Managing Stormwater as a Resource

Our opinions about the technical and financial viability of green infrastructure changed as a result of the success of our Market at Colonnade shopping center in Raleigh, North Carolina. This project’s commitment to stormwater management has opened doors for both of our firms and started transforming how we think about the return on investment (ROI) in green infrastructure.

Regency Centers—the developer and owner of the Market at Colonnade project—prioritizes sustainability in building and maintaining its national portfolio of shopping centers. However, before this project was developed, the high upfront costs of green infrastructure were usually a hindrance to designing a stormwater management system that went beyond the local regulatory requirements. But, the Market at Colonnade project has demonstrated that a stormwater management system with the lowest upfront costs may not always be the best approach for maximizing long-term value, especially in the case of higher-value properties.

The “Innovative Stormwater Management Plan” (ISMP) implemented at the Market at Colonnade was Regency Centers’ solution to maximizing the 6.25 acres (2.5 ha) of land for the 57,000-square-foot (5,300 sq m) development—an area not large enough to house the shopping center, ample parking, and a conventional detention pond or other large surface-treatment devices. By avoiding the need for an above-ground pond, we had the buildable space we needed to construct a profitable retail center. The Market at Colonnade’s ISMP includes several low-impact development strategies that work in combination to collect, retain, reuse, treat, and infiltrate stormwater runoff into the underlying soils to promote groundwater recharge.

The ISMP includes three rainwater-harvesting cisterns (totaling 43,000 gallons or 163,000 liters) that collect rooftop runoff. The cisterns, in conjunction with a subsurface gravel-and-pipe trench system approximately 2,500 feet (762 m) long, detain the first inch (2.5 cm) of runoff, which is then infiltrated into the underlying soils. All surface runoff from the site in excess of that is similarly captured, pretreated (removing oil, sediment, and debris), and temporarily stored in an underground detention chamber measuring 185 feet by 65 feet by 4 feet (56 m by 20 m by 1.2 m) that can hold an additional 350,000 gallons (1.32 million liters) of rainwater.

Harvested rainwater from the roof of the Whole Foods Market is stored in the 11,700-gallon (44,000 liter) cistern and reused inside the store for toilet flushing while additional rainwater stored in two underground cisterns (with a combined volume of 31,800 gallons or 120,000 liters) is used for the site’s landscape irrigation. Surface bioswales and bioretention areas allow for additional stormwater treatment beyond that provided by the large infiltration system. At the same time, these devices directly provide additional localized infiltration capacity to support groundwater recharge.

The cistern located beside the entrance of the Whole Foods store not only serves a functional purpose, but also acts as an icon highlighting the project’s commitment to sustainability—something the community and patrons of the shopping center can appreciate when visiting the property. At a time when the conservation of resources such as potable water and groundwater is essential, the Market at Colonnade’s ISMP demonstrates the value of a comprehensive stormwater management approach.

Research conducted by North Carolina State University, which monitored the site for a full year after construction, found that both the rate and volume of runoff leaving the site is a small fraction of what was found to be leaving a site just across the street with similar size and land use which uses a conventional stormwater management approach. Site monitoring also found that annual pollutant-loading rates for total suspended solids, total nitrogen, and total phosphorus were all 1/32nd or less than those from the adjacent site.

We are not the only ones who can benefit from a more holistic approach to stormwater management, one that uses stormwater as a resource and employs natural systems to retain, treat, and reuse it on site. That is the compelling case made in a new report, The Green Edge: How Commercial Property Investment in Green Infrastructure Creates Value, by the Natural Resources Defense Council (NRDC). According to the report, applications such as green (vegetative) roofs, permeable pavement, rainwater collection systems (usually called “rainwater harvesting systems”), bioswales, and tree planting are good not just for the environment, but also for the bottom line.

While green infrastructure techniques may not make financial sense for every project, increasingly their benefits are becoming clear and setting a new quality benchmark for the real estate industry. This is especially true as the industry focuses on infill projects that have less area available for conventional infrastructure. The business case for innovative stormwater management certainly warrants deeper consideration, and in fact may prove to provide a better ROI over the long run.

Mark Peternell is vice president, sustainability, for Regency Centers, one of the nation’s largest owners, operators, and developers of grocery-anchored and community shopping centers. Patrick Smith is design department manager for Soil & Environmental Consultants, a multidisciplinary engineering and environmental consulting firm in Raleigh, North Carolina. For more information on the NRDC’s report, go to: go.nrdc.org/greenedge.