905R91106
United States
Environmental Protection
Agency
Watershed Management Unit
Water Division, Region V
Chicago, IL
February 1991
&EPA
Regional Stormwater
Management Planning
Introduction
tormwater can cause a multitude of problems in urban settings, from
flooding to degrading the water quality of the lakes, streams, and
rivers that eventually receive it. Cities and towns within the same
basin may share similar stormwater problems, linked simply by the
hydrologic and hydraulic characteristics of their common basin.
So as communities plan to deal with the effects of stormwater they often find they
can realize economies of scale by working together on a regional level. Cities that
forgo an individual, and therefore piecemeal approach, to institute Urban Best
Management Practices (in contrast to rural BMPs) as part of a regional plan, can
produce a sound, cost-effective strategy for their entire basin.
Mill Creek/Lake Powell Watershed
his 2,600-acre forested watershed,
now relatively undeveloped, has
taken a regional approach to
planning for stormwater
management. The Mill Creek/Lake
Powell Watershed in James City
County, Virginia, foresees future
development that will transform much of its forest
into residential, industrial, and commercial land.
Without stormwater controls, this development will
lead to flooding, erosion of stream banks, and
water quality problems at Lake Powell, which
drains much of the watershed.
Several different management options were
evaluated, many of which involved regional
detention basins sited at various locations within
An aerial view of a forested watershed, with development
beginning on either side.
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the watershed. The recommended plan involves
using four regional extended detention dry ponds.
These ponds require more storage area than
regional dry ponds, but can also address
sedimentation and water quality problems.
Together, all four ponds are believed to be
capable of removing the following estimated
quantities of pollutants:
sediment 90 percent
phosphorus 40 percent
nitrogen 25 percent
lead 80 percent
zinc 45 percent
Using existing estimates for nonpoint source
loadings, this corresponds to a total annual
removal rate for the four ponds of
105 tons per year of sediment,
179 pounds per year of phosphorus,
920 pounds per year of nitrogen,
148 pounds per year of lead, and
60 pounds per year of zinc.
Removal rates can vary significantly depending
on basin size, the nonpoint source loading rate,
and detention time.
The Developer's Role
n option to be
considered for
controlling future
development is the
smaller scale BMP a
developer might install.
The developer's erosion
controls can usually be converted to
permanent facilities, optimally located
at the same place. Furthermore, the
developer would bear the original
capital costs, with the continuing
costs becoming the responsibility of
homeowners' associations and not
the County.
Although these smaller, more
dispersed, onsite facilities are usually
less economical on a regional scale,
their costs are borne directly by the
developments that generate the
needs for control.
The regional detention basin approach offers
significant advantages over the onsite detention
approach, including:
reductions in capital and operation and
maintenance costs;
A developer built this dry detention basin to control erosion in a subdivision.
reductions in the risk of downstream flooding
and erosion;
opportunities to manage existing stormwater
problems; and
flexibility to consider sites not currently
being developed.
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Local Government's Role
o succeed, a regional management
strategy requires local jurisdiction to
assume several responsibilities.
Local governments must
perform advance planning
studies to locate and develop
preliminary designs for regional
stormwater management
facilities;
finance, design, and construct the regional
stormwater management facilities before
most urban development occurs, with
reimbursement by developers over a 5- to
20-year build-out period.
In addition, some local governments may have
to maintain regional stormwater management
facilities. These maintenance activities can be
continuous, such as nuisance algae control, and/or
periodic, such as dredging.
Existing Systems
egional planning can also be used to
rectify shortcomings in an existing
stormwater system. Such a strategy
was developed in response to an
evaluation of storm sewer systems
and stormwater management
structures in eastern Loudoun
County, Virginia.
Development in this area had proceeded in a
piecemeal fashion over the past 20 years,
resulting in inadequate stormwater facilities and no
management policy. By analyzing the area from a
regional perspective, an effective strategy was
formulated.
The strategy focused on maintenance and
retrofitting. Trash, snagged dead wood, and brush
were to be cleaned out from streams. Obstructions
were to be cleared from outfalls to improve the
subsurface system. Vegetation and trash near
openings were to be removed, broken endwalls
replaced, and riprap installed at each outfall to
prevent erosion. In certain locations dry ponds
would remove some sediment and metals and
relieve peak flows.
Debris and vegetation should be cleared from stormwater outfalls.
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From Concrete to Grass
tormwater management is evolving
in orientation. In the past, controls
focused on expensive concrete
structures such as culverts and lined
channels. Today, stormwater
management strategies incorporate
soft designs such as ponds and
grassed swales, and emphasize
maintenance.
Regional planning must consider numerous
alternatives and combinations of management
practicesand their potential effects. For
example, many BMPs remove some pollutants
through infiltration, using trenches, detention
ponds, and swales. Although these techniques
slow the stormwater so as to avoid flooding and
settle out pollutants, infiltration of pollutants can
also harm ground water.
A regional stormwater management plan must
be developed on a case-by-case basis, accounting
for both long- and short-term environmental effects
and flood hazards as well as the plan's cost
effectiveness for the communities contained within
a basin.
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TERRENE
INSTITUTE
This project was funded by the U.S. Environmental Protection Agency Office of Water Enforcement
and Permits-Water Permits Division and managed by Region V Watershed Management Unit-
Water Division. Prepared by Dynamac Corporation, Entrance Engineers, Inc., and JT&A, Inc. For
copies of this publication, contact The Terrene Institute, 1000 Connecticut Avenue, NW, Suite 802,
Washington, DC 20036, (202) 833-8317.
Printed on Recycled Paper
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