The floods of 1929 and 1935 in the White Oak and Buffalo Bayou watersheds devastated downtown Houston.  They led to the creation of the Harris County Flood Control District and the Congressional authorization and appropriation of federal dollars to the US Army Corps of Engineers (USACE) to devise a plan to address flooding the Houston area.

The 1940 plan included Addicks, Barker, and White Oak reservoirs; a diversion levee along Cypress Creek (to prevent its water from overflowing into the areas draining to Addicks and Barker; the channelization of about 7 miles of Buffalo Bayou (between what is now Highway 6 and about S. Piney Point); a 2 mile long Brickhouse Gully Bypass Channel; a 25 mile long North Canal; and a 35 mile long South Canal (see plan below).

Can you identify which parts of the plan were constructed and which parts were not?

The levees built to create Addicks and Barker are not like traditional dams. Traditional dams, in places where the ground surface elevation changes radically (think Hoover Dam on the Colorado River near Las Vegas), dams can be 100’s of feet tall and made from reinforced concrete.  The water pressure against the bottom of the Hoover Dam at max pool depth is 255 pounds per square inch.  Addicks and Barker are very squat comparatively. They are made from compacted layers of dirt in a backwards facing letter “C” configuration.  They were built on ground that gently slopes to the northwest, but they were built to have horizontal tops at the same or similar elevations (except the ends).  The ends were tapered to meet the ground.  These ends were covered in a protective layer of concrete so the tapered portions could serve as emergency spillways in the event that the water collected behind the levees got too high.  Each taper is at a different elevation to make sure the rate of spillage only gradually increases (since all of the water is going to the same bayou) instead of gushing out both sides at once.  See the diagram below.

The following two diagrams provide a close-up, cross-sectional view of one of the levees. The first shows the viewing direction of each of the cross-sections, A-A and B-B.

The second shows the actual views, which hypothetical water levels illustrating the emergency spillway operation.

Many have asked me why residential and commercial development was allowed to occur to the west of the impoundment area. I think there were and are a variety of reasons.

First, the impoundment area was originally sized with the expectation that the blue conveyance systems shown in the graphic above would be constructed and that the White Oak Reservoir would be constructed to reduce the volume of water conveyed by the lower section of Buffalo Bayou. This led to the federal government only buying the acreage they thought they needed – given the discharge capacity of the blue conveyance system.

Second, as time wore on, and the city grew, the privately owned land to the west of each reservoir was not purchased by any government entity for the purpose of flood damage mitigation.  If this had been done, the land could have been set aside for that purpose.

Third, if there is one thing I’ve learned about Texans, after living here for 20 years, they value their individual property rights. This culture made it unlikely that any local government would restrict land use in the land upstream of the reservoir land.

Fourth, our laws and regulations governing real estate transactions don’t appear to require disclosure of information about flood risk unless the risk is equal to or greater than a 1% annual chance of flooding. This means that thousands of people bought homes in the area just west of the federally owned land and did not know about the flood risk associated with those locations.  Although the risk is less than 1% each year (which, as I written elsewhere, gives your home a 26% chance of flooding during a 30 year mortgage), it’s not zero.

Others have asked me why the rest of the 1940 plan was not built. I don’t know the actual answer to that question, but I suspect World War II may have had something to do with it. Also, it’s possible that some combination of local funding and Congressional funding did not materialize as planned.

Over the last week there’s been some discussion about making the levees higher, excavating the inundation pool areas deeper, or a combination of both. What do you think about these ideas?