It would be less expensive for the government to buy and preserve undeveloped land that lies in Houston’s floodplains than it would be to let development occur and face a potentially devastating bill if those properties flood, according to a new study.

Allowing development on these sites could expose Harris County to as much as $3 billion in damages by 2050, according the report, published in the journal Nature Sustainability. The magnitude of potential damages far outstrips the $400 million researchers estimated it would take to acquire the land and prevent its development.

Kris A. Johnson, one of the paper’s authors, said that while the national analysis may not take into account regional regulations and other considerations, such as elevation requirements, he hoped that it would inspire local policymakers to consider measures to reduce future flood damages.

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“Are there places where flood risk is high, but people are not yet there?” he asked. “But according to Census projections, those are the areas where people are likely to move because the population is growing? And what if we don’t develop there?”

Robert Lazaro, a communications officer at the Harris County Flood Control District, said the national study did not consider a number of local factors but agreed that buying land likely to flood plays an important role in minimizing future damages.

“We’ve found that an ounce of prevention is definitely worth a pound of cure down the road,” Lazaro said. The district buys homes in areas that are several feet deep in the floodplain, as well as undeveloped portions of the floodplain.

Growth areas hit

The analysis — carried out by members of the environmental organization Nature Conservancy, the University of Bristol in the United Kingdom, British hydrologic modeling company Fathom and the Federal Housing Finance Agency — suggested buying land may not make sense in places with low population growth. But near cities like Houston, where the demands of a growing population may encourage residents to take on the risks of living in a floodplain, it could save money in the long run.

A suit by owners of property inside the Addicks Dam reservoir highlights the gambles developers can take if not prevented from building in flood zones. A judge ruled Tuesday that the federal government is responsible for the flood damages incurred when Hurricane Harvey filled the reservoir.

“We found an area the size of Massachusetts where a dollar spent to protect areas in the floodplains would save five dollars in future damages,” Johnson said.

The paper did note that buying land may be the costliest way to preserve it and suggested changes in policy could conserve land at a much lower cost.

But Houston has bucked the idea of stopping development. When the city, under former Mayor Bill White, tried to ban new development in the floodway, the most perilous portion of the floodplain through which the waters of a 100-year flood drain downstream, property owners sued. The city quickly backpedaled.

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“There’s a balancing act,” said James Wade, property acquisition department manager for the Harris County Flood Control District. “People who own property don’t want more stringent regulations because it’s going to impact their bottom line. But you have the other side of the equation: Trying to reduce flooding and reduce damages in the future.”

Wade read the paper with interest. “The article makes a good point that one of the most effective way to prevent future damages is to purchase,” he said.

Expense underestimated

Purchasing land that has already been developed, which the department does with its voluntary buyout program, would be prohibitively expensive to do at scale, Wade said. So far, the department has completed about 3,400 voluntary buyouts, roughly 2,700 which involved land with homes on them.

Purchasing undeveloped land is less expensive, but the $400 million estimate for buying Harris County’s undeveloped floodplain land struck Wade as low. He quickly filtered a spreadsheet of all of the privately-owned land parcels in the 100-year floodplain for the ones where the Harris County Appraisal District has no record of a building, meaning they may be undeveloped. The appraisal district’s estimate for the aggregate value of that land — which the flood control district usually presumes to be on the low side — neared $6 billion.

Johnson clarified in an email that the study only looked at purchasing forest, grasslands and wetlands. It excluded areas that federal land-use data showed as even lightly developed or cultivated for agricultural use, which Johnson said could explain the district’s higher cost estimate.

“Our estimate acquisition price is only for buying natural lands in privately-held floodplain,” he wrote.

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The analysis arrived at its numbers by using a national flood hazard model developed by former PhD students at the University of Bristol (federal flood maps do not cover all of the United States, especially in sparsely populated regions) to approximate the 20-, 50-, 100- and 500-year floodplains. It then used federal projections for where development is likely to occur, and estimated the cost of damages to those developments using depth-damage functions from the Army Corps of Engineers. Costs for acquiring the land were extrapolated from data including land purchases by the Nature Conservancy and residential price estimates from the Federal Housing Finance Agency.

“Even if that land is relatively expensive to acquire, the kind of flood damages you could expect to any kind of building that would be built upon them would far outweigh the cost,” said Oliver Wing, one of the study’s authors.

This story has been revised to correct the estimated cost of damages if properties in the floodplain are developed.

rebecca.schuetz@chron.com

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