(TNS) - The flood maps that help determine where homes are built in Texas, how much insurance costs and which areas would benefit from flood control projects are based on rainfall data that hasn't been updated for as long as a half-century, meaning that development has expanded here for decades without a complete understanding of the flood risks.
Those risks have come into sharp focus following the record-setting rainfall of Hurricane Harvey, which inundated areas never touched by floodwaters before, and growing expectations among climate scientists that powerful storms will not only happen more frequently, but also pack more rain as global and ocean temperatures rise. Of the 39 Texas counties that experienced flooding during Hurricane Harvey, fewer than 10, including Harris County, have flood maps based on precipitation data from this century.
And even that data, from 2001, is more than 15 years old and doesn't capture severe storms that included Hurricane Ike in 2008, the Memorial Day floods in 2015 and the Tax Day floods of 2016.
"It's important that we do the research into how much rainfall could we get, how the watersheds are going to respond to that rainfall event, how deep, who will it impact," said Jerry Cotter, chief of water resources at Army Corps of Engineers. "We've got to do the preparedness work."
Rainfall data is one of the key components used in generating flood maps, a complex, multi-agency process that considers topography, past flooding, development and other factors. The Federal Emergency Management Agency drafts preliminary maps, which are reviewed by local governments, subjected to public comment and often disputed in contentious proceedings that can drag on for years before the final maps are adopted.
The stakes are high. Where the maps designate flood prone areas can determine the pattern of development, the value of homes, the locations of roads and highways and the designs of levees that hold back swollen rivers. In Fort Bend County, where hundreds of residents were evacuated during Hurricane Harvey over concerns that the rising Brazos River would breach levees, flood maps are still based on rainfall data from 1961.
Texas is just one of six states that has yet to update rainfall data under a program of the National Oceanic and Atmospheric Administration, which encompasses the National Weather Service. NOAA, which launched the program in 2004, requires state and local governments to pay the costs of data collection and analysis, which identifies weather patterns and determines the probabilities of heavy rains and catastrophic storms that cause flooding.
It would cost Texas about $1.5 million to build the comprehensive, up-to-date database of rainfall statistics and probabilities. So, far about $1.4 million has been raised to fund the program, including $400,000 from the Army Corps of Engineers, which launched the effort to fund the project four years ago, and $200,000 from the Harris County Flood Control District.
NOAA is expected to release preliminary rainfall data for Texas later this fall. There is no question the updated data would change Texas' and Houston's understanding of flooding, scientists and local officials say. It's just a question of by how much and where.
New data, for instance, might shrink floodplains in some areas, but expand them in others, placing residents into zones with higher flood risks that trigger federal requirements to buy flood insurance.
"It's one of the more consequential aspects of reanalyzing the data," said John Nielsen-Gammon, the state's climatologist, "because it can mean a sudden decline in property values for people who had no risk of flooding before."
Nielsen-Gammon recently warned that the state should expect the frequency of extreme rains to increase in coming years as a result of climate change, a trend that could test the safety of many Texas dams. Nearly all scientists agree that the increasing concentration of carbon dioxide in the atmosphere — much of it from burning fossil fuels — is raising global temperatures and warming the oceans, causing more water to evaporate and creating a buildup of moisture.
Hurricane Harvey, for example, carried as much as 7 percent more rain than it would have a century ago, Nielsen-Gammon estimated.
Scientists, however, disagree on whether so-called outlier events, whether record-shattering rains or extended droughts, should play into the analysis of precipitation records. Regardless, rainfall data should be refreshed for every 10 to 20 years, but often goes decades without an update, said Cotter of the Army Corps of Engineers.
Even in the Texas counties that updated rainfall data in 2001 -- including Harris, Galveston, Brazoria and Liberty — flood maps were drawn and development proceeded through the 1980s, 1990s and into the new century based on rainfall data primarily from the 1960s.
It took Tropical Storm Allison in 2001 to push Harris and the other 9 counties to update rainfall data. Allison dumped as much as 30 inches of rain over five days, flooded 73,000 homes and caused $5 billion in damage.
Harvey, which flooded more than 100,000 homes and caused, by some estimates, more than $100 billion in damage, could have the same result. But once the state gets the new rainfall data, updated flood maps could still be years away as they go through the lengthy approval process and face pressure from developers and residents who want to keep the costs of building and living in the floodplain low.
In addition, there is no requirement that local governments use the updated data to redraw flood maps, which in turn could affect property values and future development. Ataul Hannan, director of planning for the Harris County Flood Control District, said the district will weigh the impact of the updated rainfall data against these factors as it considers whether to update flood maps.
More than 90,000 residential structures in Harris, Galveston and Fort Bend counties could have been damaged by floods from the storm as of Aug. 27, according to flood models released by the Federal Emergency Management Agency. The map below features waterways, in dark blue; 100-year floodplains, in light blue; 500-year floodplains, in blue-green; destroyed structures, in dark red; structures with major damage, in orange; affected structures, in yellow; and structures with minimal damage, in gray.
Note: FEMA's methodology does not account for flooding caused by storm water backups, irrigation ditch failures, flooding from dam or levee breaks or wind damage. FEMA data obtained by Ryan Maye Handy | Map created by data journalist Rachael Gleason
He noted that the five years of drought that followed Hurricane Allison could have the effect of shrinking floodplains. On the other hand, he said, the extreme storms of the past three years might expand them and put some Harris County residents into a floodplain, requiring them to buy flood insurance.
"We have to make a call, should we do the change now, or should we wait?" Hannan said. "How do you think every five years I'm going to change the map and tell people, 'Hey, because of my new information you are now in the flood plain?'"
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