Why Houston Floods, According to Flood Control

It Rains a Lot! What Can Flood Control Do?

March 3, 2024

For years the Harris County Flood Control District has been giving public presentations that include a slide purporting to explain “Why is Harris County Flood Prone.”

According to Flood Control, the reasons Harris County floods are:

  1. It rains a lot.
  2. The landscape is flat.
  3. Soil doesn’t soak up the rain fast enough.

Strangely missing from this list is arguably the number one cause of flooding in our city: it’s covered in concrete, asphalt, and roofs, otherwise known as impervious surface. (p. 182-183) That means artificial hard stuff that doesn’t allow the rain to soak into the ground but instead increases runoff, sending stormwater really fast into our streets and overwhelming our drainage systems, natural and built. The built city also absorbs a lot of heat, influencing rain patterns, among other problems. (See “Cities Depaving for a Cooler Future.”)

Here are a few further references for the role of impervious surface in Houston’s flooding:

Buildings Partly to Blame

More Pavement, More Problems

Houston’s Urban Sprawl Increased Rainfall, Flooding During Hurricane Harvey

Houston’s Flood is a Design Problem

What Causes Floods

The Rapid Urbanization of Houston: How It Happened and Why It Matters

Why Not Mention Impervious Surface?

We asked Flood Control why their explanation doesn’t include impervious surface. A representative graciously responded:

“You are correct that impervious surfaces are a cause of flooding in our area. However, in our ‘Why is Harris County Flood Prone’ slide we are focusing primarily on the sources of flooding that our capital and maintenance projects can directly address. As you know the Flood Control District does not have regulatory authority over development and the amount of impervious surface added each year.”

Well, true. But the slide is about the reasons Harris County floods, not what Flood Control can do about it. And the flood control district doesn’t have authority over rainfall or the topography or soil type either.

But what about Flood Control’s claim that the landscape is flat and covered in “clay soils that do not soak up excess rainfall quickly”? Actually since most of it is covered in impervious surface, these points would almost seem irrelevant. But they are also not exactly true.

Is the Houston Region Really Flat? Not So Much

A 2022 study from the University of Texas confirmed what anyone who grew up around here already knows: the seemingly flat landscape is in fact sculpted with numerous sloping hills carved out by the many branching channels, otherwise known as gullies or creeks, that feed into our major streams, such as Buffalo Bayou.

A creek flowing towards Buffalo Bayou through a deep, wide gully in Houston’s Memorial Park. Photo Feb. 19, 2024, by SC

Unfortunately, many of these gullies have been filled in and houses, streets, or other structures built on top. Bad idea. Streams, like rivers, have a memory, and water will flow that direction anyway. A big problem if your house is built on top of an old gully.

Some interesting historic gullies that have been filled or developed include Spring Gully underneath what was then called Carolina Street in downtown Houston. Draining into Buffalo Bayou, Spring Gully reportedly was used as an escape route for people fleeing slavery. (p. 19) Harris Gully is another major gully that ran along what is now Kirby Drive, through Rice University, through what is now the Texas Medical Center (formerly part of Hermann Park), and into Brays Bayou.

The Franklin Avenue bridge over Spring Gully, which ran along Caroline Street, then called Carolina. St. Vincent de Paul Catholic Church in the background. Thomas Flintoff, watercolor, 1852

But What About the Soil?

Flood Control has long maintained that Houston’s natural landscape is basically like a concrete parking lot anyway: not only flat but basically impermeable. But this is wrong, as we pointed out years ago. Most of the soil in Harris County is moderately highly absorbent to very highly absorbent.

And it’s even more absorbent if we stop mowing. Plant roots facilitate the infiltration of rain where the soil is less absorbent. But mowing causes plants to devote their energy into growing taller rather than extending roots. So better to let the grass grow higher and plant native plants.

So What Does Flood Control Have Authority Over?

Basically Flood Control’s capital and maintenance projects are devoted to handing out millions of dollars in contracts to private companies to mow the grass on the banks of our channelized streams. Plus millions more to dredge and dredge and dredge again, removing sediment and valuable sand that the stream on its own would shift gradually towards to the gulf and onto the beach anyway, naturally rebuilding banks along the way, all for free. Plus millions more to widen, deepen, and damage our streams, compacting the soil and beds with heavy equipment, destroying the stream’s natural ecosystem and ability to cleanse and absorb the water. Plus cutting down forests and digging up prairies to build stormwater detention basins.

The flood control district was created by the Texas Legislature in 1937 to be the local partner with the US Army Corps of Engineers in its flood control projects. The program was to strip, straighten, and cover in concrete as many streams as possible, which is why so many Houstonians think a bayou is an artificial ditch. (Largely overlooked in the district’s enabling law is the obligation to conserve forests. p. 1) In 1979 Harris County Commissioners Court, which oversees the district, authorized Flood Control to widen its focus and build detention basins to hold stormwater. Since then the district has been criticized for focusing primarily on large, regional detention basins that require rain runoff to get there somehow. However, the district has limited authority for drainage, and it is local authorities within the county such as the City of Houston that are responsible for stormwater drainage into the streams managed by Flood Control.

Channelizing streams, collecting and moving as much stormwater as possible is an outdated, counterproductive practice, not just because of its damaging impact to the environment (and exorbitant maintenance costs) but also because it actually reduces capacity and increases flooding and erosion. The Corps of Engineers has moved away from some of its destructive practices (pp. 15-16) and local agencies across the country have adopted policies focusing on slowing, stopping, and absorbing rain before it enters our steams. (See here and pp. 32-33 and p. 78 and also China.) The City of Houston in recent years has created some tax incentives for private developers and homeowners to install Green Stormwater Infrastructure to help slow, absorb, and cleanse stormwater runoff. This includes a property tax reduction for reduced amounts of impervious surface.

Our flood control district, though bound by the limitations of its legal tools, has evolved somewhat over the years. But in the Houston region this myth about the benefits of moving as much water as fast possible continues to circulate.