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Monday
Nov202017

Who’ll Stop the Rain: Urban Design with Nature

Marines patrol past flooded Houston home. Photo by Lance Cpl. Niles Lee.

“Long as I remember, the rain been comin' down.

Clouds of mystery pourin', confusion on the ground.

Good men through the ages, tryin' to find the sun.

And I wonder, still I wonder, who'll stop the rain?”

John Fogerty’s lyrics from 1970’s “Who’ll Stop the Rain” became a devastating reality in 2017 when catastrophic storms hit the Texas Gulf Coast, Florida, and Puerto Rico. Millions of people were impacted with record amounts of rainfall – in some cases, a year’s worth in a few days! Flooding is a necessary natural phenomenon for a healthy ecosystem, but too much and in the wrong place is a deadly combination. But just how much flooding are we talking about?

Flooding by the numbers

One measurement of water volume is the cubic foot (CF). Take a container 12 inches square, fill it with water and voila – you have 1.0 CF (a little less than 7.5 gallons, weighing about 62.5 pounds).

Now multiply that by 43,560 and you get an acre-foot – the metric used for quantifying large areas of water. A single acre-foot contains 325,851 gallons and weighs 2.7 million pounds. Adds up quickly, doesn’t it?

During Hurricane Harvey, it was estimated that the Houston region was subjected to over 20 trillion gallons of flood water. That’s around 61.4 million acre-feet, weighing a staggering 83.5 billion tons (not including storm debris) – the weight of over 228,000 Empire State Buildings! No city can be expected to handle this much floodwater all at once. But 2017 was not a typical year.

Bigger, faster, stronger

Saying that 2017’s devastating storms were “1,000-year floods” is misleading and inaccurate. The 100-year storm has been the standard terminology for classifying storms, but this does not mean it occurs once a century. It is an expression of probability—a storm that has a 1 percent chance of occurring at any time. Rather, it is more accurately characterized as a 1% storm. 2017’s floods were the result of 0.5% storms, maybe even 0.1% storms.

Extreme weather associated with climate change contribute to more intense and more frequent storms. And we won’t have to wait until the Year 3017 for the next Hurricane Harvey. Even a 0.1 percent storm can occur at any time.

Consider “Rain Falls” – a fictitious suburb that’s 25 square miles with 100,000 residents. Rain Falls is 60 percent residential (1/4-acre single-family lots), 20 percent commercial (retail, office and industrial), with the remaining 20 percent evenly split between parks and roadways. While this is an oversimplification, it is within the expected range of land-uses for many suburbs.

Of those uses, only open space areas like parks can readily absorb rainwater since they are nearly 100 percent permeable. Using the runoff coefficient associated with various land-uses, it turns out that 60 percent of Rain Falls is impermeable cover (a total runoff coefficient of 0.60).

How can that be? Aren’t single-family homes surrounded by open yards? Yes, but when lots are covered by foundations, driveways, sidewalks, patios and other paved areas, the amount of open yard can be significantly reduced.

In a typical year (36 inches of rain), Rain Falls experiences 9.4 billion gallons of runoff, normally handled by creeks, retention ponds, and the stormwater drainage system. But with more frequent and more intense storms, more upstream development, and other factors, previously ’normal’ flooding can easily overwhelm Rain Falls’ storm drainage system, resulting in widespread damage.

An aerial view shows extensive flooding from Harvey in a residential area in Southeast Texas. Photo by Staff Sgt. Daniel Martinez.

Reducing flooding impact through environmentally-driven urban design

Communities that rely on development regulations written decades ago find they fail to address today’s realities, including climate change-induced severe weather patterns. When an area is overbuilt or if development has been allowed in inappropriate locations, yesterday’s storm drainage system is unable to contain more intense and more frequent storms.

Here are six ways that developers, engineers, planners, designers and officials can re-think development strategies in order to reduce the impacts of future flooding:

  1. Preserve open space through vertical density– For example, allow a 3,000-square-foot home to be designed in three levels (1,000 square feet per level), thus reducing the footprint by one third compared to the same size house in two stories. Similar strategies can be taken with apartments, offices, etc.
  2. Build fewer parking lots– Surface parking lots are cost-efficient, but highly impermeable. Encouraging structured parking—even a small two-level deck—can preserve more open space. Even something as simple as locating compatible land-uses closer together can reduce parking demand by promoting more walking and less driving.
  3. Use parks to temporarily hold stormwater– After Superstorm Sandy, some parks were designed to temporarily hold stormwater. (A standard football field built 3 feet below grade can detain about 4 acre-feet of water—about 1.4 million gallons). When finished, Dallas’ new Simmons Park will be designed to periodically flood to protect surrounding areas.
  4. Budget for basic maintenance– Cities should budget for ongoing maintenance of rivers and floodplains to ensure they are not clogged with silt and debris, which reduces their capacity to contain flooding.
  5. Update flood data– FEMA must keep flood maps up-to-date so they accurately reflect floodway and floodplain information.
  6. Scrutinize certain types of development– While politically difficult, projects that radically increase runoff, build acres and acres of surface parking, or significantly divert creeks and floodplains should be carefully weighed against projected economic benefits. Cities should use creative incentives to encourage environmentally-responsible development while protecting sensitive natural areas.

It is less expensive and safer to develop with the environment in mind rather than trying to overcome it (or worse, ignore it). There were too many stories of communities devastated by flooding in 2017. We can help lessen this impact through considerate and thoughtful design.

Or we can join John Fogerty and sing: “and I wonder, still I wonder, who'll stop the rain?”

Joe Pobiner, FAICP, CNU-A, is a master planner and urban designer in Gensler's Dallas office. He specializes in applying responsible planning and urban design principles that strengthen the physical, natural, economic, and cultural frameworks of our rapidly urbanizing planet. Contact him at joe_pobiner@gensler.com.