Apartments along Brays Bayou

Why Do We Flood?

Why do we flood? To clarify the obvious: Too much rain … creates too much runoff … in too little time … over terrain that’s too flat. Now, dig a little deeper.

Ask a slightly different question: “Why do we experience so much flood damage?” You get slightly different, but related reasons. Based on what I’ve learned while researching more than 2,000 articles since Harvey, the reasons include:

  1. Inaccurate predictions of future rainfall.
  2. Conflicting development standards and building codes.
  3. Building too close to threats.
  4. Upstream changes that undermine our assumptions.
  5. Difficulty of adapting downstream.
  6. Historical unwillingness to fund flood mitigation at meaningful levels.

Many of these self-inflicted wounds have to do with self-interest and the problems we have working together. Let’s explore each in more detail.

Inaccurate Predictions of Future Rainfall

Periodically, scientists update our predictions of future rainfall. No one is omniscient. We can only guess at what might happen in the future based on what has happened in the past. So we rely on a branch of mathematics called EVA (extreme value analysis). EVA predicts the probability of future events based on the observation of extremely rare past events. But that means the mathematicians work with extremely small data sets. And that, in turn, means their confidence is low. So, every time a new superstorm comes along, they get to re-evaluate assumptions.

During the past 20 years, our rainfall assumptions have officially changed at least twice: after Allison and after Harvey. Each monster storm pushed predictions higher. After Harvey, predictions increased 25-30% compared to Allison.

atlas 14 rainfall probabilities
Atlas 14 rainfall probabilities adopted by Harris County after Harvey.

A 100-year storm after Harvey is about equal to a 1000-year storm before Harvey.

Unfortunately, the differences are even greater for those who live near county lines where upstream rainfall standards may be based on data from the 1980s. For instance, the difference between official rainfall probabilities in Harris and Montgomery Counties approaches 40%.

And the amount of runoff new developments must retain varies accordingly. If just one development cut corners, you’d probably never see the difference. But if hundreds or thousands of upstream developments do not retain enough rain, you will.

The casualties in this upstream/downstream conflict will include those who bought homes or built businesses near the limit of the old assumptions. If they were at the edge of the floodplain before, now they’re in it. New floodplain maps based on Atlas-14 have not yet been officially released, but…

Floodplains could expand 50% or more when using new Atlas-14 numbers.

FEMA and Harris County Flood Control Estimates

We use these imperfect and shifting predictions of future rainfall to design margins of safety into new developments. So your safety depends on when your house was built. And where.

Conflicting Development Standards and Building Codes

Humans, both private individuals and public officials, act in their self interest. Therefore, they also disagree about the probability of future risk. This often brings private property rights and public interest into conflict. People who own cheap land in floodplains, or worse yet in floodways, often want to develop it.

Big floods, by their very definition, happen rarely. This often fuels arguments between developers and regulators. John Blount, Harris County’s former head of engineering showed that higher development standards and building codes reduced flood damage 20X.

But higher standards can impact developers’ profits now. And the consequences of not observing those standards may not be visible for decades.

Meanwhile, politicians are eager to attract new development. And they live in a competitive world. Higher standards and strict enforcement can cause developers to look elsewhere for more attractive opportunities. So some politicians are reluctant to adopt higher standards.

That has created a patchwork quilt of regulations in the seven-county region. After Harvey, Harris County tried to get all municipalities and counties to adopt five minimum standards. Five years later, we’ve had mixed results. See below.

Results of effort to analyze and update drainage regs in Houston region. Status reported in June 2022.

Variation in shifting and conflicting standards makes it difficult for home buyers, lenders, and even engineers to assess flood risk. “Whose 100-year floodplain are you talking about?”

And when floodplain standards do change, affected home and business owners may fight them because being in a floodplain could reduce the value of their property.

Building Too Close to Threats

People normally like water. They just don’t like it four feet deep in their homes. Water is part of our DNA. We like to live close to it; aspire to own beach homes; and vacation at the shore. We even pay premiums for homes with soothing water views – despite the higher risk.

However, those same locations can turn into raging torrents of water that destroy homes, businesses, vehicles, lives, and our illusions of safety.

Many people find understanding flood risk difficult. And even if they understand it intellectually, they may repress it emotionally.

After all, who wants to think about the possibility of dying in a traffic accident every time he/she goes to the grocery store?

So most people deny and ultimately compromise. For decades, they compromised by purchasing nationally subsidized flood insurance. Until the advent of Risk Rating 2.0 last year, flood insurance didn’t reflect the true cost of payouts.

