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Illinois Urban Manual
PRACTICE STANDARD
URBAN STORMWATER WETLAND
(acre)
Code 800
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(Source: Native Plant Guide for Streams and
Stormwater Facilities in Northeastern Illinois)
DEFINITION
A constructed system of shallow pools that create growing conditions suitable
for emergent and riparian wetland plants explicitly designed to lessen the
impacts of stormwater quality and quantity in urban areas.
PURPOSE
Stormwater wetlands are designed and installed to maximize pollutant removal
and create wetland habitat through the creation of a matrix of water, sediment,
plants, and detritus that collectively provides temporary storage of urban
stormwater runoff, and removes multiple pollutants from it through a series of
complementary physical, chemical, and biological pathways.
CONDITIONS WHERE PRACTICE APPLIES
This practice applies to watersheds in urban or urbanizing landscapes where
stormwater quality and quantity control is needed to meet the diverse management
objectives of developers and local governing units. Stormwater wetlands
typically are not located within delineated natural wetland areas. Natural
wetlands provide critical habitat and ecosystem benefits and are protected under
local, state and federal statutes. Stormwater wetlands should also not be
confused with created or restored wetlands that are used to mitigate for the
loss of natural wetlands under permitting provisions of wetland protection
requirements.
CRITERIA
For maximum effectiveness, the following basic design criteria should be
followed to achieve the major objectives of the stormwater wetland:
- Capture and effectively treat the stormwater runoff produced by 90 percent
of the storms in the urban watershed by designing a stormwater wetland to
meet seven basic sizing criteria:
- Minimum treatment volume to capture and treat an amount equal to 0.5
watershed inch.
- Surface area requirement - minimum wetland to watershed ratio:
Shallow Marsh Wetland 2%
Pond/wetlands 1%
Extended Detention (ED)/wetlands 1%
Pocket wetlands 1%
Wetland types are defined in Reference 1.
- Depth/Area allocation guidelines are shown in Table
2.
- Treatment/Volume allocation guidelines are shown in
Table 2.
- Flow path length - dry weather flow path of 2:1 for length to width ratio.
- Dry weather water balance - determine that inflow and ground water inputs
are greater than infiltration and evaporation water losses for all designs
except pocket wetlands.
- Extended detention volume - consider extended volume, time, release,
clogging protection, and water elevation.
- Pre-treat the stormwater runoff before it reaches the wetland area to
reduce water velocity, trap coarse sediments and associated pollutants.
Examples of pre-treatment structures are pre-settling basins and forebays.
- Create a diversity of depth zones within the wetland to meet the unique
growing requirements of emergent wetland plants.
- Establish a diverse and dense wetland plant community in the shortest
possible time.
- Create a functional pondscape within and around the wetland that augments
pollutant removal, creates better wildlife habitat, and promotes a more
natural appearance.
- Reduce the future maintenance burden of the stormwater wetland through
preventative management to protect its long-term function.
- Provide habitat elements that promote greater wildlife and waterfowl use
within the wetland and buffer, but avoid undesirable habitat outcomes.
- Serve as an attractive, yet safe, community amenity for adjacent
residents.
- Reduce or avoid any undesirable secondary environmental impacts produced
by the construction or operation of the stormwater wetland.
CONSIDERATIONS
Avoid conflict with natural wetlands wherever possible. Employ design
techniques to enhance pollutant removal performance of stormwater wetland
systems (Table 1). Establish the plant community by transplanting stock native
to the region and/or by utilizing mulch/topsoil from a nearby donor wetland
scheduled to be developed. Plan habitat diversity to meet the feeding,
breeding/nesting, and cover requirements for a wide range of aquatic, avian, and
terrestrial species. Check with state and/or federal agencies that issue permits
for wetlands about the regulatory status of stormwater wetlands and needed
permits prior to construction.
PLANS AND SPECIFICATIONS
Site suitability should be determined on each site by field observation by a
qualified interdisciplinary design team with expertise in stormwater
engineering, wetlands, landscaping, and pond construction. Construction
specifications should be shown in a site-specific construction plan or drawing.
Design criteria for stormwater wetland designs are shown in Table
3.
OPERATION AND MAINTENANCE
Both initial establishment and future development of a stormwater wetland
require active management of the hydrology and vegetation, as it grows in
biomass, diversity, and spatial coverage.
The design team must plan for the future operation and maintenance of the
stormwater wetland in this stage, with a strong emphasis on the first three
years. Maintenance activities must be fully vested with a responsible party
through an enforceable maintenance covenant. The covenant should specifically
include a projected schedule for inspections and forebay sediment cleanouts, and
show evidence that dedicated funding will be available to perform this function.
Inspection Criteria -The stormwater wetland should be inspected twice a
year in the first three years after construction, with an annual inspection
thereafter. Inspections should be conducted with the as-built and pondscaping
plans in hand, and should take specific note of species distribution/survival,
sediment accumulation, water elevations, and condition of the outlet. Records
should be stored so that the progressive development of the wetland system over
time can be tracked.
Sediment Cleanout - Accumulated sediment in the forebay should be cleaned
out every 3 to 5 years. Cleanouts are conducted after draining the forebay with
the help of a skid loader or backhoe. The preferred disposal method is on-site
land application at a pre-designated spoil area.
