What Causes Floods? A Look at Natural and Human-Made Triggers

Many assume floods strike only during hurricanes or near rivers, but today’s reality is more complicated and far more widespread. Floods now affect inland cities, suburban neighborhoods, and even desert communities with increasing frequency. Understanding the root causes of floods, both natural and man-made, is essential for assessing risk, planning ahead, and deciding whether flood insurance is a smart investment.

What Is a Flood?

FEMA defines a flood as “an excess of water on land that is normally dry.” This can result from a range of conditions, from overflowing rivers to clogged drainage systems. Knowing this definition matters, because many insurance claims are denied when flooding doesn’t match common assumptions. 

What’s important to understand is that flooding doesn’t always come with a storm warning and it doesn’t require proximity to a body of water.

Natural Causes of Flooding

Nature is often the first trigger and the most recognized. Common natural causes include:

• Heavy rainfall: When precipitation exceeds the ground’s ability to absorb it, water collects and overflows.
• River flooding: Snowmelt or prolonged rain can cause rivers to exceed their banks.
• Storm surges and coastal flooding: Strong winds from hurricanes and tropical storms can push seawater far inland.
• Flash floods: Sudden, intense rainfall can trigger flooding in minutes, especially in dry or mountainous regions.
• Atmospheric rivers: Narrow bands of moisture in the atmosphere can dump inches of rain in concentrated areas over short periods.
• Tsunamis and seismic activity: Less common, but often catastrophic, floods triggered by undersea earthquakes or landslides.

“Flood events are often recorded where there are people to observe them,” explains Melanie Veltman, Senior Data Manager at Guidewire. “If a stream overflows in a remote area, it likely won’t be captured in the data.”

While these natural causes vary in form and severity, what they share is unpredictability and growing frequency due to climate shifts. Events like Hurricane Harvey inundated inland regions hundreds of miles from the coast, exposing flood risk where few expected it. This data gap can obscure the full scale of risk, especially in evolving climates.

Human-Made Triggers and Urbanization

While natural conditions may start the process, human infrastructure often amplifies the damage. Increasingly, urban development is placing people and property directly in harm’s way.

Key human-made contributors include:

• Impervious surfaces: Pavement, buildings, and roads prevent rain from soaking into the ground, leading to fast-moving runoff.
• Aging levees and dams: Legacy drainage systems built for older rainfall patterns are now routinely overwhelmed.
• Deforestation and land use changes: Removing vegetation reduces the land’s ability to slow and absorb water.
• Undersized drainage systems: Older stormwater infrastructure often cannot accommodate today’s precipitation volumes.

“All over the news, there are more and more stories of catastrophic urban flooding,” says Veltman. “The engineered landscape doesn’t absorb water as quickly when there’s heavy rainfall.”

As Tammy Schwartz, Senior Director of Analytics at Guidewire, explains: “A lot of communities don’t realize their flood infrastructure is outdated. It’s why you see repeat losses in the same areas.”

How Climate Change Compounds Risk

Warmer air holds more moisture, which means storms release more water in shorter windows, overwhelming both land and infrastructure. Climate change is intensifying many natural flood triggers. Meanwhile, sea level rise has expanded coastal flood zones, and more frequent wildfires have damaged soil, reducing its ability to absorb water.

In 2023, the U.S. experienced four separate billion dollar flood events, contributing to a record-breaking total of 28 weather and climate disasters that each caused over $1 billion in damage. This volatility also complicates underwriting models built on outdated recurrence intervals. Even the “100-year floodplain,” long a cornerstone of FEMA’s flood risk modeling, may no longer represent realistic recurrence intervals. What once was expected once per century may now occur far more frequently.

Why Location Is No Longer a Guarantee of Safety

Flooding isn’t just a coastal or riverfront concern. Urban areas with poor drainage or steep terrain are increasingly vulnerable to flash floods.

