- It’s not because the water comes in. It’s because it is forced to leave again.
- Reducing impervious surfaces and improving water conveyance has a role to play
- most important step 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 finding new uses for it in place.
- China’s “sponge cities” as a dramatic example—a government-funded effort to engineer new, permeable materials to anticipate and mitigate the flooding common to that nation….
Ian Bogost The Atlantic August 29 2017 Read full article here
…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’t because the water comes in, not exactly. It’s because the pavement of civilization forces the water to get back out again.…
…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’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….
…In Houston’s 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’s 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’s “most pressing infrastructure need.” …
…Houston poses both a typical and an unusual situation for stormwater management. The 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 flooding 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 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 “million-year” flood, as some are calling Harvey’s aftermath, is difficult and costly. Stiftel confirms that it’s almost impossible to design for these “maximal probable flood events,” 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. “The Houston event seems like an illustration that we haven’t figured it out,” 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’t slowed in the last half-century. Cities have only become more desirable, spreading outward over the plentiful land available in the United States….
….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 green space, 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’s “sponge cities” as a dramatic example—a government-funded effort to engineer new, permeable materials to anticipate and mitigate the flooding common to that nation….
….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’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 place.
That 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….
….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…
…But there are some regions that just shouldn’t become cities. “Parts of Houston in the floodway, parts of New Orleans submerged during Katrina, parts of Florida—these places never should have been developed in the first place,” Debo concludes. Add sea-level rise and climate-change superstorms, and something has to give.
…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 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 flooding with a more sophisticated, long-term interest in stormwater management.