Harvey and Houston

The news for the past several days has centered around Hurricane Harvey and the unprecedented flooding produced by rainfalls of over 40” in much of the Houston area. Part of Texas by Tuesday morning, August 29, had received over 49” a new record for the landfall of a tropical storm on the mainland With rainfalls at such levels no amount of planning or engineering design could handle what is often termed, to use planning language, the “maximal probable flood event.” Of course, at 49” in my mind it is beyond the probable stage (one location set a new record for rainfall on the US mainland from one storm, at over 51” and approaching the record set in Hawaii at one location). The havoc happening in Houston is mainly the result of a few factors. This post will explore those factors and comment on whether some changes may have mitigated some of the flooding.

Houston is the fourth largest city in the nation with over 2.3 million persons occupying a large land mass of 667 square miles. For comparison purposes, the state of Rhode Island is 1,212 sq mi, or about twice the size of this one Texas city. I guess everything is bigger in Texas. Houston has a strong connection to oil. In fact almost 30% of all crude oil for the United States is refined in the Houston region. As goes Harvey so go the nation’s gas prices, and the effect so far has been a few cents, but could become larger depending upon how long the refineries are off line. What is rather unique about Houston is, it is quite flat. The greatest topographic variation, is 88 feet--yes over the whole city the maximum variation between the highest ground elevation and the lowest ground elevation is only 88 feet. I doubt any Houston streams have a waterfall. To think, we Wisconsinites thought Illinois was flat. Houston’s downtown sits about 50’ above sea level, and the south area of the city, closer to the Gulf of Mexico, is only about 40’ or so above sea level. In its downtown there is only a four foot difference in elevation. Areas in the north part of the city are higher in elevation. Being flat can make some things easier, but the lack of terrain also provides several engineering challenges in regard to storm water management. Houston does have some detention basins, and in flat areas, the basins have to be larger since there is not much ability to have depth to allow outfall. The acre-feet for storm water storage has to be met by a pond with a larger surface area. Detention ponds have been common for over thirty years in order to offset the flooding effects of development. Quite simply, when water hits pavement or hard surfaces it has to runoff, it cannot infiltrate. This leads to more flooding downstream. Detention ponds are often sized to accommodate the 100 year event. Rainfall from Harvey is well above the 100 year and 500 year event. A 100 year event actually means the storm has a 1% chance of occurring every year, or the idea is to be expected one time in a span of 100 years. But, flat land also leads to ponding of water in large storm events. When land is so flat water does not flow sufficiently fast to get to the detention ponds, or other storage areas. It takes a long time to drain out, and in the meantime it ponds. It is a simple matter of physics.

Houston Interchange

One problem Houston has is they do not have sufficient storage for the amount of impervious surfaces. Houston has two large dry reservoirs to store floodwaters. They were to create a third, but that land area was developed. They also lack sufficient upstream detention to help offset the flooding of streams. A landscape architect I know who is doing work in Texas said they really have no sense of storm water management. In a sense Houston bears some of the blame for the situation in which it finds itself. It is hard to blame anyone for having problems dealing with such a large amount of rainfall, but Houston also finds itself being unable to properly handle a 100 year flood. Designing for that event is common in Wisconsin and Minnesota. Such large storm events seem to be occurring with greater frequency. There is a reason for infrastructure planning, and Houston is a perfect example of that need. A development surely wants sufficient water supply and sanitary sewer service, but in Texas they seem to care little about storm water management.

Flooded Highway

Besides using detention ponds, Houston’s well known massive freeway systems are designed to help store storm water from these large events. The freeway is in effect a large detention pond when more than the 100 year event occurs. This helps explain the photographs of underwater freeways. About a decade ago when the Lake Delton dam broke from heavy rainfall (two back to back 100 year events) in the Baraboo—Wisconsin Dells area, the unthinkable for Wisconsin happened, a section of I-90/94 in between Baraboo and Portage was closed due to flooding. The bayous of Houston contain streams and creeks that help carry water away, but by definition they relatively flat and water drains slowly, which compounds the problem of moving water away. Some may say the answer is in decreasing impervious surfaces, but that can lead to other issues. One has to be cognizant of ground water levels, as water that goes in the ground will generally come out somewhere. Bio-infiltration facilities are popular today. These large rain gardens store water and native vegetation opens pores in the soil to allow better infiltration, but also to undertake transpiration. Bio-infiltration facilities will have water overflow out of the facility when a certain rainfall level (perhaps 100 year event) has flowed to the basin. Showing there is always a downside, use of detention ponds can cause problems, such as stream scouring due to more water being released. Even though the water is released at the pre-development rate, the impervious surfaces means more water is in the pond that has to be released and that water is then released over a greater time frame, which is not natural. Hydrologists are now finding negative effects to the stream and more erosion due to the greater volume of water over a greater period of time than in nature. Yet, with other aspects of development t becomes a balancing act.

Houston Flooding

Houston is also the largest city in the nation to not have municipal zoning. As one writer said, “It is an epitome of urban sprawl characterized by American exurbanism.” Zoning is more than simply regulating land uses. Properly accomplished it also regulates where development should or should not occur. Knowing that the nation likes growth, the federal government intended to strike a balance for floodplains by allowing some development in a floodplain in exchange for flood insurance. A Faustian bargain, and many in Houston will find out the downside when FEMA will not provide assistance to them since they never took out their flood insurance. By contrast, the city where I am the city planner, has only one building in a mapped 100 year floodplain, and that is a small storage building. There is one multi-family building in a mapped 500 year flood plan, a building constructed in 1971 before floodplain zoning came into existence. One aspect of good planning is to limit susceptibility to disasters. Unlike a tornado, flooding areas can be determined. It is a simple aspect of city planning to first examine where development should not occur; in other words locate the limitations. Some may not like the natural and cultural limitations, but such challengesforce more planning, engineering, and more creativity. Wetlands are often thought of as wasteland, but they do provide some major hydrologic functions--such as natural storage of storm water. Houston is not the first city to be built on wetlands or in floodplains. But have they learned their lesson? By some accounts, Houston now has 50% less wetlands in the city as existed 25 years ago. They should have known better within the last 25 years, but apparently did not. Development pressure likely led to wetland loss, just as they developed the area in which they were going to build a storm water reservoir. The common good being sacrificed for profit. Literature suggests that after the recession of 2008 many cities, and apparently Houston too, sacrificed environmental concerns for the sake of development.

Detention pond, not in Houston

With the amount of rainfall Harvey has handed Houston, the loss of the wetlands would not have solved the current flooding, but it may well have prevented some areas from experiencing flooding. Houston is a large city in both population and area. It is an economic engine, and while some would argue its ability to not have certain environmental restraints allowed for its development and its being an economic catalyst. But, in being such an economic engine it means more responsibility is required. More care should be taken to mitigate and limit susceptibility to disaster. The lack of concern for the environment has a way of coming back to bite people who wish to ignore the earth. Will Harvey wake Houston up to the need for better planning and regulation?

Bio infiltration facility