Getty Images/C.J. Burton

Stormwater management at the Gray’s Landing affordable housing project in Portland, Ore., starts at the top of the six-story building. As rainwater hits the vegetated roof of the 245,000-square-foot structure, much of it is absorbed or evaporated before reaching ground level. In heavier rain events, the rest of the multistage system kicks in: Water flows through roof drains to a drainage mat, then through downspouts into a second-story courtyard, says Michael Bonn, principal with Portland-based firm Ankrom Moisan Architects. Planter boxes soak up more of the water; what is not absorbed drains into a tank and flows by gravity into a series of gunnels and a vegetated wall. The setup has worked well, even in soaking rain.

In storm events, however, even the best green infrastructure may not be enough to prevent flooding, especially in a flood plain. “When I was in private practice as a developer, I thought any green space was good for development,” says Stephen Costello, chief resilience officer with the Office of the Mayor in Houston. “Now being part of the regulatory community, I think there are areas where maybe we shouldn’t be developing. Everyone’s thinking differently now, after Harvey.”

In the aftermath of 2017’s Hurricane Harvey, builders and developers are taking a good look at how they control water on their sites. Many states and local jurisdictions require that developers modify the water flow on a site as little as possible; some are requiring developers to use low-impact development techniques to contain stormwater. As a result, rather than focusing on getting water off of a site as fast as possible, some developers are trying new methods to slow and infiltrate runoff in lieu of or in addition to building traditional underground detention systems.

After Harvey, Harris County, Texas, changed its regulatory requirements for building in flood plains. While it does not prohibit development, it now requires that any new projects in the 500-year flood plain be elevated 2 feet above that flood level, as opposed to 1 foot above the 100-year flood level. “We want to make sure all residential structures are 2 feet above the 500-year flood plain and that the overall project is self-contained, meaning that in an extreme event, water is not displaced onto adjoining neighborhoods,” says Costello.

Meritage Homes anticipated this change in regulations—and went beyond what was required—for Spring Brook Village, a 900-home community in Houston still under construction on a former 115-acre golf course in the 500-year flood plain.

The builder designed a 26-acre drainage system. “There was a system of lakes on the course already; we widened and deepened those and connected them to create one really large lake system that could perform the same job the golf course did in the past,” says Kyle Davison, division president for Meritage Homes in Houston.

The water is slowly released into Brickhouse Gully downstream no faster than when the site was a golf course, he adds. Soil from the excavated lakes was used to elevate the building pads, so none had to be brought in or taken out. The system was put to the test during Harvey, and the site drained perfectly, says Davison.

Finding Solutions
A 600-acre single-family home development, Woodridge Forest in Montgomery County, Texas, is also being built in a flood plain. The county requires developers to meet the 100-year flood plain requirements, but much of the development probably meets 500-year flood plain standards, says Harry Masterson, a founding principal with Concourse Cos. In the fall, the National Oceanic and Atmospheric Administration released its Atlas 14, which found higher rainfall frequency values in parts of Texas and redefined the amount of rainfall it takes to qualify as a 100-year or 1,000-year event. Texas counties are now using this data to help manage development in flood plains; several are producing new flood plain maps as a result.

Because of this, developers in the area now try to build to 500-year flood plain standards, says Masterson. “Nobody knows what the new Montgomery County maps will say, but we exceeded the design criteria at the time, using excavated soil to raise the sections. The 100- and 500-year flood plains are so close—it may be the difference of only a couple of inches,” he says.

His firm created more than 25 acres of ponds that will store stormwater and release it slowly into the two natural creeks that flow through the property. Project planners also left vegetated buffers of 30 feet to 60 feet from the top of stream banks, lakes, and the homes. Masterson says their goal—also required by the regulatory agencies—was to ensure the rate and volume of the water leaving the property was no greater than the flow leaving the site prior to development.

Preserving open space and wetlands—the original green infrastructure—is another way to ensure permeability. Masterson says while home buyers may not want to pay for something called “green infrastructure,” they do want to pay for parks and open space. “Through municipal utility districts, people are now paying for parks, which can act as very large buffers and help infiltrate stormwater,” says Masterson. His firm is working to increase the percentage of park areas in its developments. “We’re trying to come up with a cost effective way to convey and store water that doesn’t create impervious surface.”

But as great as it is to build parks, experts note that created green space may not be as permeable as the original undeveloped site, and the percentage of impervious surface often increases with new buildings and “hard” features. If a site’s imperviousness increases from 70% to 80%, for example, rain gardens, bioswales, stormwater planters, or vegetated roofs can be added to the built area to mitigate that difference in permeability. “Green infrastructure is not only stormwater management but also comprehensive design,” says Tom Liptan, green infrastructure consultant and author of Sustainable Stormwater Management: A Landscape-Driven Approach to Planning and Design.

Challenges to Overcome
While protecting future homes from devastating flooding seems a no-brainer, there’s not one simple solution to address the issue. One challenge in the past has been convincing regulators to allow developers to try new techniques. When Liptan first proposed stormwater innovations such as planters and vegetated roofs for Portland in the early 1990s, city officials were dubious. But last summer, they passed a requirement that all new buildings with more than 20,000 interior square feet must have vegetated roofs.

