In order to understand, and possibly mitigate, observed reductions in water flow throughout the Ipswich River Watershed, the Massachusetts Department of Conservation and Recreation (DCR), using funds from a U.S. EPA Targeted Watershed Grant, retained Geosyntec to evaluate the impacts of stormwater flow using Low Impact Development (LID) stormwater management design techniques. Geosyntec designed the study to contrast the stormwater runoff patterns of well-documented conventional development stormwater management with the real-world application of environmentally sensitive site design and LID principles, thus determining the degree to which stormwater runoff is reduced in a new housing development compared to an existing conventional development. The results of this state-of-the-practice comparison would allow communities and developers to assess the effectiveness of the design types and create benchmarks for LID stormwater management. The immediate goal was to help improve groundwater base flow and develop a scientifically-based, reliable stormwater model to aid in other future LID designs within the Ipswich River Watershed. Careful lot-by-lot delineation of drainage and a complete understanding of the flow paths during rainfall events was critical for the success of this project. Partridgeberry Place, a recent residential development, was chosen as the evaluation site.
Partridgeberry Place is a compact site design with cluster homes on residential lots less than 0.20-acres in size and 74% (28 acres) of the 38-acre site as undeveloped open space. The development incorporated a variety of LID stormwater management features and Open Space Residential Design (OSRD) principles. LID stormwater management features included dry wells, vegetated swales, GrassPave, bioretention, three rain gardens on individual residential lots, reduced impervious area, and native drought-resistant vegetation.
Geosyntec's Scope of Services
Geosyntec's primary task was to characterize the stormwater runoff dynamics at the LID subdivision through on-site monitoring and estimating stormwater runoff dynamics from this development technique compared to "Pre-development Watershed." The comparison of the LID Subdivision to the Pre-development (i.e., forested) watershed provides understanding as to how closely LID design mimics pre-development watershed conditions. Through this study we interpolated the stormwater runoff effects in developments using only "Cluster Home" and "Conventional" development characteristics.
Geosyntec conducted detailed monitoring of stormwater to accurately measure and calculate flows. We measured flow into and out of a central rain garden and two detention basins. To obtain data from the forested land, we installed measurement systems on the forest floor to calculate stormwater runoff. We observed 44 storm events and collected approximately 20 inches of precipitation.
Geosyntec's evaluation results showed that LID design most closely replicates Pre-development Watershed conditions due to reduced impervious cover, preservation of open space, and incorporation of LID techniques, when compared to developments using conventional stormwater management techniques. Using LID with cluster design reduces stormwater runoff volume by 21 percent for the 10-year storm. Our results are expected to advance the state-of-the-practice in using models to design and implement LID stormwater management techniques. This study is among the first in the nation that uses actual field measurements from two land-use types in the same watershed to compare pre-development and post-development stormwater conditions.