How Floods Work: Harris County Edition

Welcome, Instapundit readers! Thanks for stopping by, and please be sure to read the comments. There are some excellent updates and additional information and observations from readers.

A lot of hand-wringing and second-guessing now and for the foreseeable future focuses on how a city as big as Houston (and the surrounding areas) could “be allowed” to flood. Setting aside the little problem of humans’ inability to steer storms and rain to or from desired locations, what we’re seeing is a combination of hydrology, urban development, and “excess” precipitation. And it is rather predictable that when you have certain combinations of the above, you get “flooding.” Flooding in this case means water in places where it is not desired, in sufficient quantities to cause damage and to endanger human and animal life. You see, Texas drains into Houston and Brownsville.

Texas can be divided into roughly three major drainage areas. Those places that eventually flow into the Rio Grande, those that feed into the Brazos and Trinity, and those that supply the Red. Many of those streams drain south and east into the plains around what is now Houston – Port Arthur.

The Colorado, Brazos, and Trinity are the problem as of August 27, 2017.

The geology of the state explains why: Houston more or less sits on the silt and gravel that has washed off the rest of the state, aside from the Panhandle and Edwards Plateau west.

The low brown is sediment that is the Coastal Plain.

So Houston was developed on wetlands, or specifically on higher areas within wetlands and streams called bayous. Buffalo Bayou is one of the best known, but there are scads of them, or were. Houston also sits on sediment that is slowly sliding down into the Gulf of Mexico and has been since the end of the last ice age. To compound the problem, in the ’50s and ’60s, groundwater pumping was tried to augment Houston and some of the surrounding towns’ water supplies, and that lowered a few areas almost below sea level before pumping stopped.

The darker yellow is the Coastal Aquifer. I live on the grey-blue Ogallala.

So the area under Houston has always been low and moist, with water coming up from below and down from “above,” above meaning north and the clouds together.

However, ground absorbs rain. Trees, grasses, and brush all slow down rain and absorb some of the water, keeping less of what falls from overloading the soil. Even bare ground can absorb some rain, but not as much as soft, grassy soil or pure sand. When the air spaces between bits or soil fill with water, the ground is saturated. Saturated ground turns from a sponge into a watercourse, and you have what is called “Hortonian Overland Flow.” That’s a fancy word for water on the surface that can’t soak in so it flows off.

Buildings, parking lots, and streets do not absorb rain. So the water must be directed somewhere, through storm sewers usually, and sent away. To make a long story really short and simple, a lot of urban areas have so much pavement (impermeable surface) as compared to absorbent ground that hard, fast rainfall causes street flooding. Denver, Amarillo, Garden City in KS, LA, anywhere you get a lot of rain in a short period of time can have minor flooding.

The problem with Houston and the surrounding area is that over the past 150 years or so, more and more has been built on the bayous and wetlands along the Texas Gulf Coast, and upstream. So not only has there been far too much rainfall for the ground to absorb (30″ in 36 hours is too much even on absorbent ground), but it is falling upstream and filling the rivers that drain into the lower areas. And the development in that part of the state has covered over the soil and streams, reducing the amount of vegetation, and so it flooded. Houston always floods. Mom Red waded to her college graduation because part of her campus always went under water. There’s a reason the Heights district in Houston is called The Heights. It sits on a relatively high area. Low, flood-prone patches was true of my Alma Mater in GA as well, and you learned to avoid the low areas (unless in my case you really wanted to see just how sticky Georgia red clay is. And yes, it stains everything.)

However, the truly awe-full amount of rain in a short time, plus lots of impermeable surfaces, plus reduced drainage, plus more water rushing in from upstream has put Harris County and a lot of the surrounding area under water. From a hydrologic point of view, sitting 800 miles uphill, it is not the least bit surprising. For the people dealing with the disaster, it is horrible and the scale is unexpected. Everything that could go hydrologically wrong, with the exception of an earthquake or sustained Cat V hurricane winds, did.

