Improving Stormwater Management Through Low Impact Development
by Nick Walker, NVSWCD Intern
(Conservation Currents, Northern Virginia Soil and Water Conservation District, Fall 2006)
The Chesapeake Bay is currently at risk from a variety of factors, many of which are caused by poorly designed or insufficient stormwater management practices and run-off. One key problem is pollution from excess nutrients (phosphorous and nitrogen), bacteria, and sediment, which is eroded primarily from stream banks following storm events. The bay also is impacted by thermal pollution in the form of heat from run-off of water over impervious surfaces, such as pavement and rooftops.
Fortunately, low impact development practices can partially reduce these risks to the Chesapeake Bay and our local water resources. Low impact development (LID) describes an approach to stormwater management and land use planning that focuses on using technology to recreate pre-development hydrology in our watersheds. Some of the LID practices in use in Fairfax County include: bioretention basins/cells, permeable and porous pavements, green roofs, tree box filters, soil amendment, and bioretention swales. These practices, and some considerations related to each, are described below:
Bioretention Basins or Cells
Bioretention basins or cells contain layers of gravel along with an engineered soil mix. [Editor's note: rain gardens are popular examples of bioretention practices.] The engineered soil mix filters pollutants from rainwater before they reach rivers and streams. These cells, particularly those that include underdrains, have proven to be highly effective in removing nitrogen and phosphorous from run-off, often by as much as 30-40%. They also remove up to 70% of fecal coliform including E. coli bacteria. Additionally, bioretention reduces the total amount of stormwater outflow; up to 60% can be removed through evaporation, soil absorption, and/or evapotranspiration.
When using bioretention to improve water quality, the following should be considered: First, the soil mix must be checked for the Phosphorous (P) Index, which is a measure of the amount of phosphorous it contains. Because phosphorous will migrate from high to low concentration areas, the mix must have a sufficiently low P-Index (usually between 10-30) in order to filter phosphorous from run-off. When installing bioretention facilities on previous farmland that has been subject to phosphorous-intensive fertilizers for decades, soil with a low P-Index often must be imported from geographically distant locations. This can increase the price of the facility. Secondly, bioretention cells must be maintained in order to be effective. As silt collects in the facility over time, the cells can clog and become ineffective. Installing a forebay or filter strip to intercept sediments can increase a cell’s effectiveness. A requirement, however, is to regularly monitor and maintain each bioretention site by removing sediment and, if necessary, replacing the filter media.
Permeable pavement can also slow the flow of stormwater and filter pollutants. Permeable or porous pavement is any pavement that water can pass through instead of beading on or running off the surface; it can be composed of gravel, asphalt, plastic, or other materials. Concrete grid pavers filled with sand or topsoil do not allow as much water to infiltrate as other types of permeable pavement, but are more efficient at removing pollutants. Permeable pavement also mitigates the issue of heat pollution endemic to ordinary pavements, which retain heat during the day and transfer that heat to run-off during precipitation. When this newly heated water reaches streams, it can be detrimental to many aquatic species.
Permeable pavement also will become less effective over time at infiltrating and filtering runoff if it is not maintained. Fortunately, the maintenance procedures often are not too difficult. Periodically running a street sweeper over the pavement, and over the paved portion of its drainage area, can greatly improve infiltration. Soil compaction caused by construction can limit the pavement’s performance. However, this is somewhat assuaged by the layer of gravel installed immediately under the pavement. When installing permeable pavement, this underlying gravel bed must be as deep as the frost line to be effective.
A simple green roof is created by adding 3-4” of soil to the rooftop surface on top of a water-proof membrane. The soil layer absorbs precipitation and reduces run-off. The shallow soil depth and increased temperatures of roofs make a green roof functionally similar to a desert ecosystem. This means that a green roof can effectively reduce run-off, but has a limited capacity to remove phosphorous. As the roof pitch steepens, less run-off is captured. This trend is also observed with land-based bioretention in steeper topography. In the case of a very steep roof, a cistern to collect rainwater may provide a better solution for controlling run-off. Although no stormwater management practice is maintenance-free, a green roof is generally easier to maintain than either a bioretention cell or permeable pavement.
Tree Box Filter
A tree box filter consists of a precast concrete box filled with filtration media that can be planted with a small tree or shrub. Tree boxes are installed between sidewalks and streets and employed in a manner similar to bioretention cells. However, a tree whose roots are contained in a tree box may suffer mortality due to the constrained root environment, or the tree may have to be moved after fifteen to twenty years and re-planted elsewhere. Soil amendment involves incorporating compost into the soil profile to increase water absorption and filtration. A bioretention swale provides filtration, but unlike a bioretention cell, it is also designed to convey run-off. Bioretention swales are often installed along streets or adjacent to parking lots where they filter and detain stormwater running off the pavement.
Because there are so many practices and applications of LID, it is currently managed through a variety of public and private partnerships. Some of the agencies involved include the Environmental Protection Agency, the Virginia Department of Conservation and Recreation (DCR), the Virginia Department of Transportation, and various local government agencies.
Dr. Bill Hunt of North Carolina State University, a researcher who has done extensive engineering studies on these LID practices, addressed the LID working group of the Potomac Watershed Roundtable in September 2006. The LID working group brings together agency staff, private business interests, and policymakers from Northern Virginia and neighboring jurisdictions to provide a common forum for LID issues. The September meeting was also attended by Virginia Delegates David Bulova and Robert Wittman. Much of the information about individual LID practices presented in this article is drawn from Dr. Hunt’s presentation to the group.
Also, in 2007, the Northern Virginia Regional Commission published an LID Supplement to the Northern Virginia Best Management Practices (BMP) or Handbook. This supplement was called the Maintaining Stormwater Systems: A Guidebook for Private Owners and Operators in Northern Virginia (1.6 MB). The publication created standardized engineering guidelines for LID practices that mitigate the effects of development on water quality and manage stormwater. Policy decisions, however, were left to local jurisdictions. It consolidated LID practices and provides a resource for Northern Virginia counties. The supplement development process was also an excellent opportunity for all those involved to learn from one another.
At the state level, the Virginia DCR has a standing “Technical Advisory Committee for Stormwater” to develop state-wide regulations. In addition, Virginia Polytechnic Institute and State University is requesting the state legislature to establish a LID research and outreach position through their College of Agriculture Life Sciences Cooperative Extension Program, which would facilitate applied LID work.
These shared standards will help bring LID practices into the mainstream and will ultimately benefit the Chesapeake Bay. At both the state and regional level, jurisdictions, agencies, and private industry are working together to protect water quality, incorporate LID into their codes and engineering guidelines, and also to implement the practices on the ground.
To learn more about low impact development, see: Rain Gardens, Green Roofs and Other Low Impact Development Practices. For more information on the Potomac Watershed LID working group, see: Potomac Watershed Roundtable.