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Low Impact Development: The Village At Springbrook Farms
Lebanon County PA

On undeveloped land, trees, forests, and fields act as natural buffers that capture rainfall and snowmelt, filter pollutants, and slow down stormwater runoff to enable it to infiltrate into the ground. When land is converted from this natural state and replaced by impervious roadways, rooftops, and parking lots, this buffering ability is lost. More polluted stormwater runoff reaches our waterways more quickly, bringing nitrogen, phosphorous, sediment, toxics, and bacteria with it—and causing flooding, erosion, and channelization along the way. Low Impact Development (LID) techniques aim to maintain the hydrologic cycle and protect water quality throughout the development process.

The Village at Springbrook Farms is a good example of what is possible when a township that takes a long term view works with a developer willing to innovate. Recognizing the problems associated with detention basins and the need to sustain groundwater supplies, South Londonderry Township requires infiltration wherever possible on new development sites. Prior to development, Springbrook Farms comprised a gently rolling landscape under cultivation for soybeans and corn. Most of the northern side of the site drained into closed depressions, while the southern side drained into surface channels during storm events.

Rather than use the mammoth detention basin that was called for in the original development plan, engineers from Cahill Associates studied the site's natural conditions and created a stormwater management system that mimicked the existing regime. The overall approach was to keep the stormwater as close to the source as possible, cleansing and recycling it with a variety of BMPs (Best Management Practices). Porous asphalt pavement was used extensively—for sidewalks, paths, and parking areas—and typically had stone-filled recharge beds built underneath to purify runoff before enabling it to seep back into the ground. Infiltration beds were also used underneath non-porous driveways, while rain gardens, vegetated swales, and other landscape features were incorporated throughout the site to mitigate any potential impacts from runoff.

It's always best if you can to keep the water on site and percolate it down through the ground right where it falls. As time goes on, people will come to desire these types of communities and we hope to be able to use that as a marketing feature here.
-Scott Campbell, Owner Brownstone Real Estate Company.

After an extensive site investigation, soil and infiltration tests took place. It was determined that the "pre-development" stormwater runoff condition was essentially zero for both rate and volume of runoff, because the many closed depressions acted as small catchments. It was agreed that rather than "over-engineer" to create a costly and ineffectual collection and conveyance system, the site would be treated as though runoff could occur during extreme weather events. For volume control, the various BMPs were designed so there would be no increase in stormwater or rainwater runoff volume for the 2-year, 24-hour storm. For peak rate control and to prevent localized flooding, the various BMPs were interconnected with a shallow piping system capable of conveying 2-year through 100-year flows without overtopping the BMPs or creating damaging flooding.

In the end, more than 100 storage/infiltration BMPs have been distributed throughout the site. Each was located and sized according to its drainage area, and accounts for both storage volume and the amount of surface area required to "spread out the water" to avoid over-concentrating infiltration. With all elements working together, Springbrook’s stormwater management system treats pollutants, re-charges the groundwater, and maintains the water table, and provides flood control, while preventing destructive effects downstream.


PROFILE

Situation: A fully integrated system of more than one hundred BMP's on a 59 Acre, 259-unit residential community. The development sits on sinkhole-prone karst topography. Infiltration was the method of choice for stormwater management.

BMPs
A site assessment resulted in numerous non-structural BMPs being incorporated into the final site design. In addition to utilizing existing flow pathways, clustering, minimizing site disturbance, and reducing impervious surfaces, a wide range of interconnected structural BMPs were put into place to manage precipitation where it falls, including:

    Benefits:
  • Infiltration
  • Pollutant reduction
  • Water Quality
  • Volume Reduction
  • Aesthetics
    Site:
  • The Village at Springbrook Farms,
  • South Londonderry Township, Lebanon County, PA
    Players
  • Brownstone Real Estate Company (developer)
  • Cahill Associates, Water Resources Engineers
  • RGS Associates, Landscape Architects
  • Abel Construction (contractor)
  • South Londonderry Township

BENEFITS

On the Springbrook Farms site, stormwater is managed through 124 storage and infiltration elements that are distributed throughout the site. These BMPs are integrated into the built landscape and add aesthetic appeal, while reducing runoff volume, filtering pollutants, and maintaining the natural hydrologic balance.

    Quality of Life
  • Aesthetically appealing landscape
  • Walking/bicycle paths with pervious pavement
  • Rain gardens and other vegetated areas with infiltration
  • Improved water quality
    Environmental
  • Volume and peak rate control
  • Increased infiltration
  • Maintained hydrologic cycle
  • Stormwater managed close to the source

PLANNING

The original site featured gently rolling topography with closed depressions and sinkholes and was underlain entirely by limestone formations susceptible to subsidence. In order to maintain the natural pre-existing infiltration regime, it was crucial to undertake a detailed site investigation, so that the stormwater management plan would avoid sinkholes (and potential sinkhole areas), avoid overconcentration in any single area, and allow for infiltration into a suitable depth of soil mantle to remove pollutants and control the infiltration rate.

The site investigation included:

  • A desktop review of geologic and hydrogeologic maps and publications; as well as aerial photographs.
  • Discussions with the previous owner concerning the history of sinkhole formation on the site.
  • A field survey to locate existing sinkholes and closed depressions.
  • A field investigation of soil probes to determine the depth of soil mantle and depth to rock across the site.
  • A series of test pits were investigated across the site to visually inspect and understand the subsurface conditions.
  • Percolation tests were performed within each of the test pits to verify the infiltration capacity of the soil.
  • A geotechnical engineer was consulted and performed additional testing near existing sinkholes and closed depressions and provided recommended sinkhole repair procedures.