Eventually, the water will flow into the Gulf, the bayous will return to their usual contained courses, and the interstates will emerge from the waters. New flood maps will be drawn up, and people will be assured that this was a “one in [X] hundred years” event. Which means hydrology people will groan, because “A one hundred year flood” only means your odds of having it happen are 1:100, not because you can mark your calendar that there won’t be another flood of this size for a hundred years. So, [DEITY] forbid, something like this could happen next year if everything goes wrong in the same way again. It is not likely, but certainly possible.

How do you prevent this kind of flooding? You can’t. You can modify building and construction codes to maximize water absorption by requiring permeable paving where possible and by insisting on certain amounts of water-absorbant green-space to counter the hard-space. You can install pumps a la New Orleans and try to bail out low areas, preemptively draining the bayous so they could take more rainfall before they overflow. You can install upstream dams to slow the flood crest, although they have the dams open right now (Tuesday afternoon) so the dams don’t overload and fail.You can insist that all new construction be on stilts. It’s a little too late to, oh, dig canals to re-direct all the rivers aimed at Harris County so they drain into the Red River. You can’t move Houston, Katy, and all the adjacent towns. In some cases, the geology will work against you. In others, the cost is so high, and retroactively changing building codes? Not popular, and not cheap. Heck, trying to get zoning in Houston is enough of a challenge.

Hydrology and geology, with a large slug of meteorology are why Southeastern Texas is a lake at the moment. It’s not the time to blame any one person, unless you want to find the grave of the man who developed the area in the first place, back in the early 1900s, and curse his bones. Now is the time to take care of people and animals, and then see to property. And one hopes, to start looking at the landscape and considering what to do the next time the water is “nine foot high and risin’.”

36 thoughts on “How Floods Work: Harris County Edition

  1. That pioneer was the smart one.
    It took a somewhat similar disaster for most of the folks to move up from Galveston and join him on higher ground.

    That his foresight wasn’t fully able to mitigate a force of nature, is only to be expected.

    😉 I’d nitpick a bit about the lack of gradient and the infiltration rates of clay soils, but I’m absolutely convinced that the less time I spend remembering GeolE 409, the happier I’ll be.
    (It’s a very bad sign when the professor walks into a packed classroom, asks “how many here are taking this class for the first time?” and three hands go up. There were another four who had only taken it once.)

    • I wrote this on the fly, so-to-speak, and the exact soil profiles of that part of the state are not something I’m familiar with in detail. And yes, with clay soils, you have to consider initial infiltration versus post-wetting infiltration.

  2. A good overview – I agree. I think there are a couple of important points you made that could be expanded on:
    – People settle in areas that are easy to get to and from and easy to live in; traditionally that meant river valleys with water available for crops, drinking, and water transport and with fertile soil good for growing – Guess what? these areas drain large areas, so they tend to flood under heavy rain. In fact, they had to to get the fertile soil that makes crops grow well!
    – People tend to accumulate where other people already are, and the people are already in places like this because of the factors mentioned above, so cities, with few exceptions, are in areas vulnerable to flooding and other natural disasters. A few new cities are NOT in those areas, but even they have trouble when it floods. For an example, look at Las Vegas.
    – Any time, any place that has inordinate, unaccustomed rains will have flooding and other problems – it is just the way things are. One thing that will be interesting to look for in future news is to see if there are any infrastructure problems from the flooding; erosion and undermining from moving water could mess up buildings and highways in the area the same way that floods in mountainous areas lead to landslides and washed out bridges.

    • Excellent additions, Jonathon, thank you. We’ve moved away from the philosophy I head from a retired farmer in Flat State who had “good corn ground” near one of the smaller rivers up there, who stated, “Rivers flood. That’s what rivers do.” Trying to find a balance between ease of access to waterways and distance from flood-risks is a challenge, to put it mildly.

      We got over three inches of rain up here in around and hour two years ago. An urban playa promptly ate a new subdivision. That intense of a rainfall-event happens so rarely that trying to plan for it wasn’t considered really necessary. The folks with houses ankle-deep in water disagreed, as you can imagine.

    • And there’s that whole “people move to places convenient for ports” thing, too. Part of the problem with port towns is they often sit directly on those places where rivers empty – and they’re usually the rivers that all the other rivers dump into, because (when you look at it from the opposite angle), distribution system!

