-funded buyout programs can help residents relocate to safer ground while converting the high-risk land into permanent open space. Communities also need resilient emergency alert systems — such as satellite-linked river monitors and automated mobile phone alerts — so people receive clear notifications to evacuate before rapid waters rise.
How might flash floods disproportionately impact underserved communities or developing countries?
Roshi: Flash floods disproportionately impact underserved communities and developing nations because human vulnerability, infrastructure design and recovery systems dictate how a physical hazard becomes a disaster. The same volume of water creates vastly different human outcomes depending on whether exposure is chosen or imposed, whether infrastructure mitigates or amplifies the flow and whether institutions restore or abandon affected populations.
First, wealth dictates whether living in a high-risk zone is a lifestyle choice or an involuntary burden. In affluent regions, some wealthy property owners choose to occupy hazardous areas — such as coastal beachfronts or scenic riverbanks — because they possess the resources to reinforce structures, afford insurance and evacuate safely. In the U.S., for instance, national flood insurance programs have historically cushioned these financial risks for oceanfront properties. Conversely, in developing nations and low-income urban centers, marginalized populations are often constrained to live in hazardous areas, such as steep slopes vulnerable to landslides or low-lying riverbanks, simply because affordable options do not exist elsewhere.
Second, infrastructure determines the extent of physical destruction. Underserved communities often experience chronic institutional neglect, receiving far less municipal investment in basic public works. Drainage systems in these neighborhoods are frequently undersized, poorly maintained or blocked by uncollected waste, turning routine rainfall into severe inundation. Furthermore, housing in marginalized areas suffers from weak code enforcement and limited protective barriers, leaving residents exceptionally vulnerable to moving water.
Finally, institutional design shapes whether the damage is temporary or long-lasting. While wealthier property owners rely on private savings, comprehensive insurance and political influence to demand rapid repairs, poorer communities face structural hurdles to recovery. Disaster assistance programs often rely on cost-benefit analyses, complex application processes and local matching-fund requirements that can favor wealthier jurisdictions and applicants with greater financial and administrative capacity. Without access to formal credit or equitable state aid, low-income families and poorer nations are frequently left with unaddressed health hazards, prolonged displacement and compounding economic loss long after the floodwaters recede.
Personal Preparation and Safety Measures for Flash Floods
What practical steps and safety measures can individuals take to stay safe before and during a flash flood?
Roshi: Preparing for flash floods requires understanding how quickly localized terrain responds to heavy weather and maintaining immediate situational awareness. Prior to traveling through or living in flood-prone regions, individuals should identify localized high ground and carry a battery-powered weather radio or satellite communication device, as standard mobile networks frequently fail during severe storms.
When physical signs of a flash flood appear — such as water suddenly turning muddy, a low roaring sound or rapidly rising levels — safety depends on precise, decisive action. Individuals should move immediately to nearby higher ground without entering or crossing floodwater, as a mere six inches of fast-moving water exerts enough dynamic force to knock an adult off their feet. Vehicles offer a false sense of protection: more than half of all flood-related drownings occur inside automobiles, and as little as 12 inches of rushing water can stall or carry away a small car. Drivers must never attempt to navigate water-covered roads or flooded dips, remaining mindful that the underlying pavement may have washed away entirely. Safe positioning on high ground must be maintained until authorities confirm the hazard has passed.
