A. Sediment traps generally require excessive surface areas to settle clay particles and fine silts. If these conditions exist on the site consider using a sediment basin (DNR Conservation Practice Standard Sediment Basin 1064) or adding polymer to the sediment trap. See WDNR Conservation Practice Standard Sediment Control Water Application of Polymers (1051) for criteria governing the use of polymers
B. To improve trapping efficiency, filter fabric can be placed on the up-slope side of the stone outlet / gabion and anchored with stone. When fabric is utilized to enhance filtering, more frequent maintenance is required to prevent clogging. When using fabric, a monofilament type fabric shall be used (such as WisDOT Type FF). The apparent opening size of the fabric, not the stone size, will dictate the flow rate through the outlet therefore outlet lengths need to be calculated since values in Table 1 are based on stone. When calculating the size of the outlet a clogging factor of 50% should be used for the fabric.
C. Consider possible interference with construction activities when locating sediment traps.
D. Provisions should be made for protecting the embankment from failure caused by storms exceeding the 10-year design requirement. Consider a stabilized and non-erosive emergency spillway bypass.
E. In general, groundwater impacts from temporary sediment traps that have storage areas in contact with groundwater are not a major concern. However, sediment trap contact with groundwater should be avoided in areas with karst features, fractured bedrock, or areas of significant groundwater recharge.
F. Sediment trapping is achieved primarily by settling within the pool formed by the trap. Sediment trapping efficiency is a function of surface area, depth of pool, and detention time. If site conditions permit, a length to width ratio greater than 2:1 will increase efficiency.
G. If site conditions prevent the sediment trap from having a three-foot depth, then an equivalent storage volume must be created through increasing the surface area.
H. For sediment traps in place longer than 6 months, consider outlets constructed of two types of stone. A combination of coarse aggregate and riprap (WisDOT light riprap classification) should be used to provide stability. A one-foot layer of one inch washed stone then should be placed on the up-slope face to reduce drainage flow rate.
VII Plans and Specifications
A. Plans and specifications for installing sediment traps shall be in keeping with this standard and shall describe the requirements for applying the practice to achieve its intended purpose. The plans and specifications shall address the following:
1. Location and spacing of sediment traps
2. Schedules and sequence of installation and removal
3. Standard drawings and installation details
4. Rock gradation
B. All plans, standard detail drawings, or specifications shall include a schedule for installation, inspection, maintenance, and identify the responsible party.
VIII Operation and Maintenance
Sediment Traps shall, at a minimum, be inspected weekly and within 24 hours after every precipitation event that produces 0.5 inches of rain or more during a 24-hour period. Sediment may need to be removed more frequently.
A. Deposits of sediment shall be removed when they reach a depth of one foot.
B. If the outlet becomes clogged it shall be cleaned to restore flow capacity.
C. Recommend provisions for proper disposal of the sediment removed from the trap.
D. Maintenance shall be completed as soon as possible with consideration given to site conditions.
E. Sediment traps shall be removed and the location stabilized after the disturbed area draining to the sediment trap is stabilized and no longer susceptible to erosion.
IX References
Flow through the stone outlet and gabion can be calculated using the following equation:
Q = ( h2/3 * L ) / [(W/D) + 25 + W 2 ]1/2
Where:
Q = total flow through stone (cfs)
h = depth of flow measured from invert of the stone outlet to the crest of emergency spillway (ft)
W = average width of weir or flow
length through stone outlet (ft)
L = length of weir (ft)
D = Average Rock Diameter (ft)
Note: For a stone outlet, the length of stone outlet (L) will vary with the depth and slope of stone outlet. For a gabion, the length of flow is fixed to gabion width. A complete discussion of this equation and its proper application can be found in:
C. McIntyre, G. Aron, J. Willenbrock, and M. Deimler. Report No. 10: Analysis of flow through porous media as applied to gabion dams regarding the storage and release of storm water runoff. NAHB/NRC Designated Housing Research Center at Penn State, Department of Civil Engineering; August 1992.
X Definitions
Stabilized (III): Means that all land disturbing construction activities at the construction site have been completed and that a uniform perennial vegetative cover has been established with a density of at least 70% of the cover for the unpaved areas and areas not covered by permanent structures or that employ equivalent stabilization measures.
Temporary (I): An erosion control measure that is in place for the duration of construction or until the site is stabilized.
