Green Infrastructure Collaborative tours

By Lake Champlain Sea Grant Staff
November 24, 2015

At the end of September the Green Infrastructure Collaborative (GIC) sponsored a trip to the University of New Hampshire Stormwater Research Center. GIC affiliates, engineers, landscape designers, researchers, and state agency employees from Vermont, New Hampshire, and New York were among those who attended. The UNH Stormwater Center is nationally recognized for their research on Green Stormwater Infrastructure (GSI). Their extensive monitoring of a variety of GSI practices both on and off campus provide valuable data on best practices for design, implementation, and maintenance relevant to our region.

UNH Stormwater Center Director Thomas P. Ballestero, PhD. P.E. and Program Manager James Houle, MA, CPSWQ led groups to on and off-campus installations and monitoring sites. One on campus installation they visited was the West Edge Parking Lot, where the UNH Stormwater Center has conducted research for over a decade. Runoff from a parking lot is directed into parallel tracks, which allow for side-by-side analysis of different technologies including: bioretention, subsurface gravel wetlands, hydrodynamic separators, and tree pits. Newer installations on the other side of campus include an infiltration swale that collects water from a large section of parking lot. Off-campus installations on the tour included an undersized gravel wetland that drains runoff from a residential neighborhood, and an infiltration swale in a busy commercial parking lot in the center of Durham.

Some of the key takeaways from the tour are:

  • Subsurface gravel wetland vegetation must be completely harvested every three years to avoid Phosphorus loading from decomposing plant material. This should be a standard component of any maintenance plan for these systems, particularly if nutrient removal is the primary goal.
  • Undersized gravel wetland installations perform better in the field than models suggest. At year two, performance of a retrofitted quarter sized system far exceeds expectations with the close to full-size phosphorus reduction. In space-constrained urban areas, installing designs that are below optimal size can have significant impact on reducing pollutant loading.
  • Manufactured hydrodynamic separators – flow-through structures that mechanically remove particles from stormwater - exhibited no better (and in some cases worse) nutrient and total suspended solids (TSS) removal than basic catch basins. (See detailed data in table below)
  • Successful bioretention and/or gravel wetland installations can serve as important seed sources for new installations. The UNH team gather perennial seeds from existing installations to spread on new sites for the best germination rates. Further, the group takes water samples from past projects to seed new sites with the active organisms needed for optimal system performance.
  • Planting plans should carefully consider location and aesthetic goals. In some cases, mowed grass can have comparable removal rates of pollutants, to that of native ornamental plants, and may have preferable maintenance regimes.

Permeable parking and road areas do not require permeable surface materials to successfully infiltrate stormwater. Functionally, the road surface simply offers an inlet to a subsurface infiltration media. Catch basins can direct stormwater into a subsurface perforated pipe network for infiltration. Given the challenges with adoption of permeable pavement technology in Vermont, this method could present a promising alternative to porous surfaces.


The graph below provides side-by-side performance analysis of both conventional and GSI practices investigated at UNH:

(UNH Stormwater Center 2012 Biennial Report, page 11)


The lessons learned at UNH are already being used in Vermont by designers, installers, and state agencies. The Green Infrastructure Collaborative will continue to link practitioners with the newest research on Green Stormwater Infrastructure in an effort to improve Vermont’s application of this cutting edge technology, for the benefit of our waterbodies. Find more detailed analysis and data on the technologies from the UNH Stormwater Center by visiting their website