Title Image Credit: NASA Earth Observatory/Lauren Dauphin
Intense rain caused flooding from Florida to Maryland
$12.3 billion—that’s the estimated total U.S. flood damage from Hurricane Debby as determined by First Street, specialists in physical climate risk data and forecasting. But how can a relatively minor hurricane cause so much damage? How do we determine flood risk in the first place? Is flooding happening more often? Let’s start with Debby…
Debby’s Overture
Hurricane Debby started as a tropical depression in the Atlantic, quickly intensifying into a tropical storm. Entering the very warm Gulf of Mexico, the storm strengthened into a Category 1 hurricane on August 4 as it approached the Florida coast.
The following cautionary note was included in our August 3, 2024 forecast for Hurricane Debby, posted by the Climate Adaptation Center’s CEO, Bob Bunting:
“Be prepared for freshwater flooding from the excessive rains.”
Sure enough, intense rainfall from Debby caused extensive flooding along the west coast of Florida. Sarasota and Manatee counties were particularly hard-hit, with total rainfall amounts of 17.78 inches in Sarasota and 18.34 inches at Lakewood Ranch.
Debby Heads North
After its journey up the Gulf coast, Debby made landfall in Florida’s Big Bend region on August 5, delivering sustained winds of 80 mph, heavy rainfall, storm surge and coastal flooding. Slowing down and weakening to a Tropical Storm as it crossed northern Florida, inland areas experienced torrential rain and flash flooding. Debby eventually reached the Atlantic Ocean off the coast of Georgia, only to come ashore a second time August 8 as a tropical storm, northeast of Charleston, South Carolina. Gradually diminishing to a post-tropical cyclone, the slow-moving storm tracked inland and northeastward, delivering heavy rain, flooding and spawning tornadoes until finally dissipating over Canada’s Maritime provinces August 10.
The figure below shows an overview of the area covered and the amount of rainfall delivered by Debby as the storm trundled up the east coast, from Florida to Maine.
National Weather Prediction Center summary map for Hurricane Debby. Note the highest precipitation estimates (15+ inches) are along the coast of South Carolina and on the Florida coast at Sarasota
Tornadoes accompanied the storm as it moved up the eastern seaboard, with tornado warnings issued by the National Weather Service from Florida as far North as New York state. The map below shows one example—NWS tornado warnings issued as Debby traveled past North Carolina.
NWS North Carolina tornado warnings for August 6 – 8 Credit: Vernon Turner/@WeathermanVern
Widespread Damage
$12.3 billion. That’s the estimated total flood damage caused by Hurricane Debby, according to a detailed analysis by First Street. Here’s the breakdown:
- 385,000 properties affected
- 160,000 (42%) of the affected properties estimated to sustain flood damage
- 124,000 (78%) of the damaged properties are located outside the Federal Emergency Management Agency (FEMA) “hundred year flood zone,” where flood insurance is required by law
- $9.7 billion of the total flood damage occurred to properties outside the FEMA flood zone and therefore unlikely to be insured.
Florida was particularly hard-hit. First Street estimated that flood damage in Florida totaled $2.6 to $4.5 billion. About 70% of the damage, in the range of $1.9 -$4.5 billion, occurred outside of the area designated as the 100 year FEMA flood zone. Because those properties are outside the FEMA-designated risk zone, most of that loss is likely to be uninsured.
Hurricane Debby’s heavy rainfall was the culprit in Florida, with rainfall exceeding the rate of a 1 in 200 year storm. (A 1 in 200 event has a 0.5% likelihood of happening in a given year.) In Florida, the highest rainfall rates were recorded in the Sarasota area (1 in 200), and northeast of Gainesville (a 1 in 240 year event).
Using USGS stream flow data, NOAA precipitation data, and 1 meter-resolution topographic data, First Street’s Flood Model (FSFM) is able to quantify flood risk and economic impact at the level of individual properties. Preliminary review of their analysis of Hurricane Debby confirms that flooded areas and flood depths predicted by the FSFM closely align with Media and property owner reports during and after the storm. The FSFM flood damage estimates are based on property-specific building characteristics and engineering-based vulnerability analysis.
FEMA and Flood Risk
FEMA is responsible for identifying areas within the U.S. that are at risk of flooding and for managing the National Flood Insurance Program (NFIP) to help property owners and businesses recover from damaging flood events.
The fundamental resources for determining flood risk are the FEMA Flood Insurance Rate Maps, or FIRMs. FEMA’s maps identify both a “500 year flood plain” (experiences 1 flood event in 500 years) and the more important “100 year flood plain” (1 event in 100 years) The high risk 100 year flood plain on a FEMA map is referred to as the Special Flood Hazard Area (SFHA). A typical FEMA flood risk map is shown below.
FEMA Flood Insurance Rate Map for Corpus Christi, Texas
Within the SFHA, property owners with federally backed mortgages are required to buy flood insurance. This is a good idea, because the 1 in 100 years (i.e., 1% in any given year) likelihood of flooding in the SFHA means there is at least a 1 in 4 chance of flooding during a 30 year mortgage. Properties in the SFHA are also subject to special building codes and NFIP floodplain management regulations.