      Nice but if info, Txred, and well layed-out.

      • Thank you. It’s been a while since I’ve written much about hydrology, but once you’re trained to “think like a water-drop” as one of my profs said, it never quite goes away.

  3. “Denver, Amarillo, Garden City in KS, LA, anywhere you get a lot of rain in a short period of time can have minor flooding.”

    …Lubbock, San Angelo…

    “Now is the time to take care of people and animals, and then see to property. ”

    Agreed. Fortunately most Texans are doing this and setting aside differences. Some of the pinheads on Twitter, not so much. If there is a silver lining in this tragedy, it’s that we’ve put aside petty differences and have come together to support each other.

    Thanks for the post. While I already knew most of the information, it was a good refresher.

    • Just because people talk loudly doesn’t mean they are representative of the population – for example, all of the recent news about violence by and between 2 opposing factions have involved at most several hundred people out of a nation of over 300 million; in science and engineering, that is less than a rounding error.

  4. That’s why I live on a hill, and wouldn’t live down there, or anywhere on the coast on a bet. It appears the current argument is whether this is an 800 year or 1000 year flood. There actually ‘was’ a logical plan to evacuate, but it was never put into motion (evac lowest areas first, 100 miles away, lather rinse repeat). I don’t care WHERE you live, 50 inches of rain WILL flood areas, the earth simply cannot absorb that amount of water in 72 hours… sigh

    • I got to wade through the results of three inches in an hour and a retention pond overflowed. Not my idea of fun, especially since mine was one of two ground floor apartments that didn’t flood.

    • One of the problems is that there is a limit to how many people get be gotten out of an area in a given time. While it depends on the roads and planning, there is still a limit – While I’m not a highway designer, I suspect that that limit would have been reached in trying to evacuate Houston. I am concerned that trying to evacuate and failing would have been worse than not trying; too many people would have been stuck in traffic when the rain and flooding came and if that happened, the death toll would have been MUCH worse.

      • Remember what happened when Florida tried to evacuate ahead of Hurricane Floyd in 1999. The interstates became parking lots. I staffed a Red Cross shelter in north Florida that filled up with people who had been on the highway for 14 hours or more and had barely made it past Jacksonville.

        And that was in a state that the storm missed completely.

      • And they knew it; in Rita, the roads out of Houston basically turned into parking lots, then water hazards full of cars. That leaves out the logistics of having enough fuel to conduct it; you would need extra supplies ahead of the storm, and refueling points along the evacuation route. Here in Dallas, we’re back to gas lines because there’s not enough fuel coming in.

    • Living on a hill say a mountain may save you from flooding but unless you are on the top It will not save you from erosion in a 10-50 rainfall. Rainfall of that magnitude was not imaginable until this storm. However any place on a hill has more problems getting a well drilled for water It is more costly to drill a deep well. So ability to have water is difficult unless the residence is closer to the ground water. Instead most places attempt to solve drainage problems Houston is flat . so drainage is a problem Even still they are draining slowly.

      I live next to a river on the Chesapeake I sit at 40 feet above the water I would not flood But my soil would erode severely and could undermine the house with a rainfall of this magnitude. We have spent years digging drainage ditches and re routing water flows away from the house just to prevent this.

  5. As someone who’s lived in Houston since 1980, litigated cases involving flood plain disclosures, and just lived through Harvey: Kudos on an excellent article.

    My only criticism is that you seem to jump to the immediate suggestion of more governmental compulsion as the preferred solution. Building and construction codes have a role, as do actions taken by governmental agencies like the Army Corps of Engineers.

    But as you point out, we Houstonians have been keenly aware of these problems, and reminded of them with great regularity, and the last such reminder, Harvey, came with exclamation points. There aren’t simple, quick, or cheap answers — but that doesn’t mean that nobody’s doing anything, nor that nobody can do anything useful or cost-effective.

