Second in a 3-part Series on the Root Causes of Flood Damage – Yesterday, I posted about three root causes of flood damage. Beyond “too much rain,” they included:
- Inaccurate predictions of future rainfall – Why we don’t expect enough and therefore underestimate risk.
- Conflicting standards and building codes – How some jurisdictions fail to update and enforce floodplain standards/building codes as a way to compete for development.
- Building too close to threats – How some people invest too much confidence in engineering stamps and government approval, and begin thinking of them as safety guarantees.
Today, I’ll discuss three more root causes of flood damage:
- Upstream changes that undermine downstream assumptions
- Difficulty of adapting to those changes downstream
- Historical unwillingness to fund flood mitigation at meaningful levels
Through this series, I hope to distill key lessons from thousands of observations during the last six years.
Upstream Changes that Undermine Downstream Assumptions
As development expands upstream, it sometimes changes the landscape in ways that increase flood risk for people downstream.
The standard in the development industry is to create “no adverse impact.” However, it doesn’t always work out that way as this tragic story dramatizes.
People living downstream from Woodridge along Taylor Gully in Kingwood had never flooded, even during Harvey. Then they flooded twice in 2019 – AFTER Perry Homes and its contractors clearcut 270 acres of forested wetlands upstream.
Then Perry Homes bought the steeply sloping land. Their engineer’s plan called for clearing a section, developing stormwater detention basins for it, then moving on to the next section.
But contractors clearcut and graded the entire area all at once, increasing flood risk. Detention basins were only 7% complete at the time of the first flood and less than 23% at the time of the second. Many Elm Grove residents downstream had just finished refurbishing their homes from the first flood when the second struck.
During the ensuing lawsuit and investigation, it came out that Perry’s soil testing firm had taken samples outside of the known wetland areas. Thus, they overestimated the rate of infiltration by up to 10X.
And LJA, their engineering firm, said there were no floodplains. In reality, the floodplains were there; they just hadn’t been mapped – a tragic misrepresentation. The flood survey simply stopped at the county line.
Contractors failed to:
- Install sediment controls.
- Plant temporary vegetation.
- Preserve a berm that protected Elm Grove from sheet flow.
- Follow requirements in the Montgomery County Drainage Manual.
- Build required detention capacity
Even had they built the entire detention capacity specified in the plans before the September storm hit, the site would have been 40% short of new Atlas 14 requirements. Montgomery County had not yet adopted Atlas 14. This would have created a deficit in perpetuity.
In addition to damaging the homes of up to 600 downstream residents, sediment discharged from the site also reduced the capacity of Taylor Gully. Harris County Flood Control (HCFCD) had to clean it out and is now looking for ways to expand channel capacity.
After all that, Perry Homes still blamed the flooding on Acts of God, a common ploy in flooding lawsuits.
Had HCFCD and the City of Houston not purchased the property and started increasing detention capacity, downstream residents would likely suffer from more flooding in the future. The only alternative would have been to raise approximately 600 homes.
Perry and its contractors eventually settled the lawsuit two years after the second flood.
Difficulty of Adapting Downstream to Upstream Changes
Luckily, the purchase of the Perry property happened while there was still open land available to increase flood mitigation. That’s not always the case.
Sometimes, homes, schools, and businesses are built before the flood potential is discovered. Then, mitigation becomes much more difficult.
Properties – or even entire subdivisions – must be bought out before channels or stormwater detention capacity can expand.
Halls Bayou provides two examples near I-69. Note the two subdivisions inside the giant detention basins that bracket the freeway.
This area used to be farmland. Then it rapidly developed. It looks like this today.
Buyouts for the two giant stormwater detention basins started in 2002 after Tropical Storm Allison.
Then HCFCD had to get permits from the City of Houston to demolish the streets. That took several more years.
Construction took three years for each basin; they were completed in 2015 and 2018.
During that time, we went through Memorial Day, Tax Day and Harvey floods. Those storms, along with Allison and Imelda damaged more than 25,000 structures in the watershed, according to HCFCD Federal Reports, making it one of the most flood-prone in the county.
Worse, we no longer have easy flood-mitigation solutions in this watershed. Even with hundreds of millions of dollars from the GLO, mitigating flood damage will be difficult, expensive and time consuming. It will also be politically controversial because it will likely displace families and even whole neighborhoods.
Historical Unwillingness to Fund Flood Mitigation at Meaningful Levels
Another root cause of flood damage is historical unwillingness to fund flood mitigation at meaningful levels.
We tend to spend more on correction than prevention as you can see from the Woodridge Village example above.
Before the 2018 Flood Bond, HCFCD sometimes had to save up years to build one detention basin.
There was a bump in funding after TS Allison in 2001. But then, for the most part, mitigation spending averaged about $120 million per year through Harvey. Then, we had a large bump in spending with the flood bond.
A greater focus on prevention of flood damage through higher floodplain development standards, monitoring and enforcement could make more money available for mitigation.
Tomorrow, I will focus on more ideas that address the other root causes of flood damage.
Posted by Bob Rehak on 11/20/2023
2274 Days since Hurricane Harvey
The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.