For "point source" discharges (construction
sites, maintenance yards, rest areas) the Clean Water
Act requires pollutant removal, depending on the type
of pollutant, that represents best conventional pollutant
control technology (BCT) or best available technology
economically achievable (BAT). In addition, these other
point sources are required to comply with local water
quality standards if they happen to be higher, even
if this means controls beyond BCT/BAT, such as "all
known available and reasonable" technology or
methods of pollution prevention (AKART). BCT and BAT
are numerically defined for most industries but not
for construction site runoff. This has meant that construction
site requirements, as well as those for municipal runoff,
have been something of a moving target.
All projects should incorporate certain minimum design
elements with respect to water quality concerns. Such
design goals include the following: [N]
- Minimize Impervious Surfaces: The intent
of this goal is to reduce the volume of runoff.
- Prevent Downstream Erosion: Stormwater drainage
systems should be designed to avoid causing or contributing
to downstream erosion.
- Stabilize Disturbed Soil Areas: Disturbed
soil areas should be appropriately stabilized.
- Maximize Vegetated Surfaces Consistent with
Existing Policies: Vegetated surfaces prevent
erosion, promote infiltration (which reduces runoff),
and remove pollutants from stormwater. See the following
section on design for sustainable, low maintenance
roadsides.
NYSDOT's Operations & Maintenance
Manual for Stormwater Facilities contains the
following general pre-construction stormwater facility
design considerations: [N]
- Discuss proposed facilities with the Maintenance
Environmental Coordinator and Residency personnel.
- Make facilities visible. Visible structures get
more attention.
- Select low-growing suitable grasses to reduce
mowing needs. Add nitrogen fixing plants such as clover
to reduce fertilizing needs.
- Incorporate reference points into basins and other
features that require clean-out in regard to an absolute
elevation. How would someone know how much sediment
has accumulated? Percentage of capacity reduction
is difficult to estimate.
- Consider mosquito control such as introducing
natural predators into a permanent pool (for example
mosquito eating fish) or placing commercial mosquito
traps. Non-native species should not have a means
of escape.
- Features to be maintained must be accessible.
Consider access roads, ramps to basin bottoms, sturdy
slopes.
- Trash racks should be accessible at normal pool
elevations.
- Do not plant trees and shrubs on embankments,
side slopes or dam areas.
- Slopes that should be mowed should be 1 on 3 or
flatter. If slope is higher than 5 feet, slope should
be 1on 4 or flatter. If steeper, explore other treatment
options.
- For non-vegetated covers, loose stone or rip rap,
which encourages the growth of weeds, should be discouraged.
Can consider using gabion lining.
- Consider effects of sediment removal from vegetated
surfaces. Can vegetative cover recuperate on its own?
- Aesthetic features of the stormwater management
facilities requested by municipalities should be maintained
by the requester. Commitments by municipalities must
be made by signed resolutions..
Low Impact Design
Low Impact Development (LID) is a stormwater management
strategy concerned with maintaining or restoring the
natural hydrologic functions of a site to achieve natural
resource protection objectives and fulfill environmental
regulatory requirements. LID employs a variety of natural
and built features that reduce the rate of runoff,
filter out its pollutants, and facilitate the infiltration
of water into the ground. By reducing water pollution
and increasing groundwater recharge, LID helps to improve
the quality of receiving surface waters and stabilize
the flow rates of nearby streams.
LID incorporates a set of overall site design strategies
as well as highly localized, small-scale, decentralized
source control techniques known as Integrated Management
Practices (IMPs). IMPs may be integrated into buildings,
infrastructure, or landscape design. Rather than collecting
runoff in piped or channelized networks and controlling
the flow downstream in a large stormwater management
facility, LID takes a decentralized approach that disperses
flows and manages runoff closer to where it originates.
