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12/16/2020 Houston's Flood Is a Design Problem - The Atlantic

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RICHARD CARSON / REUTERS

Floods cause greater property damage and more deaths than tornadoes or hurricanes.

And Houston’s �ood is truly a disaster of biblical proportions: e 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’s runoff are harrowing, with interstates turned to sturdy and mature

rivers. From Katrina to Sandy, Rita to Tōhoku, it’s easier to imagine the �ooding caused

by storm surges wrought by hurricanes and tsunamis. In these cases, the �ooding

problem appears to be caused by water breaching shores, seawalls, or levees. ose

examples reinforce the idea that �ooding is a problem of keeping water out—either

through fortunate avoidance or engineering foresight.

But the impact of �ooding, particularly in densely developed areas like cities, is far more

constant than a massive, natural disaster like Harvey exposes. e reason cities �ood

T E C H N O L O GY

Houston's Flood Is a Design Problem It’s not because the water comes in. It’s because it is forced to leave again.

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12/16/2020 Houston's Flood Is a Design Problem - The Atlantic

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isn’t because the water comes in, not exactly. It’s because the pavement of civilization

forces the water to get back out again.

* * *

ere are different kinds of �oods. ere’s the storm surge from hurricanes, the runoff

from snowmelt, the inundation of riverbanks. But all these examples cast �ooding as an

occasional foe out to damage human civilization. In truth, �ooding happens constantly,

in small and large quantities, every time precipitation falls to earth. People just don’t

tend to notice it until it reaches the proportions of disaster.

R E C O M M E N D E D R E A D I N G

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. e ground becomes inundated, and the water spreads out in

accordance with the topography. e second is covering over the ground so it cannot

soak up water in the �rst place. And that’s exactly what cities do—they 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.

e natural system is very good at accepting rainfall. But when water hits pavement, it

creates runoff immediately. at water has to go somewhere. So it �ows wherever the

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12/16/2020 Houston's Flood Is a Design Problem - The Atlantic

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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. is

process—the policy, planning, engineering, implementation, and maintenance of urban

water systems—is 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 surfaces 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—such as rainfall or

riverbank overrun events that might happen every 10 or 25 years. When these intervals

are exceeded, and the infrastructure can’t handle the rate and volume of water, �ooding

is the result.

Houston poses both a typical and an unusual situation for stormwater management.

e city is enormous, stretching out over 600 square miles. It’s 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 �ooding risk

from storm surge inundation is low. Instead, it’s 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 �ooding, 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 �ll, the roadways provide overrun. e 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. is is also why

evacuating Houston, a metropolitan area of 6.5 million people, would have been a

terrible idea. is is a city run by cars, and sending its residents to sit in gridlock on the

thoroughfares and freeways designed to become rivers during �ooding would have

doomed them to death by water.

* * *

Accounting for a 100-year, 500-year, or “million-year” �ood, as some are calling

Harvey’s aftermath, is difficult and costly. Stiftel con�rms that it’s almost impossible to

design for these “maximal probable �ood events,” as planners call them. Instead, the

hope is to design communities such that when they �ood, they can withstand the ill

effects and support effective evacuations to keep people safe. “e Houston event seems

like an illustration that we haven’t �gured it out,” Stiftel says.

12/16/2020 Houston's Flood Is a Design Problem - The Atlantic

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Many planners contend that impervious surface itself is the problem. e 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 �ooding. e

problem is, urban development hasn’t slowed in the last half-century. Cities have only

become more desirable, spreading outward over the plentiful land available in the

United States.

e 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 �oodplain, insurance allowed people to build

within it, within management constraints recommended by FEMA. In theory, �ood-

hazard mitigation hoped to direct development away from �ood-prone areas through

the disincentives of risk insurance and regulatory complexity.

Since then, attitudes have changed. For one part, initial avoidance of �oodplains created

desirable targets for development, especially in the middle of cities. But for another,

Stiftel tells me that attitudes about development in �oodplains have changed, too. “It’s

more about living with water than it is about discouraging development in areas prone

to risk.”

Sometimes “living with water” means sidestepping the consequences. Developers

working in �ood zones might not care what happens after they sell a property. at’s

where governmental oversight is supposed to take over. Some are more strict than

others. After the global �nancial 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, �oodplains 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. ese efforts dovetail

with urban redevelopment efforts that privilege mixed-use and green space, associated

with both new urbanism and gentri�cation. Recreation lands, conservation lands and

easements, dry washes, and other approaches attempt to counterbalance pavement

when possible. Stiftel cites China’s “sponge cities” as a dramatic example—a

government-funded effort to engineer new, permeable materials to anticipate and

mitigate the �ooding common to that nation.

* * *

But omas 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 �ooding. “We focus too much on impervious

surface and not enough on the conveyance of water,” he tells me. Even when reduced in

quantity, the water still ends up in in pipes and concrete channels, speeding fast toward

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larger channels. “It’s like taking an aspirin to cure an ailment,” he scoffs. Houston’s

�ooding 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 �ooding is to reduce the velocity of water

when it is channelized, so that it doesn’t deluge other sites. And then to stop moving

water away from buildings and structures entirely, and to start �nding new uses for it in

place.

at can be done by collecting water into cisterns for processing and reuse—in some

cases, Debo explains, the result can even save money by reducing the need to rely on

utility-provided water. Adding vegetation, reclaiming stormwater, and building local

conveyance systems for delivery of this water offer more promising solutions.

ough retired from Georgia Tech, Debo still consults on the campus’s local

stormwater management efforts. In one case, the institute took a soccer �eld and made

it into an in�ltration basin. Water permeates the �eld, where it is channeled into pipes

and then into local cisterns.

In Houston’s case, catastrophic �oods have been anticipated for some time. e

combination of climate change, which produces more intense and unpredictable

storms, and aggressive development made an event like this week’s almost inevitable.

e Association of State Floodplain Managers has called for a national flood risk-

management strategy, and the Houston Chronicle has called �ood control the city’s

“most pressing infrastructure need.” A lack of funding is often blamed, and relaxed

FEMA regulations under the Trump Administration won’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 surface is not the

solution to urban stormwater management, so government-run, singular infrastructure

might not be either. “It’s much more difficult, and a much bigger picture,” Debo insists

to me. “ere is no silver bullet for stormwater management.”

* * *

One problem is that people care about �ooding, because it’s dramatic and catastrophic.

ey don’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—it just isn’t treated that way. Instead of looking for holistic

answers, site-speci�c ones must be pursued instead. Rather than putting a straight

12/16/2020 Houston's Flood Is a Design Problem - The Atlantic

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channel through a subdivision, for example, Debo suggests designing one to meander

through it, to decrease the velocity of the water as it exits.

e hardest part of managing urban �ooding is reconciling it with Americans’

insistence that they can and should be able to live, work, and play anywhere.

Waterborne transit was a key driver of urban development, and it’s inevitable that cities

have grown where �ooding is prevalent. But there are some regions that just shouldn’t

become cities. “Parts of Houston in the �oodway, parts of New Orleans submerged

during Katrina, parts of Florida—these places never should have been developed in the

�rst place,” 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. “I don’t think any

of it’s going to happen,” he concedes. “Until we get people in Congress and in the

White House who care about the environment, it’s just going to get worse and worse.”

Even so, there’s reason for optimism. If good stormwater management means good, site-

speci�c 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 in�uence stormwater issues within their municipalities. Many folks

know that they have a local city council and school board, but local planning, zoning,

and urban design agencies also hold regular public meetings—unfortunately, 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 �ooding with a more sophisticated, long-

term interest in stormwater management.