Seeding For Construction Site Erosion Control
1059 (11/03)
Wisconsin Department of Natural Resources
Conservation Practice Standard
Note: Words in the standard that are shown in italics are described in X. Definitions. The words are italicized the first time they are used.
I. Definition
Planting seed to establish temporary or permanent vegetation for erosion control.
II. Purpose
The purpose of temporary seeding1 is to reduce runoff and erosion until permanent vegetation or other erosion control practices can be established. The purpose of permanent seeding is to permanently stabilize areas of exposed soil.
Ill. Conditions Where Practice Applies
This practice applies to areas of exposed soil where the establishment of vegetation is desired. Temporary seeding applies to disturbed areas that will not be brought to final grade or on which land-disturbing activities will not be performed for a period greater than 30 days, and requires vegetative cover for less than one year. Permanent seeding applies to areas where perennial vegetative cover is needed.
IV. Federal, State and Local Laws
Users of this standard shall be aware of all applicable federal, state and local laws, rules, regulations or permit requirements governing seeding. This standard does not contain the text of federal, state or local laws.
V. Criteria
This section establishes the minimum standards for design, installation and performance requirements.
A. Site and Seedbed Preparation
Site preparation activities shall include:
1. Temporary Seeding
a. Temporary seeding requires a seedbed of loose soil to a minimum depth of 2 inches.
b. Fertilizer application is not generally required for temporary seeding. However, any application of fertilizer or lime shall be based on soil testing results.
c. The soil shall have a pH range of 5.5 to 8.0.
2. Permanent Seeding
a. Topsoil installation shall be completed prior to permanent seeding.
b. Permanent seeding requires a seedbed of loose topsoil to a minimum depth of 4 inches with the ability to support a dense vegetative cover.
c. Application rates of fertilizer or lime shall be based on soil testing results.
d. Prepare a tilled, fine, but firm seedbed. Remove rocks, twigs foreign material and clods over two inches that cannot be broken down.
e. The soil shall have a pH range of 5.5 to 8.0.
B. Seeding
1. Seed Selection
a. Seed mixtures that will produce dense vegetation shall be selected based on soil and site conditions and intended final use. Section IX References, lists sources containing suggested seed mixtures.
b.
All seed shall conform to the requirements of the Wisconsin Statutes and of the Administrative Code Chapter
ATCP 20.01 regarding noxious weed seed content and labeling.
c. Seed mixtures that contain potentially invasive species or species that may be harmful to native plant communities shall be avoided.
d. Seed shall not be used later than one year after the test date that appears on the label.
e. Seed shall be tested for purity, germination and noxious weed seed content and shall meet the minimum purity and germination requirements as prescribed in the current edition of Rules for Testing Seed, published by the Association of Official Seed Analysts.
2. Seed Rates
a. Temporary Seeding (Cover Crop)
Areas needing protection during periods when permanent seeding is not applied shall be seeded with annual species for temporary protection. See Table 1 for seeding rates of commonly used species. The residue from this crop may either be incorporated into the soil during seedbed preparation at the next permanent seeding period or left on the soil surface and the planting made as a no-till seeding.
Table 1
b. Permanent Seeding
Rates shall be based on pounds or ounces of Pure Live Seed (PLS) per acre. Section IX contains some possible reference documents that provide seeding rates. Permanent seeding rates may be increased above the minimum rates shown in the reference documents to address land use and environmental conditions.
If a nurse crop is used in conjunction with permanent seeding, the nurse crop shall not hinder establishment of the permanent vegetation.
A nurse crop shall be applied at 50% its temporary seeding rate when applied with permanent seed.
3. Inoculation
Legume seed shall be inoculated in accordance with the manufacturer's recommendations. Inoculants shall not be mixed with liquid fertilizer.
4. Sowing
Seed grasses and legumes no more than ¼ inch deep. Distribute seed uniformly. Mixtures with low seeding rates require special care in sowing to achieve proper seed distribution.
Seed may be broadcast, drilled, or hydroseeded as appropriate for the site.
Seed when soil temperatures remain consistently above 53° F. Dormant seed when the soil temperature is consistently below 53° F (typically Nov. 1st until snow cover). Seed shall not be applied on top of snow.
VI. Considerations
A. Consider seeding at a lower rate and making two passes to ensure adequate coverage.
