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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. LID practices
currently 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 contain layers of gravel
along with an engineered soil mix. 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.
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 first meeting of a new Potomac Watershed regional
LID working group in September 2006. The new 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, the Northern Virginia Regional Commission
is currently
working with staff from jurisdictions in the region to develop
an LID Supplement to the Northern Virginia Best Management Practices
(BMP) Handbook. The commission’s goal is to create standardized
engineering guidelines for LID practices that mitigate the effects
of development on water quality and manage stormwater. The LID
supplement will not address policy decisions, leaving these
choices to local jurisdictions. It will, however, consolidate
LID practices and provide a resource for multiple counties currently
at very different stages in the development of their programs.
For example, Stafford County has encouraged LID for the last
three years due to rapid growth, but other counties, such as
Loudoun and Fauquier, are only beginning to consider low impact
development. By opening up dialogues among these counties, the
supplement development process will also be an excellent opportunity
for all those involved to learn from one another and to better
understand LID.
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 establish
an LID research and outreach position through their College
of Agriculture & Life Sciences Cooperative Extension Program,
which would facilitate applied LID work.
Creating shared standards and jointly addressing the policy
issues related to LID, will help bring LID technology into the
mainstream and will ultimately benefit the Chesapeake Bay. At
both the state and regional level, jurisdictions and agencies
are working together with private interests to protect water
quality and incorporate LID into their codes and engineering
guidelines.
To learn more about low impact development, visit
Dr. Bill Hunt’s web site or attend the next LID working
group meeting on January 26, 2007. For more information on the
Potomac Watershed LID working group, contact Bob
Slusser of Virginia DCR. |