Mowing - The maintenance access, maintenance bench and embankment should
be mowed once a year to prevent woody growth. All remaining areas can be managed
as a wet meadow or forest.
REFERENCES
Schueler, Thomas R.;1992. Design of Stormwater Wetland Systems: Guidelines
for Creating Diverse and Effective Stormwater Wetlands In the Middle-Atlantic
Region. Metropolitan Washington Council of Governments, Washington, DC
Washington State Department of Ecology, 1991. Stormwater Management Manual
for the Puget Sound Basin (Public Review Draft), WA
NRCS IL - August 1994
TABLE 1 - From Table 7, Schueler, 1992.
DESIGN TECHNIQUES TO ENHANCE
POLLUTANT REMOVAL PERFORMANCE OF STORMWATER
WETLAND SYSTEMS |
- Increase the Volume of Runoff Treatment
- Capture greater percentage of annual runoff volume
- Provide for longer residence time in wetland for most storm events
- Increase the Surface Area to Volume Ratio
- Increase the total area of the wetland, or
- Increase the internal structural complexity of the wetland, by adding
complex microtopography and establishing extensive and dense wetland plant
cover
- Increase the Effective Flow Path Through the Wetland
- Extend distance between the inlet and outlet berms
- Maximize sinuosity of dry weather flow path with high marsh wedges
- Create some areas with extremely shallow flow path (i.e., high marsh)
- Use multiple cells within the wetland system
- Provide Runoff Pre-Treatment and Energy Dissipation
- Use forebay or pond cell near inlet, with broad crested weirs to spread
flow between cells
- Utilize Redundant Pollutant Removal Pathways
- Provide extended detention to keep removal rates reliable during
non-growing season, or
- Utilize permanent pool to increase algal uptake and sedimentation
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GUIDELINES FOR THE ALLOCATION
OF DEPTH ZONES AND TREATMENT VOLUME IN STORMWATER WETLAND SYSTEMS
Stormwater Wetland Systems |
TABLE 2 - From Table 11, Schueler, 1992.
TARGET
ALLOCATIONS |
Design
No. 1 Shallow
Marsh |
Design
No. 2 Pond/
Wetland |
Design
No. 3 ED
Wetland |
Design
No. 4 Pocket
Wetland |
Percent of
Surface Area |
Forebay |
5% |
0% |
5% |
0% |
Micropool |
5% |
5% |
5% |
0% |
Deepwater |
5% |
40% |
0% |
5% |
"Low Marsh" |
40% |
25% |
40% |
50% |
"High Marsh" |
40% |
25% |
40% |
40% |
"Semi-wet" |
5% |
5% |
10% |
5% |
|
Percent of
Treatment Volume
|
Forebay |
10% |
0% |
10% |
0% |
Micropool |
10% |
10% |
10% |
0% |
Deepwater |
10% |
60% |
-- |
20% |
"Low Marsh" |
45% |
20% |
20% |
55% |
"High Marsh" |
25% |
10% |
10% |
25% |
"Semi-wet" |
0% |
0% |
50% |
0% |
-
Deepwater - One to six feet below normal pool
(includes forebays, micropools, pool and channel)
-
Low Marsh - Six to 18 inches below normal pool
-
High Marsh - Zero to six inches below normal pool
-
Semi-wet - Zero to two feet above normal pool
(includes ED)
Note: The allocations are only general guidelines and will vary
according to design and site constraints.
TABLE 3 - From Table 16, Schueler, 1992.
DESIGN CRITERIA FOR STORMWATER WETLAND DESIGNS |
DESIGN
CRITERIA
|
Design No. 1 SHALLOW MARSH |
Design No. 2 POND/ WETLAND |
Design No. 3 ED WETLAND |
Design No. 4 POCKET WETLAND |
Wetland/Watershed Ratio |
.02 |
.01 |
.01 |
.01 (target) |
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Minimum Drainage Area |
25 acre |
25 acre |
10 acre |
1-10 acre |
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Length to Width (minimum) |
1:1 |
1:1 |
1:1 |
1:1 (target) |
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Extended Detention |
No |
No |
Yes |
No |
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Allocation of Treatment Volume (pool, marsh, ED) |
40/60/0 |
70/30/0 |
20/30/50 |
20/80/0 |
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Allocation of Surface Area (deep, low, high)
|
20/40/40 |
45/25/30 |
20/35/45 |
10/40/50 |
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Cleanout Frequency |
2-5 yrs. |
10 yrs. |
2-5 yrs. |
10 yrs. |
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Forebay |
Required |
No |
Required |
Optional |
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Micropool |
Required |
Required |
Required |
Optional |
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Outlet Configuration
|
Reverse-slope pipe or hooded broad crested weir
|
Same |
Same |
Hooded broad crested weir |
Propagation Technique |
Mulch or Transplant |
Mulch or
Transplant |
Mulch or
Transplant |
Volunteer |
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Buffer (feet) |
20 to 50 |
25 to 50 |
25 to 50 |
0 to 25 |
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Pondscaping Plan Requirements |
Emphasize wildlife habitat marsh micro-topography, buffer |
Emphasize wildlife habitat and high marsh wedges |
Emphasize stabilization of ED zone, project pondscaping zones |
Pondscaping plan optional |
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