In July 2023, Vermont experienced historic, rainfall-driven flooding on July 9–10, causing widespread devastation—without a coastal hurricane involved. Communities that once believed they were safe now find themselves filing disaster claims for the first time.

Stormwater runoff, rain and snowmelt that flows over impervious surfaces like streets and rooftops, is a growing problem. Without adequate zoning updates and infrastructure investments, neighborhoods that once felt safe may now be exposed.

“Flood risk is evolving, but many policies and perceptions haven’t caught up,” Schwartz notes. “Communities that weren’t built for today’s weather are finding themselves underwater.”

Mapping Risk: From FEMA to HazardHub

Homeowners often rely on FEMA’s Flood Insurance Rate Maps (FIRMs) to understand their flood zone. These maps are a useful starting point and determine flood insurance requirements for properties in Special Flood Hazard Areas (SFHAs) like Zones AE or VE.

But FIRMs have limits. They often don’t account for smaller-scale variations or the latest precipitation trends.

Tools like the HazardHub Flood Score integrate multiple data sources, including topography, drainage patterns, elevation, and localized storm history to deliver parcel-level flood risk analysis.

How to Access and Use Flood Zone Maps for Your Property

HazardHub Flood Score

What is Base Flood Elevation (BFE)?

Base Flood Elevation (BFE) is a key metric used in floodplain management and insurance underwriting. FEMA uses BFE to help determine where Special Flood Hazard Areas (SFHAs) begin. If your home’s lowest floor sits below the BFE, you are at a significantly higher risk of flood damage, and your flood insurance premiums may reflect that elevated risk. Conversely, elevating your home above the BFE can reduce premiums and, more importantly, reduce the likelihood of catastrophic loss.

Beyond insurance implications, BFE serves as a guideline for construction, zoning, and community resilience planning. Local governments may require new developments to be built at or above the BFE to reduce future flood exposure. FEMA's Elevation Certificate helps document how a property’s elevation compares to the BFE and is often required when applying for or adjusting a flood insurance policy.

Ask your agent or local building department for your property’s BFE and how your elevation compares.
 

The Role of Base Flood Elevation (BFE): Real Life Examples

After Hurricane Sandy in 2012, many homeowners in New Jersey and New York were required to elevate their homes to meet updated BFE requirements if they wanted to rebuild and maintain insurance coverage.

In towns like Toms River, NJ, some homes were raised 3 to 6 feet above the original grade. Those who elevated saw a reduction in annual flood premiums by thousands of dollars, while those who didn't faced steep premium hikes or became ineligible for affordable coverage.

These changes were about more than just safety. They also determined whether homeowners qualified for continued subsidized NFIP rates and local rebuilding permits.

Why Insurance Uptake Remains Low—And Risk Remains High

Despite growing exposure, many homeowners still don’t carry flood insurance. FEMA reports that only about 30% of residential homes in Special Flood Hazard Areas (SFHAs) carry an active NFIP policy, meaning the majority in high-risk flood zones remain uninsured. Many homeowners assume disaster aid will be enough, but in most cases, federal help covers only a fraction of the loss.

This low uptake leaves many vulnerable to financial ruin. FEMA estimates that just one inch of water in a home can cause over $25,000 in damage. Without insurance, recovery is often slow, partial, and dependent on limited federal aid.

Don’t Wait for the Water to Rise

Floods aren’t always dramatic, but they are consistently destructive. Whether triggered by nature or shaped by human development, flood risk is increasing across the U.S.

Next steps:

• Look up your FEMA flood zone classification
• Get a parcel-level flood score using the Guidewire HazardHub Quiz
• Talk to your insurer about potential gaps in your coverage
• Take note of mitigation opportunities around your home or property

Understanding the causes of flooding is the first step toward preparation. Take time to explore your property’s risk using both FEMA maps and real-time tools like the HazardHub Flood Score. And if you haven’t already, consider whether flood insurance could be the difference between rebuilding quickly or not at all.