Tag Archive for: flood map accuracy

Flood Map Accuracy

On December 6, 2022, The Washington Post ran an article titled “America Underwater: Extreme floods expose the flaws in FEMA’s risk maps.” The lengthy story by Samuel Oakford, John Muyskens, Sarah Cahlan and Joyce Sohyun Lee cross-referenced photos and videos with FEMA flood maps from areas around the country that flooded last summer.

The basic premise: FEMA’s flood maps “are failing to warn Americans about flood risk.” The authors then claim, “The resulting picture leaves homeowners, prospective buyers, renters and cities in the dark about the potential dangers they face, which insurance they should buy and what kinds of development should be restricted.”

There’s certainly room for improvement in FEMA flood maps.

FEMA Map from National Flood Hazard Layer Viewer. Note how mapping stops at Montgomery County line, one of the issues cited in The Post article.

But is Climate Change the Reason for Inaccuracy?

However, the authors blame climate change for the inaccuracy far more than other contributing factors which are far more obvious.

FEMA is supposed to update flood maps every 5-10 years. It’s hard to imagine climate change invalidating them in that time period.

Climate is an average of weather occurring over much longer time periods. Depending on whether you talk to a meteorologist or a geologist, the time period could range from 30 to millions of years.

At least five major ice ages have occurred throughout Earth’s history: the earliest was over 2 billion years ago, and the most recent one began approximately 3 million years ago and continues today (yes, we live in an ice age!). Currently, we are in a warm interglacial that began about 11,000 years ago. The last period of glaciation, often called the “Ice Age,” peaked about 20,000 years ago. At that time, the world was on average probably about 10°F colder than today.

Interestingly, one day after The Post article, the New York Times ran a story about the DNA of animals found frozen in the permafrost of northern Greenland, just a few hundred miles from the North Pole. The 135 different species scientists found there paint a picture of an arctic once lush with life typical of warmer climates today.

But another thing puzzles me. I see climate change often mentioned as the reason for drought. The US Geological Survey states, “Climate change has further altered the natural pattern of droughts, making them more frequent, longer, and more severe.” But The Post uses almost identical language to blame climate change for frequent flooding in many of the same general areas at the same time. Which is it?

And to what degree can climate change explain flood map inaccuracy? Many more obvious reasons exist that are less of a stretch for any inaccuracies.

Reasons Listed in Post Article for Inaccuracy of Maps

Here’s a list of the references in The Washington Post story used to explain inaccuracies found within FEMA maps. I’ve broken them into two groups so you can see the weight they gave to climate change.

Climate-Change References:
  1. “As climate change accelerates, it is increasing types of flooding that the maps aren’t built to include.”
  2. “Extreme precipitation events are growing increasingly common.”
  3. “A warming climate allows storms to carry more moisture, producing greater rain or snow in a short period of time.”
  4. “Climate has changed so much that the maps aren’t going to keep up.”
  5. Maps are out of date, some decades-old “in a changing climate.”
  6. “The effects of a changing climate.”
  7. Climate change impacts are getting worse.
  8. Climate change is “pushing FEMA’s maps beyond their limits.”
  9.  A gap exists between the data that goes into FEMA maps and current climate conditions.
  10. Climate change baseline is changing.
  11. “Climate change velocities are high.”
  12. “Maps do not take climate change into account.”
  13. “Overestimating the rarity of some events even before climate change…”
Other Possible Explanations Mentioned by The Post:
  1. “Communities may resist expanding designated flood zones because it adds costs and can hamper development.”
  2. Not all areas that flooded are mapped yet.
  3.  “Local communities often resist the expansion of federal flood zones”
  4. “Maps do not forecast flooding. Maps only reflect past flooding…”
  5. “Local governments have been opposed to any maps that show an increasing risk.”
  6. Relatively high imperviousness of gentrifying areas.
  7. Maps don’t reflect intense bursts of rainfall in a short period and the resulting street flooding.
  8. Impervious surface is replacing porous surface.
  9. Maps cover mainly coastal and riverine flooding.
  10. “Rain combining with melted snowpack.”
  11. FEMA flood maps don’t even attempt to model urban flooding
  12. “City neglected drainage problems.”
  13. Local opposition to expanding the floodplain.
  14. No sense of urgency to update maps.
  15. “Multiple compounding factors contribute to the flooding”

However, the article makes no mention of the mathematical limitations of Extreme Value Analysis, the key to understanding the uncertainties associated with rainfall probabilities.

Floods Can Also Be Explained Without Climate Change

The second group of references in The Post article seems far more immediate, compelling and easily provable when explaining any inaccuracy found in flood maps. They’re certainly typical of what I have found in the Houston area.

For the past five years I have been researching instances of flooding in and around Harris County. I published more than 250 articles on different aspects of the 2019 Elm Grove floods alone. And I don’t recall one person ever blaming those on climate change.

Elm Grove did not flood during Harvey, but did flood on two much smaller rains in 2019. The difference? Clearcutting and insufficiently mitigated upstream development. Contractors clearcut approximately 270 acres immediately north of Elm Grove without building sufficient detention capacity before the rains fell.

Similar stories – with variations – have played out over and over again throughout the Houston region. For instance, we see developers filling in wetlands. Exaggerating the infiltration rates of soils. Underestimating impermeable cover. Building in floodplains. Building to outdated codes and floodplain regulations. Being grandfathered under old regulations. Various jurisdictions refusing to update regulations. And more.

Regardless of your position on climate change, this discussion dramatizes the needs to:

  • Understand your local flood risk and the factors that affect it
  • Buy flood insurance.

Hopefully, Harris County Flood Control District’s MAAPnext project will address data deficiencies discussed in The Post article. But it will be years before those maps become official. And when they do, the landscape will have already changed.

Posted by Bob Rehak on 12/12/22

1931 Days since Hurricane Harvey

The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.