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Yale Climate Connections
In an era of increasing heavy downpours, rising sea levels, and deterioration of flood protection infrastructure, it’s critical to know your property’s flood risk. In the U.S., every state has had multiple flood disasters in recent years, and we provide here a comprehensive look at the tools that can help you understand how flooding could affect you. In addition, consult part one of this multipart series, “How fast are the seas rising?” and part two, which reviewed eight great books on flood risks in some of the most endangered U.S. places: Miami, New York City, New Jersey, Charleston, Norfolk, Houston, New Orleans, Houston, and the Florida Keys.
Why not just look at FEMA flood maps?
The best tools for a quick overview of your property’s flood risk: Risk Factor, ClimateCheck
Tools for researching the flood history of your property: Redfin, realtor.com, Coastalresilience.org, NRDC.org, NOAA storm events database
Tool to determine your elevation: USGS National Map Viewer
Tools for researching river flooding risks: NOAA flood gages, USGS WaterWatch
Tool for researching levee failure risk: National Levee Database
Tool for researching dam failure risk: the National Inventory of Dams
Tools for researching future sea level rise from Climate Central, NOAA, NHC, EPA
Tools for exploring isolation from sea level rise
Tools for understanding the coming regime shift in sea level rise flooding from NASA
A word on Great Lakes flooding
Tools for estimating flood insurance costs
The back story: The traditional way to know your flood risk has been to consult the latest Federal Emergency Management Agency (FEMA) flood map through its Flood Map Service Center.
What it does: These maps show the expected 1-in-100-year and 1-in-500-year flood risk areas, which are regions that have a 1% per year and 0.2% per year chance of flooding, respectively. If you are in a FEMA 1-in-100-year flood risk area, you will be required to buy flood insurance from the National Flood Insurance Program in order to secure a federally backed mortgage. If you are within a FEMA 1-in-100-year flood risk area, you could assume there is at least a 26% chance of a 1-in-100-year flood occurring over the span of 30 years.
Is the data good? FEMA maps have significant problems. They are often out of date, available only as decades-old paper maps in some locations, and many local governments oppose efforts to update the maps for fear that it will depress property values and discourage development. FEMA maps do not factor in climate change and also do not account for the risk of “pluvial” floods. This is an increasingly common type of flooding in urban areas that have been extensively paved over when climate change-boosted extreme rainfall events dump huge amounts of water on regions that drain poorly.
Because FEMA is not using the most recent rainfall statistics, which show a sharply increasing risk of heavy precipitation events in the past 20 years, the number of contiguous U.S. properties at risk of 1-in-100-year flooding is a factor of 2.2 higher than FEMA flood maps show, according to a 2023 analysis of NOAA’s 795 continental U.S. rain gauges from the past 20 years done by the nonprofit First Street Foundation. The Washington Post published a searchable table in June 2023 that allows you to view the report’s change in 1-in-100-year flood events. For example, Houston’s Harris County is now estimated to experience what FEMA calls a 1-in-100-year flood event every 13 years (Fig. 1).
A 2020 study by the First Street Foundation, though based on an older version of the flood model, has detailed state-by-state pages of flood risk.
The back story: There’s a huge demand for information on climate change risks of all kinds, including flooding, and a new industry has sprung into being to provide these ratings. Though these new companies are providing valuable information, the industry has a bit of a Wild West feel. As an article by Lee Harris in the American Prospect warned, beware of “risk modelers who are selling precision in an inherently imprecise business, and formulating questions that sound technical but convey little meaningful information.”
So buyer beware! These tools are new and unproven, and rely on data that is not publicly available and use methods that are often “black boxes.” I was only able to perform a cursory check of these tools, and an in-depth evaluation could well reveal major flaws.
Risk Factor from the nonprofit First Street Foundation provides the most-used freely available tool for determining climate risk of an individual property. But as pointed out in Madison Condon’s excellent 2023 essay, Climate Services: The Business of Physical Risk, the nonprofit Flood Coalition cautions that “while First Street Foundation’s nationwide data is useful for ‘risk awareness,’ it is not a substitute for detailed property-level stormwater modeling, and should not be used in regulatory decision making. This same context and clarification is lacking on First Street’s own site.”
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Condon argues that “actionable and transparent information about our climate-changed future is a public good that the private sector cannot be depended upon to provide equitably or reliably” and points out that “angry homeowners are resorting to posting one-star Google reviews of First Street Foundation, complaining about faulty methods and lack of recourse to challenge their listing’s Flood Factor.” She calls for the establishment of a National Climate Service to provide a publicly available option to the private risk agencies — and to guide federal regulators, including financial regulators.