The LID effort in Prince George's County was one of
the first. It began with the development and use of
bioretention cells, which are created by replacing
existing soil with a highly porous soil mixture, grading
the area to form a shallow depression, and replanting
the area with specially selected vegetation. Such vegetation
must be able to tolerate temporarily saturated soil
conditions as well as the pollutants contained in the
local runoff. When it rains, bioretention areas collect
the runoff and then filter out the pollutants as the
water passes down through the soil. After initial experimentation,
the County initiated a full-scale effort to incorporate
the LID approach; the Prince George's County manual
was expanded into a nationally distributed LID manual
published in 2000. LID site design attempts to minimize
runoff volume and preserve existing flow paths, in
order to minimize infrastructure needs, in contrast
to conventional site design, which may generate increases
in runoff volume and energy, resulting in concentrated
flows that require larger and more extensive stormwater
infrastructure.
Examples of LID site design strategies and practices
include:
- Grade to encourage sheet flow and lengthen flow
paths.
- Maintain natural drainage divides to keep flow
paths dispersed.
- Disconnect impervious areas such as pavement and
roofs from the storm drain network, allowing runoff
to be conveyed over pervious areas instead.
- Preserve the naturally vegetated areas and soil
types that slow runoff, filter out pollutants, and
facilitate infiltration.
- Direct runoff into or across vegetated areas to
help filter runoff and encourage recharge.
- Provide small-scale distributed features and devices
that help meet regulatory and resource objectives.
- Treat pollutant loads where they are generated,
or prevent their generation
Bioretention, dry wells, filter strips, grassed swales,
infiltration trenches, and inlet pollution traps/removal
devices, and permeable pavers and pavement are some
of the common LID tools. Greater use of soil amendments
increase the capacity of soil to absorb moisture and
sustain vegetation, curbside or in swales, which in
turns removes water through transpiration.
Resources for LID design include:
EPA maintains a low impact development methods website
among its on-line non-point source pollution resources.
[N]
The Department of Defense has also created a Low
Impact Development manual, which reviews LID principles,
the cost, maintenance, and other considerations of
various LID BMPs, in Chapter 8. [N]
Recent DOT research has shown that some of the initial
assumptions guiding LID have not been valid and that
the concept can be more widely applied, in less space
than previously thought. One particularly important
study found that: [N]
- Over the range of slopes and system specific conditions
studied, runoff did not correlate positively to slope
(i.e., higher runoff was not observed at higher slopes).
Most importantly, the current design guideline requiring
LID slopes to be not steeper than 7:1 is not justified
and thus eliminates many areas with acceptable levels
of LID infiltration or increases the LID length well
beyond what is necessary at many roadside locations.
- Saturated conductivity (Ks) was found to be the
most critical design parameter. Required minimum values
of Ks (102 mm/hr) found in existing natural dispersion
guidance were found to be not physically based, again
eliminating areas that are well suited for LID application
or resulting in LID lengths greater than needed in
some situations. As a result of this research, a simplified,
user friendly design equation (LID Design Equation)
was developed and shown to be an accurate means of
determining LID length by comparison to field data
and calibrated model simulations using a finite difference
solution to the hydrodynamic wave equation dynamically
coupled with the Green-Ampt infiltration equation.[N]
Federally Sponsored Stormwater
BMP Manuals
Detailed selection guidance and information on BMP
effectiveness will be available from NCHRP 25-20(01)
in late 2006. Existing federally sponsored stormwater
BMP manual include the following.
Guidance on Low Volume
Road Design
Erosion and sedimentation control is a primary consideration
in the design and maintenance of low volume roads.
Some of the best guidance for transportation agencies
has been developed by the USDA Forest Service and the
PennDOT supported State Conservation Commission Dirt & Gravel
Road Program. The Forest Service and U.S. Agency
for International Development Guide covers best environmental
practices in the following areas:[N]
State Sponsored Stormwater
BMP Manuals
Almost every state DOT has a guide to development
of such plans and design of stormwater BMPs. The U.S.