The site analyses showed that much of the site drained to the closed depression areas, with no apparent surface runoff occurring. Runoff infiltrated within each small catchment, sometimes forming shallow ponds which slowly infiltrated. Direct surface runoff did not occur beyond the closed depressions for much of the site under normal conditions, so the "before" or "pre-development" runoff condition was essentially zero for both the rate of discharge and the volume of runoff. Because of this topography, conventional stormwater collection and conveyance systems would have required excessive grading and excavation, a costly and undesirable approach in soils over limestone formations.

Based on discussions with the local reviewing agencies, the site was analyzed as though runoff could occur from the closed depressions during extreme events, and the following stormwater management strategy was applied:

  • For volume control, the various BMPs were designed so that that there would be no increase in the volume of runoff after development for the 2-year, 24-hour storm event.
  • For peak rate control and to help with flood control, the various stormwater BMPs were interconnected with a shallow piping system capable of conveying the 2-year through 100-year flow rates without overtopping the BMPs or creating damaging flooding.

This strategy was achieved by the use of 124 storage/infiltration elements distributed throughout the site. These elements are integrated into the built landscape and include vegetated infiltration beds, pervious concrete sidewalks, vegetated infiltration swales, rain gardens, infiltration beneath standard driveway parking areas, porous asphalt pathways and pavement, and similar landscape/stormwater elements.

BMP’s

Stormwater management is achieved by the use of 124 storage/infiltration elements distributed throughout the site. These elements are integrated into the built landscape and include vegetated infiltration beds, pervious concrete sidewalks, vegetated infiltration swales, rain gardens, infiltration beneath standard driveway parking areas, porous asphalt pathways, and similar landscape/stormwater elements. These elements are designed to reduce runoff volume and maintain the natural hydrologic balance. The system is designed such that there is no increase in the volume of runoff after development for the 2-year frequency, 24-hour duration storm event, and no increase in peak flow rates for the 1-year through 100-year storms.

Each BMP was located and sized based on the drainage area to that BMP, with consideration of both the storage volume and the surface area required to "spread the water out" and avoid over-concentrating infiltration. The design attempted to keep the loading ratio (the ratio of impervious area draining to the BMP to the BMP area itself) at or below 3 for many of the BMPs, with a maximum ratio of approximately five.

During the design process, detailed routing of one sub-area was performed by carefully routing each BMP based on its stage-storage-discharge characteristics to confirm that control of volume and travel time would reduce peak flow rates in large storms.

It was critical that rigorous erosion and sediment control measures were maintained and that overall site construction and stormwater BMP construction efforts were closely coordinated to minimize site disturbance, protect soil integrity, etc. When limestone pinnacles were encountered, BMPs were adjusted to avoid excavation or placement of infiltration directly on the limestone.

Specific BMPs utilized include:

Total available stormwater storage by BMP type

BMP type Storage Volume (CF)
Rain Gardens 6,509
Surface Infiltration Beds 70,870
Infiltration Beds 41,543
Porous Pavement Beds 2,506
Porous Asphalt Pathway 13,847
Trench Beds 46,852
TOTAL 182,125
  (4.18ac.ft.)

 

REGULATORY

This development took place on a rolling landscape that had been under cultivation for row crops. It was challenging from a stormwater management perspective because it held closed depressions and sinkholes and was essentially shaped like a big bowl; prior to development, stormwater did not leave the site.

South Londonderry Township requires that infiltration regimes be maintained on new development sites if at all possible. The developers of the Village at Springbrook Farms recognized they were responsible for maintaining the natural hydrologic cycle and saw the opportunity to meet the zoning requirements with innovative stormwater management BMPs, rather than build massive basins that would require extensive excavation, grading, and piping systems.

 

RESOURCES

Center for Watershed Protection: www.cwp.org
CWP is an invaluable resource for stormwater managers, and offers a wide range of publications, tools, and links to materials that have local, regional, and national appeal. Of particular relevance for those interested in Low Impact Development is Chapter 3 of the 2008 publication " Managing Stormwater in Your Community: A Guide for Building an Effective Post-Construction Program".

Chapter 3: Land Use Planning as the first BMP (PDF)

The entire publication is available here

Builders for the Bay was a partnership between CWP, the Alliance for the Chesapeake Bay, and the National Association of Homebuilders aimed at encouraging site design principles that reduce the environmental effects of residential and commercial development on the Chesapeake Bay. The partnership aims to form alliances that engage local governments and developers in a process to assess their current codes and ordinances—and develop model principles to guide future development.

The program is on hiatus, but a series of roundtables were convened in several of Pennsylvania's Bay counties with positive results. The Blair Country consensus document is a good example of the comprehensive approach recommended by the program:

Recommended Model Development Principles: Blair County (PDF)

Natural Lands Trust: www.naturallands.org
NLT works to protect the natural assets of the Delaware Valley by building a network of interconnected open space throughout the region through acquisition, planning and land management. In addition to its regional effort, NLT conducts the Growing Greener: Conservation By Design™ program, which is a statewide program aimed at helping municipalities and developers build new homes and businesses while protecting important local open space.

The Growing Greener overview highlights the four key actions of the program (PDF)

This publication makes Conservation Design recommendations for the Chesapeake Bay watershed (PDF)

More Low Impact Development Resources

Low Impact Development materials from U.S. EPA

Municipal Guide to Low Impact Development (NAHB brochure) PDF

Low Impact Development in Puget Sound (Puget Sound Action Team, WA) PDF

Better Site Design: A Handbook for Changing Development Rules in Your Community, PART 1 (Center for Watershed Protection) PDF

Better Site Design: A Handbook for Changing Development Rules in Your Community, PART 2 (Center for Watershed Protection) PDF

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