What it does: Risk Factor allows you to type in an address and see the specific flood risk for that property for the next 30 years (called the Flood Factor), on a scale of one to 10. The rating is determined using the First Street Foundation Flood Model, which considers flooding from high-intensity rainfall, overflowing rivers and streams, high tides, and coastal storm surge, using a moderate global warming scenario. A map of their predicted change in flood risk for the U.S. is shown in Fig. 2. The tool also evaluates wildfire, excessive heat, and wind risk for your property.
Who’s behind it: the nonprofit First Street Foundation, which launched riskfactor.com (also called floodfactor.com) in 2020. Real estate websites Redfin and realtor.com use riskfactor.com. Kerry Emanuel, one of the world’s top hurricane scientists, is on the organization’s board of advisers.
Cost: Risk Factor is free for noncommercial purposes. Risk Factor Pro is available for free for one noncommercial property; otherwise, it costs $100 per property. The pro version provides excellent historical flood data on an interactive map (see a sample screenshot for a property in Crawfordville, Florida, in Fig. 3).
ClimateCheck is a startup competitor to Risk Factor.
What it does: It gives a risk rating for flood, precipitation, heat, drought, and wildfire on a 1-100 scale relative to the risk for the rest of the contiguous U.S.
Cost: For a limited time, the site is allowing users to generate a “Premium Report” — a PDF file that is emailed to you that contains the flood risk in detail, with separate risk factors given for river/rainfall flooding — called fluvial/pluvial flooding — storm surge, and high-tide flooding. Maps showing the potential inundation of the property and surrounding area from each of these types of floods are included.
Who’s behind it: Daniel Swain, a climate scientist I often quote on extreme flood issues, is on the board of advisors for the company. This service is used by real estate website Redfin.com.
Is the data good? These numbers all seemed quite reasonable for the one coastal property I tried, except for the high tide flood numbers, which were misleadingly high. The report claimed that the property currently experienced 114 high tide flood days per year, which would increase to 120 days per year by 2050. In reality, the property currently does not experience any significant high tide flooding.
Another service is freehomerisk.com.
What it does: It provides an “A” through “F” letter grade for hazards related to river floods, storm surge, hurricane winds, thunderstorms, hail, tornadoes, lightning, wildfire, earthquakes, nuclear power plants, radon, crime, and the distance to the nearest fire station.
Cost: Free
A fourth option is the startup climatealpha.ai.
What it does: They have some interesting data not available on the other services for “Readiness” and “Vulnerability.” For example, there are some good stats on infrastructure, and on how much of the land is paved over, which impacts flooding.
Cost: Free for individual property owners
Is the data good? Their ratings for flood and heat risk were too low compared to riskfactor.com and climatecheck.com for the two properties I examined and did not appear realistic.
The back story: The past flood history of a property is crucial to understanding its future flood risk, but it’s hard to find out if a particular property has filed previous flood insurance claims with the National Flood Insurance Program because of privacy concerns. There is no federal law requiring a seller to disclose this information, and many states do not require it. FEMA has a detailed pdf file showing disclosure laws by state, though.
The tools: Real estate websites Redfin and realtor.com have links to First Street Foundation flood risk info for their listings, including whether a National Flood Insurance Program policy is required — and the potential cost of that policy. Redfin also includes storm risk from ClimateCheck.com.
Coastalresilience.org has a Repetitive Loss Property option on its interactive map for Florida, Alabama, Mississippi, Louisiana, and Texas that shows insured properties that filed two or more insurance claims of more than $1,000 within 10 years of each other with the National Flood Insurance Program. This data is only up-to-date through 2010, though. A sample for Wakulla County, Florida, is shown in Fig. 5.
Though not as detailed, NRDC.org has Repetitive Loss Property data by county through 2018 for the entire U.S.
NOAA has a Storm Events Database with data back to 1950 that you can search to find all extreme weather events that have impacted a county.
If you are a homeowner, your flood insurance certificate should tell you your elevation based on the best available USGS surveys for the area. If not, go to the U.S. Geological Survey website called the National Map Viewer, enter an address, or just zoom into the area of interest. This website has detailed instructions on how to use the USGS tool to find your elevation.
The back story: If you live in or near the flood plain of a river, it would be good to familiarize yourself with its flood history.
Start by looking at the nearest NOAA flood gage.
What it does: Each gage has a hydrograph showing current water levels and forecast for the coming few days. The page also has a “Historic Crests” section that shows the all-time high floods along with a “Recent Crests” section so you can see how recent floods compare to historic ones. A “Flood Impacts & Photos” section provides details on which area will flood for a given water level.