As you might expect, most property owners who are not located in an SFHA assume because they aren’t required to purchase flood insurance, it’s safe to go without. Hurricane Debby and other recent storms suggest that’s not a good idea. As First Street pointed out, 78% of the properties damaged by Debby lie outside the hundred year flood zone.
FEMA Flood Maps Have Problems
To begin with, FEMA’s flood risk maps are often out of date, sometimes by decades, even though FEMA is required by law to update the maps every five years. Lack of adequate funding is a perennial problem.
From a national perspective, the maps are incomplete, and some areas are not mapped at all. As of 2022, FEMA had only developed flood maps for 61% of the continental U.S., leaving thousands of communities unmapped.
In the same vein, it’s estimated that only a third of the nation’s streams, rivers and coastlines have been mapped by FEMA. Mapping projects are often executed years apart in adjacent states or counties, leading to discontinuities at state/county boundaries where the environment has changed in the intervening years.
The current flood risk maps are binary—on one street you’re in the 100 year zone, on the next street you’re not. In reality, there isn’t an abrupt change in risk at a defined boundary of the 1 in 100 year zone (or the 1 in 500 year zone), but a gradual transition from higher risk to lower risk. FEMA plans to move to a more graduated risk presentation in future.
Perhaps the biggest flaw in the maps is that they only capture river and coastal flooding. As Debby showed, flooding can also be caused by an outburst of intense rainfall, called “pluvial” flooding. (River flooding is referred to as fluvial flooding.) This is a particular problem in cities, where impervious surfaces combine with heavy rainfall to overload the city’s stormwater drainage system.
Rainfall-driven flooding contributes to a significant increase in risk for inland communities, a considerable number of which are in currently unmapped regions even though they experience flooding. When fully accounting for fluvial and pluvial flood risks, First Street estimates that approximately 17.7 million properties across the country are at 1-in-100-year risk level or greater— more than twice the number included in the FEMA 1 in 100 risk zones.
This brings us to…
The Impact of Climate Change
Perhaps the biggest problem with the current flood risk maps is that they don’t account for climate change, because they are based on historic data, not current climate reality and predicted changes.
Predicting inland flood risk fundamentally requires an ability to predict precipitation intensity, duration and frequency. The NOAA Office of Water Prediction (OWP) maintains “Atlas 14,” the latest edition of “The Precipitation Frequency Atlas of the United States.” NOAA Atlas 14 estimates are used to “design, plan, and manage civil engineering and transportation infrastructure.” However, those estimates are based on historic rainfall data, in some cases going back 200 years.
Meanwhile, climate change is rendering those estimates (and FEMA flood risk maps) obsolete and inaccurate.
Heavy rainfall events have become more frequent and severe in the U.S. as a result of climate warming. As we’ve discussed previously, as air temperature increases, more water vapor can be held by the atmosphere to be released during rainfall events. For every 1°C increase, 7% more water vapor can be carried by the same volume of air. As temperatures have increased, we are seeing changes in the intensity, duration and frequency of rainfall events.
A 2023 study of “The Precipitation Problem” shows the extent to which climate change has increased flood risks in the continental U.S.:
- 51% of the population live in areas where a 1 in 100 year flood (as expected from Atlas 14 data) is now a 1 in 50 year flood (a 2% chance of occurring in any given year)
- 21% live where their 1 in 100 year flood can be expected to happen every 25 years
- 20 counties, home to over 1.3 million people, are expected to experience their currently predicted 1 in 100 year flood at least once every 8 to 10 years
Unfortunately, some of the biggest changes in flood risk resulting from climate change are occurring in some of the most heavily populated areas of the U.S., as shown in the figure below. The map shows how climate change has affected the likelihood of Atlas 14-based 1 in 100 year rainfall events in different parts of the country.
Credit: First Street Foundation
First Street’s analysis shows that in Baltimore a 1 in 100 year storm based on Atlas 14 is now likely to occur every 14 years. By 2053, that same storm will occur every 12 years. People living in Dallas or Washington will find that their “once in a century” storm is now a once in 21 years storm.
Remnants of Hurricane Debby caused flooding in Baltimore Credit: City of Baltimore, Office of the Mayor
NOAA is well aware of the shortcomings of Atlas 14. In 2022, the Bipartisan Infrastructure Law included $492 million for NOAA’s OWP to overhaul Atlas 14, accounting for climate change. The revised precipitation database will constitute Atlas 15.
Unfortunately, Atlas 15 will not be complete until 2027. Meanwhile, new infrastructure projects built today using Atlas 14 data “are instantly decades out of date” according to First Street.
In the Meantime…
While we’re waiting for better flood risk data from the government, First Street has made its model publicly available at riskfactor.com, where anyone can type in an address and receive user-friendly risk information for any property in the U.S. Redfin and Realtor.com have also incorporated the data.