Scientists Quantify Flood-Damage-Reduction Benefit of Texas Wetlands

A study by Samuel D. Brody, Sammy Zahran, Wesley E. Highfield, Himanshu Grover, and Arnold Vedlitz called “Identifying the impact of the built environment on flood damage in Texas” quantifies the flood reduction benefits of wetlands along the Gulf coast. The authors studied property damage in 423 floods between 1997 and 2001. They identified the effect of several built-environment issues, including wetland alteration, impervious surface and dams on reported property damage while controlling for biophysical and socio-economic characteristics.

Their statistical results suggest that naturally occurring wetlands play a particularly important role in mitigating flood damage. But how much? The results vary by location, of course, but in one county, they discovered that a 3.4X increase in wetland alteration permits correlated to a 10X increase in flood damage.

Wetlands adjacent to San Jacinto East Fork upstream from Lake Houston

Importance of Understanding Causes of Flooding

Floods damage more property than any other type of natural disaster in America – billions of dollars every year. However, the author’s say, there is lack of research on the relationship between the built environment and flood impacts in the eastern portion of Texas. Say the authors, “Such information is critical given the continued development of coastal areas and the increasing vulnerability of human populations to inland coastal flooding.”

What Study Correlated and How

The authors correlated flood property damage (total dollar loss adjusted by the consumer price index) to variables such as:

  • Precipitation (day of flood)
  • Precipitation (day before flood)
  • Percentage of county in 100-year floodplains
  • Duration of flood
  • Dams
  • Percent impervious surface
  • Wetland alteration
  • FEMA Community Rating System
  • Median Household Income

They measured everything by counties. Watershed data would have been better for floods, but damage is not often aggregated by watersheds.

Researchers measured “wetland alterations” by counting wetland permits issued by the US Army Corps of Engineers in Galveston under Section 404 of the Clean Water Act. They simply counted the number of permits issued up to the day of any given flood. This enabled them to measure the cumulative impact of wetland alterations over time.

FEMA’s Community Rating System encourages city and county leaders to promote responsible development over time. The more an area reduces flood damage through regulations, mapping and 18 flood mitigation measures, the higher the discounts that residents earn on flood insurance.

The researchers used multivariate regression analyses to find which factors most influenced the degree of flood damage in eastern Texas. Multivariate regression identifies the degree of influence that multiple variables have over each other. It measures correlation, not causation.

Major Findings

Specific characteristics of the human-built environment in eastern Texas have an important influence on property damage resulting from floods, even when controlling for biophysical and socio-economic factors. Below are four major findings from the study.

First. the amount and duration of precipitation associated with a given storm flood largely governs flood damage.

When looking at biophysical variables, timing of precipitation is particularly important. “Heavy precipitation the day before the actual flood event is by far the strongest predictor of total property damage,” say the authors. This may be because of the delay in the rise of water or the saturation of soil. “It is important for decision-makers and the public to understand that heavy precipitation followed by sunny skies can still result in significant flood damage the next day.”

Second, the most important built-environment indicator of flood damage is the the alteration of naturally occurring wetlands.

“Impervious surfaces have long been criticised for their contribution to increased flooding and associated damage. However, the most significant impact may not depend solely on the total amount of imperviousness in a watershed or drainage basin, but rather on where exactly these built surfaces are placed. Altering or removing a wetland to construct car parks, roads and rooftops, for instance, effectively eliminates its ability to capture, hold and store water run-off.”

For example, comparing two identical rainfalls four years apart in De Witt County showed a 10X increase in damage. During that time, the number of wetland permits granted increased from 5 to 17 – 3.4X.

Similarly, comparing two 1.5 inch rainfalls in Wharton County, damage doubled while the number of wetland-alteration permits increased from 17 to 26.

Incredibly, Galveston County experienced a 20X increase in flood damage based on two 0.09 inch rainfalls three years apart. During that time, the number of wetland permits increased from 546 to 921.

“Developments initially believed to be safe from flood threats become an unexpected target of expensive flood damage over time,” say the authors. “The planning goal in this situation is to allow development to proceed without reducing the hydrological function and value of wetland systems.”

The authors suggest that achieving this objective will involve identifying and protecting wetlands through local land use policies. They include zoning restrictions, land acquisition programs, clustered development, density bonuses and more. Net economic benefits to a locality may result by reducing costs related to repair of damaged structures and mitigation solutions.

Third, wetlands may be more effective than dams in mitigating property loss over time.

Dams are extremely costly mitigation alternatives. And they can encourage development in flood-prone areas out of a false sense of security. See more below.

Fourth, FEMA’s Community Rating System reduces property damage.

Communities ranked high by FEMA for mitigation measures experience significantly lower amounts of flood-related property damage. “In fact, CRS participation appears to reduce community-wide flood damage more than dams, which are far more costly,” the study claims.

Economic Tradeoffs

In comparing the relative effects of various variables on flood damage, the authors also assessed economic tradeoffs of various mitigation measures.

For instance, they compared the cost in property damage in a flood to the price of wetland permits and dams. They found that 129 wetland alteration permits cost as much property damage per flood as one dam saved.

“Given the expense of building dams, their negative environmental ramifications, and the possibility of structural failure, protecting naturally occurring wetlands may be a more rational policy alternative,” says the study.

Posted by Bob Rehak on 1/30/21 based on a study by Samuel Brody, et. al.

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