Other tools: NOAA also has a page with mapping of how high river floodwaters will get for a small set of gages in 31 states. The USGS WaterWatch website has a ton of flood-related information at their network of gages.
Caveat: Although extreme precipitation is increasing because of climate change and will continue to increase as the planet warms, that doesn’t necessarily mean that extreme river flooding will also increase. For example, climate change might decrease springtime river flooding in locations where flooding is dominated by snowmelt — if less snowfall happens to occur because of a warming climate.
An example of the complex pattern of recent flood trends in the U.S. was given by a 2023 paper, Controls on Flood Trends Across the United States (Fig. 6). A 2021 paper, Global Changes in 20-Year, 50-Year, and 100-Year River Floods, found that since the 1970s, the 20-year and 50-year extreme river floods globally have mostly increased in temperate zones but decreased in arid, tropical, polar, and cold zones. 
Floods are influenced by a variety of factors, including, very importantly, how wet the soils are. When droughts occur, climate change makes soils drier, and when heavy rains fall on dry soils, it usually takes a greater amount of rain to induce flooding (though very dry, drought-baked soils can be impervious to water, increasing runoff). A 2015 study in the journal Climatic Change found that very heavy precipitation — in the 99th percentile — in the contiguous U.S. resulted in 99th-percentile flooding only 36% of the time. The odds of 99th-percentile flooding increased to 62% when the soils were already moist, though. In addition, when reservoirs are low because of pervasive drought, the risk of flooding and dam failures is reduced, since reservoirs can store a lot of flood water. This is one reason flood damage was not higher in California in early 2023 despite a near-record amount of precipitation.
The back story: Urban levees in the U.S. are generally constructed to withstand a flood with a recurrence interval between 0.1% and 2% per year (between a 1-in-1000-year and 1-in-50-year flood).
Find out if the property you are interested in is protected by a levee using the U.S. Army Corps National Levee Database. After identifying a levee you are interested in, click on the “SYSTEM” tab, then look in the “Flooding” section to see the “Incipient Overtopping Annual Exceedance Probability (AEP)”. An AEP of .01 means a 1% chance of failure per year (or 26% chance in 30 years).
Is the data good? Not all levees will have a rating, and there are some conflicts in the database. For example, the AEP for the levees protecting downtown New Orleans claims a 1-in-1,00o-year protection level, but the overview text says the levees will only protect against a 1-in-100-year storm surge event.
Levees protecting rural areas typically have much lower failure thresholds, with some privately owned ones offering only 1-in-10-year protection. Multiple levees of this nature failed in the Stockton, California, region and on the Salinas River near Monterey during the January 2023 floods. Extreme floods are notorious for exposing the unforeseen flaws in levees, and you should not be overconfident that levees rated to a particular level of protection will perform as designed in a flood of a level never endured before. In an interview with the Los Angeles Times, Jeffrey Mount, a geomorphologist and senior fellow at the Public Policy Institute of California, said, “There are two kinds of levees: Those that have failed, and those that will fail.”
The back story: If you live downstream from a dam, it is a good idea to know the risk of that dam failing and the potential inundation that would occur.
Is the data good? Unfortunately, the conditions of most U.S. dams are not available or are kept secret from the public, listed as “not available” in the National Inventory of Dams because of terrorism concerns, and no potential inundation maps are made available.
However, the U.S. Army Corps of Engineers updated the database in 2021 to include hazard ratings, condition assessments, and inundation maps for more than one-quarter of the dams, according to a 2022 AP report. A sample inundation map that the Army Corps made available for the Whittier Narrows Dam in California is shown in Fig. 8, as described in more detail in our February 2023 post, If a megaflood strikes California, these dams might be at risk.
The back story: According to a 2022 report from NOAA, updated yearly, sea level along the U.S. coastline is projected to rise, on average, 10-12 inches (0.25-0.30 meters) in the next 30 years (2020- 2050), which will be as much as the rise measured over the last 100 years (1920-2020). The 2020-2050 rise is anticipated to be, on average: 10-14 inches (0.25-0.35 meters) for the East Coast; 14-18 inches (0.35-0.45 meters) for the Gulf Coast; four to eight inches (0.1-0.2 meters) for the West coast; eight to 10 inches (0.2-0.25 meters) for the Caribbean; six to eight inches (0.15-0.2 meters) for the Hawaiian Islands; and eight to 10 inches (0.2-0.25 meters) for northern Alaska. These estimates are based on an intermediate scenario for future heat-trapping greenhouse emissions and associated climate change, so they are not a worst-case projection.