    The Galveston Hurricane of 1900 — still the greatest natural disaster in American history by far, with 9/11 and the San Francisco flood/fire as distant runners-up — prompted spectacular macroscopic changes, the result of which included the future commercial and industrial development of Houston and its port at the expense of Galveston. Harvey, thank God, will have but a fraction of that hurricane’s death toll, or, in more recent experience, Katrina’s, in large part because we have so much experience dealing with storms and flooding already.

    There’s room here for crowd-sourced, creative solutions — emphasis on the plural, solutionS, because there will be many, and they will be diverse: What works as effective flood mitigation and control in one place may be useless in another a few hundred yards away. Harvey has shown us pockets of disaster cheek-by-jowl with pockets of salvation; the difference from past storms and floods is one of intensity, not a difference in kind.

    Again, thank you for this informative essay!

    • You are very welcome! I listed “governmental compulsions” because those are the ones that strike most people as the easiest and what the media and politicians lean toward. The nudging solutions, such as re-engineering water channels to return them to a more absorbent and “natural” flow pattern, or creating a master zoning and drainage plan to slowly work with the landscape under the hardscape in order to increase drainage without unduly affecting landowners or disrupting current streets and sewers takes a great deal of foresight, patience, and time. And I’m not certain it has ever been attempted on a scale larger than places like Johnstown and Rapid City, where compulsion was used to keep people from rebuilding in the central flood plain, and then the rivers were naturalized into parks with floodways.

  6. 1. Houston is a seaport. It is the second largest seaport in the US. If it were not a seaport it would not exist – or if it did it would be the size of Liberty, TX (a former riverport). Seaports, especially those on coastal plains, flood. As the saying goes, if you can’t take a joke you should not have come.

    2. At least 90% of the houses in the Greater Metropolitan Houston Area did not flood. Total estimates for Harvey seem to be between 80K-100K buildings with water damage. There are 2.3 million households in the GHMA.

    3. If your house is within 1/2 mile of a bayou or creek in the GHMA it will almost certainly flood at some point unless it is build up higher than the flood line. (And it is the only one built higher than the flood line. If everyone raises their houses it raises the flood line.) If your house is within 3/4 mile of a bayou or creek it may flood.

    4. Any house can flood if the local city government pays insufficient attention to drainage. If they let the drainage ditched get overgrown, clogged with debris, or build structures on it, the drainage system will serve as a retention pond when it needs to be a drainage ditch.

    My house did not flood. It is just over 3/4 of a mile from Clear Creek. Two years ago other houses in my neighborhood did flood (including some across the street) because the city let the drainage ditch in the area get clogged. They cleaned it up afterwards and a much bigger rainfall caused no problems. On the other hand, the restaurants and other stores closer to the Creek on FM-518 did flood, so I have to drive a couple of miles farther to get a pizza or gas up the car.

    • Many of my relatives used to live in Houston, mostly on the northern side (North Shepherd Road). During the summers when Sib and I stayed with my grandparents and Great-Aunt and Uncle, I remember my Grandfather and Great-uncle going out and removing debris from the creek behind their houses, and walking the road ditch to make certain nothing blocked it. I recall several hurricanes that left water in the ditch and the creek full, but didn’t get near the houses. The falling pine trees, however, kept things exciting.

  7. I moved from California to Houston in the late 90’s and lived in the Atascocita area for a couple of years. What I noticed was that Houston didn’t have road drainage sewers/ openings like California did.It seemed like most of the water was expected to be absorbed by the ground or diverted into culverts.I understand the topography aspects , bayous and such, but it seems like the biggest controllable issue was the lack of formal systems to channel the water that they knew would come every year..

    Another interesting note was that Houston was the only city that I have seen that has permanent number markings under the freeway underpasses to let you know how deep the water is so you don’t try to drive through it if it is too high.

    • For the sewers, being so close to sea level sewers wouldn’t help. You saw how Houston solved the problem. The roads are made to be drains. To protect the houses. That’s why there are marking there.

      This was just to much rain. Really for almost anywhere in some places.

      I got flooded out. My retention ditch couldn’t hold and get rid of the water fast enough . 41 inches of rain

      • I’m sorry to hear that you got flooded. I hope the damage is limited and that you can get things dried out and repaired quickly, without too much fuss.

