Buckeye's Pembrook Woods development receives recognition in the...

Natural Resources Defense Council Report

Pembrook Woods Low Impact Development

This case study was provided by

Michael Clar

President, Ecosite, Inc.

Developers conceived the Pembrook Subdivision using a low-impact approach right from the start. In doing so, they created an economically desirable development that protects the environment and exhibits the benefits of a multifunctional landscape. Pembrook is a half-acre plot residential development located in northern Frederick County, Maryland. It is the first low impact development (LID) subdivision permitted in Frederick County and one of the few comprehensive LID subdivisions in the country. To date, most projects that have incorporated LID practices and principles are limited to a single lot in scope and therefore, do not realize the greater environmental benefits of the management practices spread across a drainage area.

In Pembrook, developers addressed runoff using "volume control" techniques as opposed to the more traditional "peak discharge" approach that uses a network of catch basins and pipes to convey runoff from an entire development to stormwater management ponds. The volume control approach allowed developers to replicate predevelopment runoff patterns using micro-scale integrated management practices that capture and treat rainwater close to where it hits the ground. The use of LID practices and principles throughout the development enabled developers to eliminate the use of two stormwater management ponds that they had envisioned in an earlier site conception. This elimination represented a reduction in infrastructure costs of roughly $200,000. In place of the stormwater management ponds, the developer preserved two-and-a-half acres of undisturbed open space and wetlands, which aid in the control of stormwater runoff. This also resulted in a considerable saving in wetlands mitigation impacts.

Extensive use of LID site foot-printing techniques allowed the site design to preserve approximately 50 percent of the site in undisturbed wooded condition. This design feature was very beneficial to maintaining pre-development hydrologic conditions. Site foot-printing also enabled developers to gain two additional lots by using a LID design, increasing the 43-acre site yield from 68 to 70 lots. This "density-bonus" added roughly $100,000 in additional value to the project.

Developers also reduced effective impervious cover and saved money by converting approximately 3,000 linear feet of roads from an "urban road" section to a "rural road." They did so by replacing curbs and gutters with vegetated swales and reducing paving width of the road from 36 to 30 feet. The use of swales saved the developers $60,000 in infrastructure construction and the reduced road width lowered paving cost by 17 percent, while at the same time reducing overall imperviousness.

In order to satisfy County criteria for adequate downstream conveyance, developers conducted a downstream impact analysis. The analysis examined the ability of a LID site design to maintain predevelopment peak discharge conditions for a range of storms including the 1, 2, 10, 50 and 100-year storms This analysis was important because many public works personnel perceive innovative LID stormwater management techniques to be capable of addressing water quality issues, but insufficient to provide downstream peak discharge control for the larger flood flows. The developers had initially based site LID hydrologic analysis on the 1-year storm (2.5 inch rainfall), which is part of the criteria for water quality control in Frederick County. The downstream analysis revealed, however, that the 1-year storm design was not sufficient to maintain predevelopment peak discharges for the 10, 50 and 100-year storms. They then used an incremental iterative procedure to determine additional control requirements to provide necessary downstream control. This analysis showed that increasing the design storm to a 2-year storm (3.0 inches of rainfall), provided required downstream protection over the complete range of flood events (10, 50 and 100 year storms).

The results of this study have great significance for future stormwater management policy and design criteria. These results clearly illustrate tremendous advantages achieved by incorporating a runoff volume control approach and LID technology. It also demonstrates that conventional stormwater management designs that use a peak-discharge detention approach along with stormwater management ponds are not as effective as a LID approach. The hydrologic flaws associated with the peak-discharge detention approach are numerous, and include:

• Peak discharge control does not typically address the maintenance of groundwater recharge.
• Peak discharge approaches alter the frequency and duration of flood flows resulting in stream channel degradation.
• Peak discharge approaches can actually exasperate downstream flooding conditions due to the super-positioning of runoff hydrographs.
• Peak discharge approaches, particularly the use of regional facilities, provide no protection for streams above the regional facilities.

Using an integrated LID stormwater management approach reduces or eliminates many of these problems.

Contact: Michael Clar, President, Ecosite, Inc.

6470 Dobbin Road, Suite F, Columbia, MD 21045

Telephone: 410-730-5787

General Information: ecosite@yahoo.com