Do You Know Your Home’s True Flood Risk?
11/18/24 – Do you know your home’s true flood risk? In October, National Geographic ran an insightful article by Meryl Davids Landau titled “Many Americans are buying homes in flood zones – and don’t realize it.”
The article begins, “The Flood Insurance Rate Maps (FIRMs) used by FEMA are based on antiquated data and obsolete models.”
Flood Maps Fail to Predict True Flood Risk
Landau then examines a number of storms and locations where FEMA rate maps failed to predict flood damage. She says, “The FIRM maps have extensive problems” from the data they include and exclude “to the limited assumptions around how the maps would be used.”
She quotes Susan Crawford, a senior fellow for sustainability and climate at the Carnegie Endowment for International Peace, as saying “millions of homes should be labeled as high flood risks but aren’t.”
Beyond Climate Change
Landau’s analysis of flood risk goes far beyond climate change – the usual bogeyman.
She tracks the history of FIRMs back to the 1960s and their original objective: to encourage home ownership. Landau examines the data that FIRMs are based on and their built-in sampling bias. She says the location of flood gages near major rivers was expedient at the time. However, basing flood maps on those locations ignores flooding from other sources. Those sources include:
- Smaller waterways such as streams, creeks and tributaries
- New residential and commercial development
- Growth of impervious surfaces
- Insufficient drainage infrastructure and mitigation
I touched on all these sources and more in a post titled “Why Do We Flood?”
Landau also touches on political pushback. She told a poignant story about a community that fought new flood risk maps because political leaders feared it would reduce the tax base. Because of this and other complexities that many people remain unaware of, they buy homes without understanding their true flood risk.
Sources Indicate 3X More Homes at Risk than FEMA Shows
Landau quotes Sam Brody, a professor at Texas A&M, who is developing a different approach to modeling with the Texas Division of Emergency Management.
Rather than focus on historical flooding, Brody incorporates these other factors in his models.
High flood risk areas flagged by the Texas system are termed “damage plains rather than flood plains, and they extend for many miles beyond FEMA’s hazard zones,” says Landau. “In fact, the new model determined that three times more structures in the Texas Gulf Coast are actually at high risk of getting inundated.”
So what’s a prospective home buyer to do? Check a variety of sources, starting with FEMA. If you’re going to bet your life savings on a new home, check a variety of sources:
- Start with the historical standard, FEMA National Flood Hazard Layer Viewer
- Next check First Street Foundation’s “Risk Factor website”. Warning: It’s a pay site with free trial.
- Zillow began integrating First Street’s data into listings last month.
- Visit the Institute for a Disaster Resilient Texas “Buyers Aware” website, a cooperative venture of Texas A&M and the Texas Division of Emergency Management. It covers 41 counties in SE Texas including all those in the Houston area.
Flood Risk Reports on My House Varied Radically
When I checked my address, I found radically different estimates of my flood risk.
- FEMA showed me far outside of the 500-year floodplain
- First Street showed me outside of the danger zone, but not quite as far away. It said I had “mild” flood risk.
- Buyers Aware rated my flood risk as “moderate high.”
Buyers Aware said, “…based on the flood risk variables outlined below, our analysis ranks this site as having a moderate-high flood risk. This is near the top of the highest risk categories included in our model. We strongly encourage you to purchase flood insurance and explore other actions to mitigate your flood risk.”
While this property may not be in the FEMA regulatory floodplain (sometimes referred to as the “100-year floodplain”) our analysis indicates that flood risks may be higher than what is currently measured by the regulatory maps.”
In the National Geographic article, Landau says, “Since the 1990s, over 50% of flood loss in Texas has occurred in areas outside of SFHAs [FEMA’s Special Flood-Hazard Areas].”
Buyers Aware factors in:
- Land elevation
- Distance to coastline
- Distance to streams
- Imperviousness
- Soil Characteristrics
- Height above nearest drainage
Buyers Aware also showed me a map of the neighborhood and told me that within the area shown, “$22,813,736 of flood insurance claims have been paid in the last 10 years.”
That total reflected only properties with active NFIP policies in place. I know people nearby who had five- and six-figure damage during Harvey. But they didn’t have flood insurance policies because they thought they were beyond the 500-year flood zone.
Other Interesting Data that “Buyers Aware” Includes
Buyers Aware also told me that FEMA has 3890 active NFIP policies in my area.
Next they informed me about a 7% increase in impervious surface upstream from me.
Buyers Aware also recommended I buy flood insurance and told me where I could find more information about it. They also listed several flood mitigation strategies such as:
- Improving drainage
- Retrofitting
- Flood barriers
- Property elevation
For the Record
For the record, I live 2.1 miles from the San Jacinto West Fork. During Harvey, floodwaters stopped at our driveway. And that’s the main reason I write about flooding in my retirement.
Also for the record, FEMA has set a goal to update all of its FIRMS every ten years and to include climate change in the analysis.
Finally, Harris County Flood Control District’s MAAPnext effort (not yet fully vetted by FEMA) addresses many of the issues addressed by National Geographic and Landau.
But local pushback will always be a problem everywhere. Montgomery County announced an ambitious effort to update its drainage criteria manual after Harvey, but has yet to officially adopt the changes it considered.
Posted by Bob Rehak on 11/18/24
2638 Days since Hurricane Harvey
