Natural drainage systems can cut NU’s development costs
In a South Carolina case study, “light-imprint” infrastructure reduced anticipated engineering expenses 31 percent.
In the December issue, we reported that new urbanist developers are increasingly turning to “natural drainage systems” — techniques that allow much of a community’s stormwater to soak into the ground rather than be piped to rivers, lakes, treatment plants, or large, unsightly detention ponds. A newly completed study led by Tom Low shows that these more natural methods could sharply reduce engineering costs for traditional neighborhood developments (TNDs).
Low, the Charlotte, North Carolina-based director of town planning for Duany Plater-Zyberk & Company (DPZ), organized a team that examined the financial effect of using natural drainage techniques in Griffin Park, a TND about to get under way in Greenville County, South Carolina. The techniques — sometimes described as “high-performance infrastructure” or “low-impact development” — have been dubbed “Light Imprint New Urbanism” by Low. Because they require less pipe, less paving, and less massive excavation, grading, and tree clearing, they could save developers a substantial sum of money.
Low’s team looked at the financial consequences of introducing a natural drainage system in the 42-acre first phase of the 300-acre project being developed by Jelks Little LLC. As the table to the right shows, this system would generate some extra expenses, such as $16,900 for a fence protecting existing mature trees during the erosion-control phase of the project. Twenty “rain gardens” — small, slightly depressed areas that can soak up stormwater — would cost $102,400, more than twice as much as the large detention pond that a conventional engineering approach would call for.
Reductions in other expenses, however, would more than offset those costs. Instead of installing 9,434 linear feet of pipes at a cost of $291,794, there would be only 4,182 feet of piping, costing $129,349 — a savings of more than 50 percent. Storm water inlets would fall from 101 to 24, saving $192,500, or more than 75 percent. Additional savings would be realized on sidewalks, curbs and gutters, road paving — reduced in width from 26 feet to 24 feet — and surfacing alleys with crushed stone rather than asphalt or concrete.
Altogether, engineering costs would drop by 31 percent. The cost per lot would fall 30 percent, to $6,234 from $8,934. (The changes would cut the number of lots by two, to 174, by creating additional green space. The lots are worth about $50,000 to $80,000 each. Xavier Iglesias, senior project manager at DPZ, says the revenue loss would probably be more than offset by the increased value of neighboring lots, which benefit from being close to a green.)
At the edge of the development, standard engineering would call for a large detention pond — a feature that is often unattractive and deep enough to require a barrier of chain-link fence. Low’s team would replace the pond with smaller, three-stage filtration basins, which would clean the runoff before releasing it into creeks — much as was done in Woodsong, a TND in Shallotte, North Carolina, that Low planned several years ago for developer Buddy Milliken.
The filtration basins would fill with water after downpours, but would otherwise look appealing and green, even if they weren’t suitable for active recreation. (The initial section of Griffin Park will also have a neighborhood green, covered in grass, which people can play on; it will not be designed to collect stormwater.) Rain gardens, planted areas that could be used for recreation, would be throughout the neighborhood.
“DPZ and our engineering consultants have been doing this approach to drainage for a long, long time,” Low says. “We just never documented it.” Robert Davis, developer of Seaside, Florida, concurs. “All of Seaside uses natural drainage,” Davis says. “A few spots use French drains, which in turn percolate into the soil (mostly sand) in which they are buried.”
Contributors to the study were Andres Duany; environmental engineer Georgio Tachiev of Florida International University; engineer Stephen Davis of Davis & Floyd; and landscape architect Guy Pearlman and designer Patrick Kelly, both of DPZ. The study is expected to be posted on DPZ’s website, www.dpz.com.
Natural drainage methods must vary from one location to another, adjusting to the character of the soil, intensity of development, and other factors. A complete soil analysis is essential to verify that the soil will absorb enough rainwater quickly, Davis says. Rain gardens do not work well in conditions such as clay soils.
Low says there’s a pressing need for information on how to blend natural drainage and New Urbanism because much of what’s been written about low-impact development has been based on large-lot conventional suburban subdivisions. A large suburban lot may have room for a rain garden, but a more compact TND may not be able to position infiltration areas on every property; instead, collective rain gardens may have to be scattered about, each serving a part of the neighborhood.
Natural drainage techniques can be organized along the rural-to-urban Transect. In a “sub-urban” area (the T3 zone), all of the rainwater may be handled through swales and other natural methods of infiltration. In the “general urban” (T4) zone, “you do rain gardens and you do some pipe,” Low says. In an “urban center” (T5) zone, where buildings cover much of the land surface, it may be necessary to bury large pipes that would hold a sizable volume of stormwater below ground until it gradually is absorbed.
A critical task is to determine how much of the standard engineering apparatus can be done away with in a particular development — and then win government approval. Stuart Sirota, principal in TND Planning Group in Baltimore, says regulatory agencies sometimes require unreasonably large riparian buffers for urban projects or they impose design standards that yield densities too low to produce good urbanism.
“We need to eliminate the ‘gold-plating’ of the engineering” — the insistence on installing more than is necessary, Low asserts. If governments require developers to install all the components of a standard stormwater drainage system even when natural techniques are going to be relied upon, the combined cost will be too high, making the natural system uneconomical.
One purpose of the South Carolina study is to amass knowledge that will be readily available to new urbanist developers and local governments. At Griffin Park, which is expected to have five phases spread over 300 acres, Low expects that some of the proposed Light Imprint techniques will be introduced in the first phase; he hopes the full array will be implemented in later phases.
If implemented in too single-minded a manner, a natural drainage system can conflict with New Urbanism. Some advocates of natural drainage, eager to create uninterrupted greenways, try to eliminate many street connections. Low warns that if many streets are dead-ended, the basic structure of the neighborhood will be compromised. Natural systems, Low believes, must be balanced against elements essential to a walkable neighborhood.
Trees, which are not often thought of in terms of stormwater control, play a critical function, according to Mary Vogel, principal in PlanGreen in Washington, DC. “A shade tree with an extensive crown, growing along a street, probably does more than any manmade technology to manage stormwater,” she says. “Trees should be valued highly and given adequate space at both the canopy and the root level.”
Vogel lists other important natural drainage tools: native plant perennial landscapes; grass filter strips; bottomless planter boxes designed to capture runoff from buildings; public spaces designed to maximize filtration; “stormwater art” including fountains, weep walls, sculptures, and cascades; green roofs; balcony planter boxes; porous pavement; and cisterns that capture rainwater.