Portland-based developer Ed McNamara says another roadblock is getting civil engineers to design innovative systems because they are worried about getting their designs approved. He advises finding someone in the city (or other regulatory agency) to work with early on. “Even if it’s something that’s not allowed yet, see if they’re open to looking at it,” he says.

Another hurdle is the perception that green infrastructure is costly. But Liptan says that’s a myth. “If you have to have a landscape anyway and you can design it to manage the water when you’re paying for landscaping, you’ve just gotten something for nothing,” he says. “There are many examples of where developers have saved thousands of dollars because they used the green approach and leveraged the landscape.”

Even at the single-family level, costs can be minimal. Florida-based home builder Jimmy Brattain, president of Design Works Florida, designed a custom luxury home that captures 80% of the rain that falls on the home in three 900-gallon tanks. Two of the tanks provide low-flow irrigation for the landscape; the third is used for flushing toilets. It includes UV purification and is connected to the city sewer system.

Because the project was new construction, the rainwater harvesting system did not add much to the overall costs, Brittain says. He’s now building his own home in Florida and keeping rainwater on the property using leach fields, which he says are inexpensive to build. “We’re putting as much water back in the aquifer as possible,” he says. “In Florida, with all the water restrictions, 2,700 gallons is a lot.”

Cost Savers
Instead of using costly, high-maintenance underground rainwater retention, more developers are incorporating less-expensive strategies such as rain gardens and bioswales into projects, or are creating ponds or lakes that double as recreation features, says Dale Bowyer, senior water resources engineer with the San Francisco Bay Regional Water Quality Control Board.

Tony Woody, vice president at Thomas and Hutton and project manager for a new residential development by The Beach Co. near Charleston, S.C., is one of those developers. His firm’s 2,000-acre Kiawah River development will feature spots for picnics and campfires, trails and village greens, a waterfront lodge, a swim club, a woodland chapel, guest cabins, a boat landing, and a dockside restaurant, among other amenities.

Kiawah River

Woody says it is almost impossible to find buildable land in the low country of South Carolina that is not subject to some degree of flooding. At Kiawah River, The Beach Co. elevated buildings, roads, parking areas, and other common areas to provide separation from the seasonal high water table and the design flood event, Woody says, and to buffer against coastal flooding from sea level rise. The natural creeks on the site were preserved and riparian buffers left between the creek and the development; vegetated swales will also help absorb heavy storms. Some of the man-made bodies of water that existed on the site prior to the new development were rehabilitated.

Other increasingly popular stormwater solutions include rain gardens, bioswales, pervious pavers, stormwater planters, green walls, and green roofs. But before adding green infrastructure to a site, developers can use more basic strategies to help absorb stormwater. “As much as possible, try not to disturb native riparian vegetation,” says Laurel Warddrip, industrial and construction stormwater unit chief with California’s State Water Resources Control Board. “It helps absorb rain and filter pollution and sediment. Having a setback from sensitive riparian areas, combined with other best management practices, can help developers show that the runoff leaving the area is no greater than before they built the project.”

Warddrip also recommends builders protect native soils. “If you just grade all the top soil off and fill it and apply treatment chemicals, you can create an almost impervious surface,” she says. “But if you protect the native soils or put back good soil and prepare to grow plant life, you’ll build that sponge back faster.”

Other Approaches
One way to avoid the headaches of a flooded homesite is to avoid buying land in flood-prone areas, Liptan points out. “But if you already own it and it is zoned for development, try to make the site as porous as possible,” he advises.

Another strategy for capturing stormwater—and saving money on water supply—in new residential developments is rainwater cisterns. Station Place Tower, built by REACH Community Development, is a mixed-income, affordable housing project in Portland with 176 units of senior housing and 26,000 square feet of commercial space. Rain is captured in a 20,000-gallon tank on the first floor, treated, and pumped to the first eight floors for toilet use. Brian Bieler, director of asset management with REACH, says this captured water saves 250,000 to 275,000 gallons of water per year.

Other buildings are starting at the top when it comes to managing stormwater, similar to the Gray’s Landing project in Oregon. In the Pacific Northwest, vegetated roofs annually capture 50% of the rain that falls on them, about half of which evaporates back into the atmosphere, notes Liptan. This evaporation occurs even in winter, although less than in other seasons.

At the Ramona Apartments in Portland’s Pearl District, Liptan worked with local developer McNamara to design a 33,000-square-foot vegetated roof on the five-story wood-framed building. With help from civil engineers, they devised a system to treat stormwater runoff in an 8,000-square-foot, U-shaped courtyard that sits on a concrete podium above underground parking. Stormwater flows into two long landscaped areas along the edges of the courtyard, and from there it is filtered through a layer of sand under a raised playground and wooden walkway in the courtyard before flowing into the city’s stormwater system. On another large residential project, he was able to use infiltration planters since the site did not have underground parking.

McNamara admits he initially was nervous about green infrastructure. But he has now built several residential properties in Portland using it, including infiltration trenches, vegetated roofs, and stormwater planters. He advises working closely with a project’s architects and engineers—and not being afraid to innovate.

“Start early if you want to do it,” he says. “You really have to figure out the grades. Pick your team of civil engineers and contractors that are receptive and will brainstorm with you and find ways to work with you.”