There are several great tools for viewing coastal inundation from sea level rise.
Climate Central’s Surging Seas Risk Zone Map is top-notch.
What it does: In the U.S., a “stats” button at the upper right of the map allows you to download detailed fact sheets about the current and future flood risk for the nearest NOAA tide gage. These fact sheets can also be accessed via the Surging Seas Risk Finder page.
The NOAA Sea Level Rise Viewer has another great interactive tool. Also, NOAA’s Coastal Flood Exposure Mapper has an interactive map that allows you to view different levels of sea level rise, storm surge from various Saffir-Simpson categories of hurricane, FEMA flood zones, and regions susceptible to high tide flooding.
NOAA’s Coastal Inundation site also has a Coastal County Snapshots page that gives county-level stats on what different levels of sea level rise will do and a State of High Tide Flooding and Annual Outlook page that shows the expected yearly number of high tide flood days for 90 U.S. cities for five different climate change scenarios.
The National Hurricane Center hosts storm surge risk maps that allow you to view the flooding above ground level for hurricanes at the five Saffir-Simpson scales. Be sure to look at not just your property but also the escape routes you would take in an evacuation to see if those roads might flood before your property does. Hurricanestrong.org also has a link to find your hurricane evacuation zone for any coastal county.
EPA has a sea level rise interactive map that shows where inundation of toxic waste sites might occur.
Remember that if you live near the edge of an ocean cliff, you may not be safe from sea level rise. Higher oceans cause increased rates of erosion that will cause inland retreat of cliff faces, resulting in the loss of land into the sea. This is a complex process dependent upon the geology of the region, and there is no good general resource to provide guidance on what properties might be at risk. For example, a 2011 study on 11 California counties estimated that 4.6 feet of sea level rise would erode cliffs horizontally by an average distance of 108-197 feet. However, projected land losses varied significantly within each county.
The back story: Long before a region vulnerable to sea level rise becomes permanently inundated by rising waters, it will suffer a significant loss of value and be partially abandoned because flooding of adjacent roads or critical infrastructure (like power plants, water facilities, schools, and fire stations) will cause isolation.
When a property, or the infrastructure that supports it — a road, for example — floods too frequently, the market treats it as “effectively inundated,” leading to a drastic fall in value. Even properties that are on high ground will be devalued if they are served by a main road or infrastructure that floods.
A 2017 study defined the threshold for “chronic inundation” from sea level rise-induced flooding as occurring 26 times per year — on average, once every other week. Chronic inundation can disrupt people’s routines, livelihoods, homes, and communities if it occurs over 10% or more of its land area, excluding wetlands and areas protected by federal levees.
A 2023 paper in the journal Nature Climate Change and a companion website provide the risk of isolation and potential “effective economic inundation” for each county in the U.S. For example, for Lancaster County, Virginia, a two-foot rise in sea level will inundate 2.4% of the population — but isolate 5.5% of it (Fig. 9). For the Florida Keys, the disparity is stark: a two-foot rise in sea level will inundate about 22% of the population but isolate 44% of it.
A Union of Concerned Scientists website, When Rising Seas Hit Home, has state-by-state fact sheets for coastal states and a mapping tool that shows regions expected to experience chronic inundation of at least 26 times per year by 2030 and beyond (see example in Fig. 10).
The back story: Tides are caused by the relative positions of Earth, the moon, and the sun. The moon has a natural 18.6-year cycle in its orbit that brings lower and higher tides than usual depending upon where it is in this cycle. For example, in St. Petersburg, Florida, the tidal range over this 18.6-year cycle is 1.9 inches (4.7 cm); the range is as much as 7.7 inches (20 cm) in some coastal regions. From now until 2033, the moon’s 18.6-year cycle favors higher tides in places where two high and low tides per day occur (semidiurnal tides). This is the case for the Northeast U.S. coast.
But starting in 2033, the moon will be in a position favorable for bringing higher tides where one high tide and low tide per day dominates (diurnal tides or mixed tides); lower tides will occur where semidiurnal tides dominate. This shift will bring rapidly increasing high-tide floods to the coasts of the Gulf of Mexico, the Southeast U.S., the West Coast, and Hawaii. This acceleration in the mid-2030s is obvious for St. Petersburg (Fig. 11), plotted using NASA’s fantastic Flooding Analysis Tool and Flooding Days Projection Tool.
