It\u2019s not because the water comes in. It\u2019s because it is forced to leave again.<\/b>
\nFloods cause greater property damage and more deaths than tornadoes or hurricanes. And Houston\u2019s flood is truly a disaster of biblical proportions: The sky unloaded 9 trillion gallons of water on the city within two days, and much more might fall before Harvey dissipates, producing as much as 60 inches of rain.
Pictures of Harvey\u2019s runoff are harrowing, with interstates turned to sturdy and mature rivers. From Katrina to Sandy, Rita to T\u014dhoku, it\u2019s easier to imagine the flooding caused by storm surges wrought by hurricanes and tsunamis. In these cases, the flooding problem appears to be caused by water breaching shores, seawalls, or levees. Those examples reinforce the idea that flooding is a problem of keeping water out \u2014either through fortunate avoidance or engineering foresight.
But the impact of flooding, particularly in densely developed areas like cities, is far more constant than a massive, natural disaster like Harvey exposes. The reason cities flood isn\u2019 t because the water comes in, not exactly. It\u2019s because the pavement of civilization forces the water to get back out again.
* * *
There are different kinds of floods. There\u2019s the storm surge from hurricanes, the runoff from snowmelt, the inundation of riverbanks. But all these examples cast flooding as an occasional foe out to damage human civilization. In truth, flooding happens constantly, in small and large quantities, every time precipitation falls to earth. People just don\u2019 t tend to notice it until it reaches the proportions of disaster.
Under normal circumstances, rain or snowfall soaks back into the earth after falling. It gets absorbed by grasslands, by parks, by residential lawns, by anywhere the soil is exposed. Two factors can impede that absorption. One is large quantities of rain in a short period of time. The ground becomes inundated, and the water spreads out in accordance with the topography. The second is covering over the ground so it cannot soak up water in the first place. And that\u2019s exactly what cities do\u2014they transform the land into developed civilization.
Roads, parking lots, sidewalks, and other pavements, along with asphalt, concrete, brick, stone, and other building materials, combine to create impervious surfaces that resist the natural absorption of water. In most of the United States, about 75 percent of its land area, less than 1 percent of the land is hardscape. In cities, up to 40 percent is impervious.
The natural system is very good at accepting rainfall. But when water hits pavement, it creates runoff immediately. That water has to go somewhere. So it flows wherever the grade takes it. To account for that runoff, people engineer systems to move the water away from where it is originally deposited, or to house it in situ, or even to reuse it. This process\u2014the policy, planning, engineering, implementation, and maintenance of urban water systems\u2014is called stormwater management.
According to my Georgia Institute of Technology colleague Bruce Stiftel, who is chair of the school of city and regional planning and an expert in environmental and water policy governance, stormwater management usually entails channeling water away from impervious and the structures built atop them. In other words, cities are built on the assumption that the water that would have been absorbed back into the land they occupy can be transported away instead.
Like bridges or skyscrapers designed to bear certain loads, stormwater management systems are conceived within the limits of expected behavior\u2014such as rainfall or riverbank overrun events that might happen every 10 or 25 years. When these intervals are exceeded, and the infrastructure can\u2019 t handle the rate and volume of water, flooding is the result.
Houston poses both a typical and an unusual situation for stormwater management. The city is enormous, stretching out over 600 square miles. It\u2019s an epitome of the urban sprawl characterized by American exurbanism, where available land made development easy at the edges. Unlike New Orleans, Houston is well above sea level, so flooding risk from storm surge inundation is low. Instead, it\u2019s rainfall that poses the biggest threat.
A series of slow-moving rivers, called bayous, provide natural drainage for the area. To account for the certainty of flooding, Houston has built drainage channels, sewers, outfalls, on- and off-road ditches, and detention ponds to hold or move water away from local areas. When they fill, the roadways provide overrun. The dramatic images from Houston that show wide, interstate freeways transformed into rivers look like the cause of the disaster, but they are also its solution, if not an ideal one. This is also why evacuating Houston, a metropolitan area of 6.5 million people, would have been a terrible idea. This is a city run by cars, and sending its residents to sit in gridlock on the thoroughfares and freeways designed to become rivers during flooding would have doomed them to death by water.
* * *
Accounting for a 100 year, 500 year, or \u201cMillion year\u201d flood, as some are calling Harvey\u2019s aftermath, is difficult and costly. Stiftel confirms that it\u2019s almost impossible to design for these \u201cmaximal probable flood events, \u201d as planners call them. Instead, the hope is to design communities such that when they flood, they can withstand the ill effects and support effective evacuations to keep people safe. \u201cThe Houston event seems like an illustration that we haven\u2019 t figured it out, \u201d Stiftel says.
Many planners contend that impervious surface itself is the problem. The more of it there is, the less absorption takes place and the more runoff has to be managed. Reducing development, then, is one of the best ways to manage urban flooding. The problem is, urban development hasn\u2019 t slowed in the last half century. Cities have only become more desirable, spreading outward over the plentiful land available in the United States.
The National Flood Insurance Program, established in 1968, offered one attempt at a compromise. It was meant to protect and indemnify people without creating economic catastrophe. Instead of avoiding the floodplain, insurance allowed people to build within it, within management constraints recommended by FEMA. In theory, flood-hazard mitigation hoped to direct development away from flood-prone areas through the disincentives of risk insurance and regulatory complexity.