  8. Nice article. I agree with the issues of development and would note that lots of communities originated near water for shipping and water supply reasons, and have expanded from there.

    A little clarification on the watersheds, though. Harris County actually has mostly self-contained watersheds. The big ones noted (Brazos and Trinity) pass by the county to the west and east, respectively.

    Near the coast, there are a lot of smaller watersheds. We are in the San Jacinto River watershed, and Buffalo Bayou is a major tributary. Hcfcd.org has good maps of the watersheds.

    Fort Bend County, to the west, is experiencing flooding from the Brazos. They have built levees along the river and there are many homes behind the levees.

    • Thank you for the clarification. I did not have the fine-scale maps on hand to really do justice to the drainages in the region when I was writing this.

  9. Great article! The only thing I can find wrong is repetition rate. This could happen again next week. There are more storms forming where Harvey came from.

  10. I live in one of the neighborhoods along Buffalo Bayou that is still experiencing flooding will for another few weeks. My house did not flood, but a number of my neighbors are still getting hammered by water standing in their homes.

    Many of these homes are 50-60 years old and have NEVER flooded before. They did not flood until the Corps of Engineers decided that draining the reservoir was more important than not flooding people’s homes. I understood that when the reservoirs were about to spill over, but they crested long ago. So now, many of my neighbors are still flooded because the Corps has chosen to drain the reservoirs rather than let their houses drain. And standing water like that for weeks in those homes is going to absolutely destroy them. (I think they are going to have good eminent domain claims. If so, the Corps is going to crap because houses in some of these neighborhoods are worth $10-$20 mil apiece.)

    But my point is, “government solutions” can be as much or more of the problem than mother nature. If the government let mother nature haver her way, at least my neighbors could be repairing their homes. Now, they are going to have to wait and replace them. If you give that power to the government, then you need to constrain its decision making so that is makes the right decisions.

    • Indeed. I have some very strong opinions about some of the…things the Army Corps and Bureau of Reclamation do for water management. This being a family blog, I keep those thoughts to myself.

      • Then perhaps we should be cautious about suggesting governmental solutions, since that is to whom the federal government will entrust those solutions. At least at the federal level.

        • As I said in my reply to Mr. Dyer above, I listed governmental solutions because those are what people think of first, and what will be considered or set aside first, unfortunately. In an ideal world, hydrologists, civil engineers, developers, residents, utilities, and other interested parties would organize on a watershed level, look at the landscape and the hydrology, propose ideas, and bat things back and forth, then model (if possible) the most feasible solutions, and see about how to implement them with municipal, county, and state governments, and the federal government only as pertains to truly federal-level considerations (keeping waterways open like the Intracoastal Waterway).

          However, in the world we currently live in, the federal government has managed navigable waterways since the early 1800s, and took on itself management of swamps and other wetlands in the 1970s (originally for wildlife preservation). Taking that control back at a local and watershed level is both rare and difficult.

  11. While disaster can always strike, there are known steps to mitigate the problems.

    I read, years ago, that the Chinese had a simple system for judging the government in one region, where the river tended to flood: the river should be kept low (dredged) and the walls (dykes? levees? I’m not sure of the correct term), high.

    While the geography of Houston may make this harder, it does sound as if this obvious advice was neglected.

    Near where I live (not Texas), one city ignored a problem, where a street often flooded until with a very heavy rain, houses were damaged. The stream overflowed; after the fact, dredging was considered urgent.

    If I remember correctly, on Long Island, which is rather flat, when a new housing development was built, at least one lot was fenced and dug out as l large pit, for storm water collection. This would not work at sea level, but it would work in parts of Houston and help control the water; Long Island is not very much above sea level.

    One can only hope people learn from the floods; this is an illustration of the old saying:

    We get old too soon and smart too late.

    • Your ‘pit’ solution is standard practice in new development. They are called retention or detention ponds, depending on their purpose and what they do with the water. If they don’t have a pond at a local development, they have a series of large pipes buried under the parking lot.

      In Portland, Oregon, they dug large holding tanks at every intersection and use them as storm water infrastructure.

      Like any improvement built for remediation, the cost was high.

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