Over a 10-year period beginning in the 2030s, 71% of U.S. Pacific Island, California, and Gulf of Mexico locations will experience at least a tripling, and 59% at least a quadrupling, of minor high tide flood days per year. These days will tend to cluster within one season, rather than be spread out through the year, as seen in Fig. 11 for St. Petersburg, Florida.
Because high-tide floods involve a small amount of water compared to hurricane storm surges, there’s a tendency to view them as a less significant problem overall. But high tide flooding impairs stormwater and wastewater systems, infiltrates coastal groundwater aquifers with salt water, and stresses coastal wetlands and estuarine ecosystems.
“If it floods 10 or 15 times a month, a business can’t keep operating with its parking lot under water. People lose their jobs because they can’t get to work. Seeping cesspools become a public health issue,” said Phil Thompson of the University of Hawaii, lead author of the 2021 paper.
According to the excellent NOAA 2022 Sea Level Rise Technical Report (see the companion interactive mapping tool that shows sea level rise projections for each U.S. tide gauge location), only about 1.0-2.3 feet (0.3-0.7 m) in height difference separates minor/disruptive “nuisance” high tide flooding from infrequent, moderate/typically-damaging and major/often-destructive high tide flooding.
In 2013, water levels on all of the Great Lakes reached record or near-record lows. But by 2019, the tables had turned, and all of the lakes hit record or near-record highs, causing hundreds of millions in coastal damage. According to climate scientists Ricky Rood and Drew Gronewold of the University of Michigan, “rapid transitions between extreme high and low water levels in the Great Lakes represent the ‘new normal.’ Our view is based on interactions between global climate variability and the components of the regional hydrological cycle. Increasing precipitation, the threat of recurring periods of high evaporation, and a combination of both routine and unusual climate events — such as extreme cold air outbursts — are putting the region in uncharted territory.”
As of early June 2023, water levels on the Great Lakes ranged from five to 10 inches above normal. Lake Superior was 0.5 feet below its all-time record water level for June, but the rest of the lakes were about two feet lower than their June record. The relatively high (but not record-high) water levels of 2023 will cause increased coastal damage during large storms, but these impacts will be far below what was experienced during the record-high water levels of 2019-2020.
The back story: Before you buy a property, it is good to know how much flood insurance will cost through the National Flood Insurance Program, or NFIP. NFIP will insure residential properties for up to $250,000.
What is Risk Rating 2.0? NFIP rates will be changing in the coming years because of the new Risk Rating 2.0 reforms, implemented in 2022, which aim to help pull the program out of debt, which has risen to billions of dollars. The idea is that the debt arose because NFIP rates had long been set too low for the actual — and rising — flood risk across the nation, so rates are generally going up to ensure that future payouts will not exceed premiums over time.
Risk Rating 2.0 rates are based on the risks at the exact location of a property, using its likelihood of flooding, elevation, distance from flood sources, cost of rebuilding, flood-proofing of the building, and vulnerability of levees protecting the building. This rate-setting formula has drawn considerable criticism since it is partially based on proprietary data sets held by private companies, which prevents ratepayers from knowing the justification for their price changes. The new policy has led to an increase in flood insurance rates for about 77% of all policyholders.
Ratepayers in Louisiana will eventually see a 122% increase in premiums, according to Nola.com; in the Florida Keys, rates will at least triple in the long run; in Southeast Florida, rates will increase by over 200% for portions of Miami-Dade, Broward, and Palm Beach counties. Note that these are average costs; since Risk Rating 2.0 is building-specific, rates may vary substantially from the average.
How to see risk rating changes: In April 2023, FEMA released a spreadsheet detailing the new Risk Rating 2.0 changes. One can view the data by state (Exhibit2), ZIP code (Exhibit3), or county (Exhibit4). For example, for Florida ZIP code 33316, covering the portion of Fort Lauderdale between the airport and the ocean, the average yearly insurance cost under the old system was $1,519, and the new cost will be $4,198 — a 276% increase. But since rate hikes will be capped at 18% per year (25% for properties that have flooded repeatedly), it will take six years of price increases before the final new rate is reached.
An excellent May 2023 article in theconversation.com, by Antonio Arenas Amado and Lu Liu of Iowa State University, has a great interactive map that allows you to view current and projected flood insurance rates by county.
Bubble trouble: Climate change is creating a huge and growing U.S. real estate bubble
Book review: “The Great Displacement” is a must-read
‘Where should I move to be safe from climate change?’
A brief introduction to climate change and sea-level rise
Many coastal residents willing to relocate in the face of sea level rise
Bob Henson contributed to this post.
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Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a…
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