Since then, attitudes have changed. For one part, initial avoidance of floodplains created desirable targets for development, especially in the middle of cities. But for another, Stiftel tells me that attitudes about development in floodplains have changed, too. \u201cIt\u2019s more about living with water than it is about discouraging development in areas prone to risk.\u201d
Sometimes \u201cliving with water\u201d means sidestepping the consequences. Developers working in flood zones might not care what happens after they sell a property. That\u2019s where governmental oversight is supposed to take over. Some are more strict than others. After the global financial crisis of 2008, for example, degraded local economies sometimes spurred relaxed land-use policy in exchange for new tax bases, particularly commercial ones.
In other cases, floodplains have been managed through redevelopment that reduces impervious surfaces. Natural ground cover, permeable or semi-permeable pavers, and vegetation that supports the movement of water offer examples. These efforts dovetail with urban redevelopment efforts that privilege mixed-use and greenspace, associated with both new urbanism and gentrification. Recreation lands, conservation lands and easements, dry washes, and other approaches attempt to counterbalance pavement when possible. Stiftel cites China\u2019s \u201c sponge cities \u201d as a dramatic example\u2014a government-funded effort to engineer new, permeable materials to anticipate and mitigate the flooding common to that nation.
* * *
But Thomas Debo, an emeritus professor of city planning at Georgia Tech who also wrote a popular textbook on stormwater management, takes issue with pavement reduction as a viable cure for urban flooding. \u201cWe focus too much on impervious surface and not enough on the conveyance of water, \u201d he tells me. Even when reduced in quantity, the water still ends up in pipes and concrete channels, speeding fast toward larger channels. \u201cIt\u2019s like taking an aspirin to cure an ailment, \u201d he scoffs. Houston\u2019s flooding demonstrates the impact.
Instead, Debo advocates that urban design mimic rural hydrology as much as possible. Reducing impervious surface and improving water conveyance has a role to play, but the most important step in sparing cities from flooding is to reduce the velocity of water when it is channelized, so that it doesn\u2019 t deluge other sites. And then to stop moving water away from buildings and structures entirely, and to start finding new uses for it in situ.
That can be done by collecting water into cisterns for processing and reuse\u2014in some cases, Debo explains, the result can even save money by reducing the need to rely on utility-provided water. Adding vegetation, collecting stormwater for reuse, and building local conveyance systems for delivery of this water offer more promising solutions.
Though retired from Georgia Tech, Debo he still consults on the campus\u2019s local stormwater management efforts. In one case, the institute took a soccer field and made it into an infiltration basin. Water permeates the field, where it is channeled into pipes and then into local cisterns.
In Houston\u2019s case, catastrophic floods have been anticipated for some time. The combination of climate change, which produces more intense and unpredictable storms, and aggressive development made an event like this week\u2019s almost inevitable. The Association of State Floodplain Managers has called for a national flood risk management strategy, and the Houston Chronicle has called flood control the city\u2019s \u201cmost pressing infrastructure need.\u201d A lack of funding is often blamed, and relaxed FEMA regulations under the Trump Administration won\u2019 t help either.
But for Debo and others, waiting for a holistic, centralized approach to stormwater management is a pipe dream anyway. Just as limiting impervious is not the solution to urban stormwater management, so government-run, singular infrastructure might not be either. \u201cIt\u2019s much more difficult, and a much bigger picture, \u201d Debo insists to me. \u201cThere is no silver bullet for stormwater management.\u201d
* * *
One problem is that people care about flooding, because it\u2019s dramatic and catastrophic. They don\u2019 t care about stormwater management, which is where the real issue lies. Even if it takes weeks or months, after Harvey subsides, public interest will decay too. Debo notes that traffic policy is an easier urban planning problem for ordinary folk, because it happens every day.
So does stormwater\u2014it just isn\u2019 t treated that way. Instead of looking for holistic answers, site-specific ones must be pursued instead. Rather than putting a straight channel through a subdivision, for example, Debo suggests designing one to meander through it, to decrease the velocity of the water as it exits.
The hardest part of managing urban flooding is reconciling it with Americans\u2019 insistence that they can and should be able to live, work, and play anywhere. Waterborn transit was a key driver of urban development, and it\u2019s inevitable that cities have grown where flooding is prevalent. But there are some regions that just shouldn\u2019 t become cities. \u201cParts of Houston in the floodway, parts of New Orleans submerged during Katrina, parts of Florida\u2014these places never should have been developed in the first place, \u201d Debo concludes. Add sea-level rise and climate-change superstorms, and something has to give.
Debo is not optimistic about resisting the urge toward development. \u201cI don\u2019 t think any of it\u2019s going to happen, \u201d he concedes. \u201cUntil we get people in Congress and in the White House who care about the environment, it\u2019s just going to get worse and worse.\u201d
Even so, there\u2019s reason for optimism. If good stormwater management means good, site-specific design, then ordinary people have a role to play, too. Residential homeowners who install a new cement patio or driveway might not even realize that they are channeling water down-grade to their neighbors, or overwhelming a local storm drain. Citizens can also influence stormwater issues within their municipalities. Many folks know that they can have a local city councils or school boards, but local planning, zoning, and urban design agencies also hold regular public meetings\u2014unfortunately, most people only participate in this aspect of local governance when they have an axe to grind. For the average American concerned with the deluge, the best answer is to replace an occasional, morbid curiosity with flooding with a more sophisticated, long-term interest in stormwater management.<\/p>\n

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