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A study by the National Oceanic and Atmospheric Administration has redefined experts’ understanding of how often Texas gets pummeled by heavy storms.
NOAA in September released the results of their analysis that drew upon data from weather stations across Texas. The study, known as Atlas 14, Volume 11, added decades of additional rainfall data.
The new values will have implications for engineers and local governments as cities adopt stricter building codes. It could also mean that more homeowners will be required to buy flood insurance through the National Flood Insurance Program.
“This is a game changer for us,” said City Transportation and Capital Improvements Assistant Director Nefi Garza, who leads a team of stormwater engineers and operations managers. “It changes everything we do.”
The updated study took NOAA analysts three years, said Mark Glaudemans, director of the Geo-Intelligence Division of NOAA’s Office of Water Prediction. It replaces earlier NOAA studies for Texas from the 1960s and 1970s.
Funding for the effort came from the U.S. Army Corps of Engineers-Fort Worth District, the Texas Department of Transportation, the Bravos River Authority, Harris County Flood Control District and the cities of Austin and Fort Worth, according to NOAA.
“We know there’s a user community out there that’s desperate for relatively up-to-date precipitation frequency information,” Glaudemans said.
Garza said officials with the City, Bexar County, suburban cities, and San Antonio River Authority have all been discussing the data since its release. City staff are planning to incorporate numbers from the study into the City’s drainage codes by spring, Garza said.
Garza said he’s asking engineers that submit plans to his office for approval to consider the new NOAA data when designing their projects, but they won’t be required to design to those higher storm values until the code is changed.
Designing for heavier storms will also make City projects more expensive, he said, though flood control projects included in the City’s 2017 bond will still be designed to current standards.
“Because those bond projects were voted on, they have a specific scope,” Garza said. “I can’t go in there and increase the scope.”
Comparing storm numbers
Currently, the City’s drainage codes are based on numbers from a 2004 U.S. Geological Survey and Texas Department of Transportation study. Before the NOAA update, this was the best-available data for Texas, according to Garza and other engineers.
After incorporating decades of additional weather data, the NOAA analysts calculated that heavier storms have become common across most of Texas.
For example, consider a storm that might drop 10 inches of rain over 24 hours. According to previous data, the probability of a storm of that magnitude hitting Bexar County each year is 1 in 100 (known as a 100-year storm).
The 100-year storm is an important measure of storm intensity because it’s often used in local building codes as the upper limit for what engineers have to prepare for. San Antonio’s drainage codes require engineers to study rainstorms at the five-, 25-, and 100-year levels. City codes currently consider a 10-inch storm over 24 hours to be a 100-year storm.
According to NOAA’s updates, a storm of nearly 10 inches is now considered twice as likely as previously thought. NOAA’s updated data show a storm that drops 9.86 inches of rain over downtown San Antonio has a 1 in 50 chance of happening every year (a 50-year storm).
NOAA’s new data are also more precise than those from previous studies. One document from 1963 shows that a 100-year, 24-hour storm will drop between 9 inches over most of Bexar County and 10 inches near its southern border.
By comparison, NOAA’s new analysis shows a storm of that likelihood will drop 12.8 inches in Fair Oaks Ranch, 12.4 inches in Shavano Park, 11.6 inches in downtown San Antonio, or 11.3 inches in Elmendorf.
So why does it appear that heavier storms are getting more common across most of Texas?
“That is what the data indicate,” Glaudemans said. “When we bring in 40-whatever years of data from probably the most data-rich period in history … it just shows that the rainfall depth has increased for a given recurrence interval and duration.”
Could it be because of climate change?
“We do see it,” Glaudemans said. “But this study in no way is to say that it exists or doesn’t exist.”
Garza said he also wanted to know whether climate change is a factor in these shifting values, so he compared them to the rate of change in climate projections recently completed for San Antonio.
That study projects that 100-year, 24-hour storms will go from 10.3 inches in a 1971-2000 period to up to 13 inches in the 2011-2040 period.
“I’m interested in … the rate of change. Is that consistent with the climate change projections?” Garza said. “The amount that it changed is consistent.”
Flood plain maps could change
The new NOAA values could also affect home and business owners who might be required to buy flood insurance. Officials with the Federal Emergency Management Agency, or FEMA, and the River Authority say they’ll be used for future revisions of flood insurance rate maps.
FEMA maintains these maps as official records of areas at most risk of flooding. Property owners within the 100-year flood plain are required to have flood insurance to receive mortgages from federal regulated or insured lenders.
The new rainfall data will be used in areas where detailed historical flood data are lacking, FEMA spokesperson Kathryn Van Marter-Sanders said in an email. It’ll help engineers determine how much rain will fall in a certain area over a certain time and by how much streams and rivers will overtop their banks.
“One would expect the new flood plains to increase in size, as there will be more calculated runoff with the higher precipitation standards for the 1 percent [100-year] event,” Van Marter-Sanders said. “However, that is not a given, and each new study will take into account all factors in play when determining the extent of the … flood plain.”
As an official partner, the River Authority conducts these studies under an agreement with FEMA. River Authority senior engineer Erin Cavazos said they’ve been processing the new NOAA data since its release to prepare to begin updating the flood maps, a process that will take approximately three years.
“It’s been a long time that we’ve needed this [NOAA] update,” Cavazos said. “From a flood risk management point of view … knowing how much it’s changed over the last 30 years helps us make our design more resilient in the next 30 years.”