Flooding in Houston

Houston is just one of the downstream ends of Texas river basins. Our climate also draws moisture up from the Gulf of Mexico, bringing an average of over 50 inches of rain per year, including tropical storms carrying large quantities of rain in just a few days.

Major flooding in Houston usually happens when runoff from upstream meets heavy rain in the area. Minor flooding, however, is extremely common, especially on roadways and in low-lying areas.

It may seem strange or even counterproductive for the roads to be used as channels for drainage. However, the soil in Houston is heavy with clay, and the land is mostly flat. Collecting water and directing it downstream is the only way to get rid of it when floods come.

The city is very dependent on cars and large portions of the city are paved, making absorption of heavy rainfall still more difficult. Since flooding has been worse in recent years, rainfall detention ponds are now required features of new construction to help compensate for impervious concrete.

It's tempting to blame flooding in Houston on recent climate change, but our area has flooded from the earliest days of its founding.

“We had advance notice that the road [from Houston, going west] to the Brazos was bad, owing to continuous rains. This information was correct. Hardly had we left the city when the flat Houston prairie loomed up as an endless swamp. Large puddles of water followed one another and at several places a large section of land was under water. … The wagons sank deeply into the mud, compelling the horses to pull them slowly, step by step. Often the wagons became so mired that it required the help of member of our company to push them out of a bog.”

Ferdinand Roemer writes of his 1845-1847 journey in Texas: mit besonderer rücksicht auf deutsche auswanderung und die physischen verhältnisse des landes... (Oswald Mueller, trans., 1935).

Hours later, after leaving one wagon behind for the night, the travelers camped at Piney Point. The next day they found it impossible to ford Buffalo Bayou, and had to detour around the flooding.

History records sixteen major floods in Harris County from 1836 to 1936. Studies suggested that the city (now the downtown area), situated at the confluence of Buffalo and White Oak bayous, could be better protected from flooding if water from upstream could be detained.

The December, 1935 flood was a turning point in Houston's history. Water in the downtown area rose over 40 feet above mean tide level. Even 12 miles to the west, far into the countryside, Jesse and Edith Moore were rescued from the roof of their log cabin, the waters of nearby Rummel Creek having risen as much as 20 feet.

Responding to the successive floods of 1929 and 1935, the State established the Harris County Flood Control District, and US Congress authorized Federal spending for reservoirs to protect the city. The US Army Corps of Engineers constructed Barker Dam on Buffalo Bayou during the war, between 1942 and 1945. Addicks Dam, which holds back water from tributary Langham Creek, was finished in 1948. Nothing was built to retain White Oak Bayou.

The original design of the reservoirs included an open spillway. At the time, these dams were some 15 miles away from the city center and were surrounded by fields, ranches, dairies and sawmills. As the city grew, the risk of flooded fields turned into the risk of hundreds of flooded homes. In the 1960s, floodgates were added so that outflow from the reservoirs could be limited, and urban property better protected.

Each dam holds back stormwater in a reservoir. Each has a gated outlet (“spillway”) which releases water into Buffalo Bayou, where it will flow to the Houston Ship Channel and on to the Gulf of Mexico. When heavy rain is expected, the outlets’ floodgates are closed. Water is allowed to pool and is released after the rain has subsided, with the goal of emptying the reservoir as fast as practical before the next rain.

In normal operation, while water remains in the reservoirs, it is released to achieve a flow rate of no more than 2,000 cubic feet per second (~56.6 m³/s), measured downstream at the Piney Point flow sensor.

There seems to be some margin for error above that value. After the Memorial Day flood of 2015, 3,000 cubic feet per second were released; after the Tax Day flood of 2016, it was 3,700. From the Flooding Impacts document: “Some streets and homes downstream of the reservoirs flood when the combined release rate from the reservoirs exceeds approximately 4,000 cubic feet per second.”

In the event a reservoir fills up all the way, water can flow uncontrolled around the ends of the dam, which are designed to be auxiliary spillways. This never happened until Hurricane Harvey, when water flowed over the auxiliary spillway at the north side of Addicks Dam.

This may concern the Kempwood homeowner because Addicks Reservoir is due west of Kempwood. However, drainage ditches were established outside the dam to direct overflow. The more favorable slope is south and then westward (toward Turkey Creek). The alternate is east through Spring Branch, alongside Emnora, then zigzagging south to meet the creek system that leads to the Spring Branch waterway.