That subsidy insulated both buyers and sellers. It allowed developers to build in risky, marginal areas without fear of finding homes unsaleable.

The availability of subsidized flood insurance fueled whole industries that made money off of cheap floodplain land.

Among those industries – engineering. Houston has hundreds of engineering companies that study ways to build “safely” near water. The vast majority are highly ethical. But what if you live downstream from a project with an engineering firm that wasn’t? Life can change in an instant.

Upstream Changes that Undermine Downstream Assumptions

Upstream changes cause downstream consequences. I once owned a home in the Dallas/Fort Worth area overlooking a creek. Engineers and surveyors certified that the home was two feet above the 100-year floodplain when built.

But Plano, just upstream from us, was the fastest growing city in America at the time. A new shopping mall with 80+ acres of parking changed things quickly. On small rains, water started coming up to our back door. One day, after a moderate rain, I saw a pickup floating down the creek.

After I complained to our City engineer, he requested the Army Corps to resurvey the creek. The Corps found that the upstream development had changed the floodplain.

Instead of being two feet above the 100-year floodplain, we were now 10 feet below it.

Unwilling to live with that risk, we sold our house, took a loss on it, and moved to Kingwood – two miles from the San Jacinto West Fork on one of the highest points in Harris County.

But soon Conroe became the fastest growing city in America! And Montgomery County became the second fastest growing county in the region. During my 40 years here, I’ve watched floods get higher and higher until, during Harvey, the Cajun Navy launched rescue boats from our driveway.

If all those new upstream developments really had “no adverse impact,” how did that happen? Was Harvey just a freak, monster storm? Yes. But manmade changes upstream also exacerbated the flooding. Engineering firms eager to deliver cost-effective answers for clients pushed the envelope. For instance, I could cite examples where engineers:

If enough companies push the envelope as these did, gradually flood peaks change, as shown in the graph below.

Red line shows how Brays Bayou watershed would have handled Tropical Storm Allison in 1915 before development. Blue line shows time of accumulation in 2000. Note faster, higher flood peak.

Does anyone really know how all that new impervious cover upstream affects flood peaks downstream? Some areas still use flood data from the 1980s.

Difficulty of Adapting Downstream

Lack of upstream regulation and enforcement save upstream residents money. But they cost downstream residents. The burden of mitigation costs falls on Harris County residents, not our surrounding counties. And mitigation costs billions of dollars!

More than one third of that goes to right-of-way acquisition, in large part, because people built too close to bayous.

HCFCD spending since 2000 by project category.
Apartments along Brays Bayou
Part of the $480 million Project Brays. Note proximity of housing to bayou.

To implement new mitigation projects, HCFCD must buy-out whole subdivisions and move entire neighborhoods, as it did along Halls Bayou. That’s a difficult, expensive, time-consuming process. And that exposes people to risk longer than would otherwise be necessary.

It’s easy to build a floodwater detention basin or widen a channel before development, but a Herculean task afterwards.

Another aspect of the difficulty: political conflict. Scarcity of funds has pitted neighborhoods and races against each other, as each vies for funding. Harvey united us in our resolve to finally do something about flooding. But recovery has torn us apart.

Historical Unwillingness to Fund Flood Mitigation at Meaningful Levels

Until Harvey, we have consistently underfunded flood mitigation efforts at the State, county and local levels. Unless we’re dealing with a flood disaster, it seems we have more pressing issues. Even after Harvey, we are now spending more money on bike trails in Harris County than on flood mitigation in the Lake Houston Area.

Harris County Flood Control’s budget before Harvey was so small that the District often had to save up multiple years to build one detention pond.

During the 2011 drought, State Representative Dan Huberty went to City of Houston Mayor Annise Parker. Lake Houston was so far down, the West Fork was a trickle between Humble and Kingwood. Huberty suggested that that would be a great time to get sediment buildups out of the river. You could have done it with trucks, restored the river’s conveyance, protected people from flooding and avoided water quality issues associated with dredging in our water supply. Parker declined.

So why do we flood? It’s not just the rainfall. Many of our wounds are self inflicted.

Future Posts in Harvey-Anniversary Series

My next post in this series will focus on a Harvey-Mitigation 5-Year Report Card. Another will focus on why some Harvey survivors still experience PTSD, and how it could affect them well into their seventies.

Posted by Bob Rehak on 8/25/22

1822 days since Hurricane Harvey