Watershed Texas

There are many things that we must think about as we begin to consider how to rebuild Houston. But one thing stands above the rest, literally: elevation. Elevation is the number one predictor of flooding and flood damage.  Water seeks the low spots; we need to seek the high spots. It is just that simple.  Elevation needs to be our watchword.  Elevation needs to be the metric but which we gauge all new development as well as all redevelopment.

Elevation is about getting people and structures above the level of the floodwaters. All of the practices and policy issues that involve getting people out of harm’s way involve elevation.

Elevation as the number one issue holds for all kinds of flooding. Storm surge requires the highest degree of elevation. Elevating houses to greater the 15-17 feet above the surface on barrier islands is required to withstand all but the strongest surges.  Harvey, however, was not about storm surge; it was about inland flooding.

We have two kinds of inland flooding—overbank flooding from bayous and creeks, and what is sometimes called street flooding or sheet flow flooding. Let’s examine how elevation works in each kind.

Overbank Flooding

Overbank flooding is pretty straightforward. Waters rise in the bayous and creeks, and spill out into the river valley, or what we most commonly call the floodplain. These incised river valleys have existed since time immemorial, and they can be fairly deep—8-10 feet and deeper. These are the areas where we see people wading in chest-deep water. These lowlands are where the water rescues occur.

There are a few ways we can elevate out harm’s way in the floodplains. No.1, for new buildings we can elevate by moving laterally to high ground away from the floodplain. This is the simplest remedy of all. Just don’t build in the floodplain.  When I say floodplain, I mean ALL the floodplains—100 yr and 500 yr. And if we really want to be safe, add a few feet of elevation above the 500 yr floodplain. Many if not most experts in the field now recognize that relying on the 100-year floodplain, as it is currently delineated, may not provide adequate protection. It is very common to see flooding on the 500 yr floodplain, and really not uncommon to see floods at even higher levels. A prudent approach therefore would be to elevate well above the 500 year floodplain.

The second way to elevate out of a flood zone is to elevate vertically, with the house over but not in the floodplain. This would involve building on some kind of fairly high piers or stilts. Not so hard to do for new homes, but a pretty risky proposition for homes built on slabs at grade. Very expensive, and slabs on grade were not designed to be supported on stilts.

The third way to elevate is to lower the 100-year floodplain. This is the classic engineering approach—straighten the channel, harden the banks, add more retention and conveyance, etc. The idea is to increase drainage and narrow the size of the floodplain.  This approach has the “advantage” of taking land out of the official 100-year floodplain, making it available for development. Land is expensive, and there is thus an obvious attraction to reducing the floodplain. There is however, a growing chorus of critics for this approach. It is argued that leaving ample room for the river to meander may be the safest, cheapest, and most effective approach. Seeing the damage from Harvey does inspire some serious reconsideration of whether or not engineering to the 100-yr standard has any validity anymore. It is very expensive to engineer our drainage systems to a greater than 100-yr standard.  It appears that it would be very much cheaper just to give the river room to meander.

A perverse way of lowering the floodplain is to defend against the floodwaters with levees or dikes. We have several areas like this in the region, most notably in the Brazos bottom near Sugar Land in Ft. Bend County.  We saw these levees stressed, perhaps nearly to the breaking point, with Harvey.  A number of experts question the wisdom of building in the river bottom, where levees are required, with so much other well-drained land available.

Street and Sheet Flow Flooding

Street flooding is the kind of flooding we experience when our streets fill with water, and is perhaps the most common kind of all since our streets are designed to channel water. A five-inch storm that occurs over a few hours is not at all uncommon here and it will quickly fill the streets from gutter to gutter. Similar to this is sheet-flow flooding that often occurs in the upper reaches or headwaters of the drainage system.  It appears to me that there was a whole lot of this kind of flooding during Harvey. Not particularly deep, but if the floor of your house is at or near the grade of the street, you are going to get water in the house with a storm like Harvey, or even the Tax Day floods. Elevation make all the difference in these kinds of areas.

Sheet flow and street flooding areas are unlikely to be designated as FEMA floodplains. This kind of flooding is not as serious as overbank flooding, but it  can be a very serious nuisance. Two or three inches of water in your house may as well be two to three feet given the amount of labor to dry things out and to redo the sheetrock.

Elevating a lot with fill dirt, for new construction, could raise these areas above all but the worst sheet flow flooding. Elevation can easily be raised 2-3 feet above grade with fill dirt.  Building on a pier and beam foundation could raise the structure another 3-4 feet above the ground.  Just about anywhere in the Houston region, at least outside the floodplains, could easily be elevated like this out of harm’s way.

Buy Outs

Buying up damaged houses in the floodplains and other low lying areas in a sense is elevating these houses out of the floodplain, even though they may be destroyed in the process. We need a serious program to buy out houses deep in the floodplain. Some thought will have to be given to the best way to do this as it can be traumatic for families to lose their homes this way. But the buyout should enable them to buy or build on much higher ground. We need to be thinking in terms of several tens of thousands of homes, not just the hundreds or few thousands that buyout programs have dealt with in the past.

Elevating is not going to be feasible everywhere. Very many people are going to want to stay in their damaged houses, particularly if the damage was minor. It may not be worth buying them out—until the next big one strikes, anyway.

Choices

We have the most choices to make for new development. This is important because half of the built environment of 2040 has yet to be built. Let’s make elevation the key priority at least for new development. There is plenty of land outside the floodplains that we can build on. There is enough good and usable land that we could easily withstand another Harvey without the catastrophe we recently lived through, if elevation were our mantra. And this includes protecting sensitive natural areas and prime farmland.  This would not be hard to do. Having the political will to do so is another story. Our future as a great city, however, depends on us generating that political will. Can we rise to the occasion?