Highway officials will dig a little deeper — into the Texas soil and taxpayers’ wallets — to address new rainfall risks and keep critical roads open during floods.

Armed with new Atlas 14 maps, the estimates produced by the National Weather Service for rainfall and where flooding is most likely, Texas Department of Transportation officials are adjusting the designs of projects so they can meet the new standards.

The end result should be roadways better suited to the rainfall realities of the Houston area. Those larger detention ponds, higher berms and bigger concrete culverts come at a higher price, however.

“We want to be able to acknowledge and adapt to those changes, but it means in some places more detention or building up,” said James Koch, director of transportation planning and development for TxDOT’s Houston office.

Widening Interstate 45 in Texas City alone, part of a two-decade series of projects making the freeway at least four lanes in each direction from Houston to Galveston, could see its cost soar by more than $135 million to build larger detention ponds and longer bridges through the marshy expanse between La Marque and Galveston Island.

Other projects are seeing smaller cost increases despite massive design changes.

Along FM 723 near the Brazos River in Fort Bend County, for example, TxDOT plans to elevate the road 5 feet, essentially putting the road atop a berm to keep it out of the river when it swells. That is expected to add $30 million to the project price tag.

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In Atascocita, a planned widening of FM 1960 is expected to cost an additional $25 million, in part for more erosion control along the planned six-lane road and for design changes that will extend retaining walls for bridges, project driveways and intersections.

State officials largely have relied on updates to flooding and rainfall maps developed in 1998, with a major update in 2004, when designing projects. The new Atlas 14 maps, part of a National Weather Service program nationwide to develop better data, changes all of that and incorporates major recent rains in Houston from 2015 to 2017. The Atlas maps came out last year.

For Harris County, the new estimates mean a 100-year storm — rainfall with a 1 percent chance of happening in any given year — increases from 13 inches of rain in a 24-hour period to up to 18 inches in a 24-hour period.

That means the 13 inches of rain that once was considered rare is much more common, and the region’s one-in-100 chance rainfalls could be much more devastating and involve larger deluges.

Changes prompted by the new maps also will be built into projects where construction is not imminent, such as plans to widen Texas 36A, rebuild Interstate 45 in downtown Houston or extend the Grand Parkway, officials said.

Other major projects under construction already address some of the high-water issues. The tollway work along Texas 288, expected to finish this summer, includes reconstruction and expansion of the detention ponds at 288 and Loop 610, long a flood-prone passing. The massive I-45 project proposed for downtown Houston includes drainage changes north of Interstate 10 to alleviate flooding at North Main, something Houston Mayor Sylvester Turner has said is paramount to TxDOT receiving the city’s support for the project.

For projects just months away from construction and already listed in upcoming highway spending plans, however, the changes mean sometimes major adjustments to their costs. The changes also affect construction schedules and the type of road that will be left behind.

“Some will have to take a little longer,” said Alan Clark, director of transportation planning for the Houston-Galveston Area Council, the agency that doles out federal transportation money in Harris County and the seven surrounding counties.

Still, Clark said, the changes are necessary and provide needed improvements, giving the region “a more resilient transportation network.”

Storms in the past five years have focused additional attention on roadway resilience, beyond the dozen or so well-known flooding spots along area freeways. High water is common in some spots, such as I-45 at Mount Houston Road along the frontage road and Loop 610 at Clinton on the main lanes of the freeway.

During the Memorial Day flooding in 2016 and remnants of Tropical Storm Harvey in 2017, crews scrambled to address submerged streets and roads in uncommon places. Interstate 10 near the Sam Houston Tollway and the tollway just south of I-10 both closed as water topped them.

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With so many stretches of Houston-area freeways planned for construction, that new reality of where storms can stymie routes led to re-evaluation of upcoming plans. The new rainfall maps brought that into greater focus, Koch said.

“It’s all over Texas,” he said. “But the coast has the most significant changes.”

Koch said Beaumont and Corpus Christi officials are seeing similar changes in planning for their projects.

The changes in Houston will not be cheap, officials acknowledge, based on the first round of projects, Clark said.

Still, funding the increases does not constrain spending as much as change the math on what officials have to spend in the future. TxDOT and H-GAC plan projects years in advance setting aside money, so cost increases simply mean they will have less money to spend next year when they balance the books.

The largest changes, so far, are slated for I-45 at the Texas City Wye, the interchange where the freeway, Texas 6 and Texas 146 converge in a tight tangle of crisscrossed connecting ramps near the Bayou Vista subdivision in which homeowners are as likely to have a boat dock as they are a driveway.

The new maps show a need for increasing or adding new “embankments, drainage, retaining wall and bridge structures” to the project, at a potential cost of $106 million. North of the Texas City Wye, more embankments and detention could raise costs by $31.9 million and take the total for the assorted projects south of La Marque to around $350 million or more.

Many of the estimates are in their early stages and could change as the project nears construction, said Danny Perez, a spokesman for TxDOT in Houston.

“We will continue to do all we can to advance the project while making certain the project not only improves mobility but keeps motorists safe,” Perez said.

For FM 723 and area residents, the elevation change could mean the difference between the road remaining open during heavy rains or not. During the 2016 Memorial Day and Harvey in 2017, the highway was submerged for days, cutting off many north-south trips.

Elevating the road out of the Brazos’ flood stage of 52 feet above sea level, along with splitting the project to widen the road to two lanes in each direction from FM 1093 to the Brazos River, is expected to add $30 million to the total cost and put the tab for widening the road in Harris and Fort Bend counties at nearly $220 million.

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Across the region, drivers had differing perspectives on the promise of costlier but better roads. In Pearland, Stephen Cash, 33, said transportation should spend what is needed to keep roads operating during heavy rains, saying he never understood why some roads flood so easily.

“So, yeah, do it right, even if that means putting in more basins,” Cash said.

Others remained suspicious, not only of the additional cost, but the promise that highway officials can build a better road.

“Same folks who built the new Bay Area interchange (with I-45) that floods every time,” said James Murdoch, 54, Webster.

Recent catastrophic storms such as the Tax Day, Harvey and Tropical Storm Imelda floods have left many residents concerned that berms and freeways contribute to flooding by impeding the flow of water, Murdoch said.

Koch said he knows it will take some convincing, but officials can make the needed changes without causing problems elsewhere. Larger berms to elevate FM 723, he said, can have slopes and passages that accommodate for the river.

“That flow between both sides won’t see any different water than you see today,” he said.

In many cases, even when there are significant added costs, commuters may not even notice the difference, Koch said. Larger culverts and pipes, after all, are buried.

“The bridge would be there and the culvert would be there,” he said. “This is just ordering a bigger size.”

dug.begley@chron.com