EPA Region 10: The Pacific Northwest provides web links
to Stormwater
BMP manuals from various State agencies: According
to a 2003 survey by the author, 54 percent of all the
states have developed a Highway Runoff Manual; Caltrans,
FDOT, Illinois DOT, MoDOT, Ohio DOT, and TxDOT completed
revisions in the last two years. Almost 30 percent
of state DOTs have developed manuals for stormwater
management at non-highway facilities (AR, CA, FL, GA,
IH, MO, MT, NH, NV, WA) and stormwater manuals for
construction(AR, CA, FL, GA, IH, IA, IN, LA, MI, MO,
MT, NM, OH, WA.) [N]
Following is list of manuals available on-line:
California
Georgia
Idaho
Illinois
- Illinois Department of Transportation. Erosion
and Sediment Control NPDES for Standard Specifications
for Road & Bridge Construction.
Maine
Maryland
Massachusetts
Michigan
Minnesota
Missouri
Montana
New Hampshire
New Jersey
New York
North Carolina
Ohio
Oregon
Pennsylvania
South Carolina
Tennessee
Texas
Utah
Virginia
Washington
Wisconsin
Wyoming
State Stormwater BMP Manual Builder
The Stormwater Manager's Resource Center has developed
a "Manual
Builder" on-line, a toolbox for developing
a stormwater treatment practice (STP) design manual.
It includes stormwater treatment plan design and construction
criteria and provides schematics and graphics for each
practice. It also provides information about maintenance
requirements and the typical local review process for
treatment plan design and construction. As manuals
need to be customized to meet the needs of the state
or community where they are being applied, the manual
builder does not prescribe one specific set of criteria,
but instead presents a series of options for stormwater
managers to choose from. A good manual contains specific
guidance on how to select, size, design, construct,
and maintain practices at each development site.
A typical manual will contain the following elements:
- Basic Stormwater requirements.
- Procedures
for Reviewing Stormwater Plans. In this section,
the manual outlines the process a community should
go through to review the stormwater plan.
- Basic
Sizing Criteria. The manual needs to identify
minimum sizing criteria for practices to meet groundwater
recharge, water quality, channel protection, and
flood control requirements.
- List
of Acceptable Practices. The manual should include
a list of practices that can meet water quality requirements.
An engineer can choose from this menu of practices
to treat stormwater runoff from a new development.
- Performance
Criteria. The performance criteria provide required
minimum elements and guidance to ensure that practices
are designed and maintained to ensure practice longevity
and performance.
- Guidance
on STP Selection. This section presents criteria
to guide the design engineer to select the best practice
for the site, based on characteristics such as soil
type, site slope, and the local watershed conditions.
- Stormwater
Credits. Stormwater credits are reductions in
stormwater volume requirements given in exchange
for incorporating site design techniques that minimize
the need for STPs on the site. Many manuals do not
incorporate credits, because they can increase the
burden of review on local governments substantially.
- Design
Examples. Design examples step the engineer through
designs for a representative group of STP design
variations. They should illustrate how to select,
size, and locate the practice on the site.
- Construction
Specifications. Construction specifications detail
specific materials and construction standards that
ensure that the practice will function as designed.
- Checklists
for Construction Inspection. These checklists
outline what minimum elements are needed for each
practice group during construction.
In addition to extensive design guidance available
in both manual and on-line formats, a number of BMP
selection and evaluation systems are emerging. NCHRP
25-25(01) is designing a BMP effectiveness and evaluation
system that will be available in late 2004. MDSHA has
developed an evaluation system for all stormwater facilities
and criteria for improvements. In the late 1990s WSDOT
and FDOT also developed systems for categorizing outfalls
and, in the case of WSDOT, assessing which projects
provide the best return on investment in terms of environmental
effectiveness and pollution reduction.[N]
WSDOT's system included a condition indexing methodology
and support program that enables users to quickly evaluate
and compare projects and generate benefit-cost ratios
for projects. [N]
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