2. Schedules and sequence of installation and removal
3. Standard drawings and installation details
4. Control structure detail and layout
5. Sizing of sediment storage area
6. Maintenance requirements
B. All plans, standard detail drawings, or specifications shall include sequence for installation, inspection, and maintenance requirements. The responsible party shall be identified.
VIII. Operation and Maintenance
A. Sediment basins 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.
B. Sediment shall be removed to maintain the three foot depth of the treatment surface area as measured from the invert of the principal outlet. Sediment may need to be removed more frequently.
C. If the outlet becomes clogged it shall be cleaned to restore flow capacity.
D. Provisions for proper disposal of the sediment removed shall be made.
E. Maintenance shall be completed as soon as possible with consideration to site conditions.
IX. References
Chapter
NR 333, Dam and Design Construction.
Hann, Barfield, and Hayes. Design Hydrology and Sedimentology for Small Catchments. Academic Press Inc., 1994.
Robert E. Pitt, Small Storm Hydrology.
USDA, Natural Resources Conservation Service, Ponds – Planning, Design, Construction. Agriculture Handbook No. 590, Revised September 1997.
WDNR Conservation Practice Standard 1001 Wet Detention Basin.
X. Definitions
Active Storage Volume (V.B.3) – Is measured from the invert of the lowest outlet to the invert of the emergency spillway.
Stabilized (III) – Means protecting exposed soil from erosion.
Treatment Surface Area (V.B.1) – Is the surface area of the sediment basin measured at the invert of the lowest outlet.
Figure 1
Clarification of Sediment Basin Terminology
Figure 2
Approximate Detention Basin Routing for Type II Storms
Rainfall Quantities:
Table 1 provides a summary of the 1-year, 24-hour rainfall totals using NRCS mandated TP-40 which has not been updated since 1961. Table 2 provides a summary of more current data from the Rainfall Frequency Atlas of the Midwest published in 1992. Local requirements may dictate the use of one dataset over the other.
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See PDF for table 
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See PDF for table
Attachment 1
Sample Sediment Basin Design Problem
The proper sizing and design of a sediment basin will often require iterative calculations. The technical standard for sizing sediment basins was written to give the designer as much flexibility as possible in designing the basin while meeting water quality requirements. The governing equation relates the surface area of the sediment basin to the outflow and critical particle settling velocity. The larger the sediment basin outflow, the larger the surface area required to settle the particle. As the outflow is reduced, a smaller surface area is required however the required storage volume dictates how small a surface area can become through the storage depth or hydraulic head acting on the outlet.
The particle settling velocities are listed in the standard requiring the designer to either start with a desired outflow based on an outlet size or an estimated starting surface area. The sample equation below starts with an estimated surface area.
Sample Problem:
A 10 acre site is being developed into condos. Eight acres of the site are being disturbed while 2 acres of forest are remaining undisturbed. The dominate soils on the site are silt loam. The 1-year, 24-hour design storm is 2.25 inches.
Step 1: Calculate runoff volume and peak using TR-55 or approved method.
From TR-55 the curve number (CN) for the disturbed area is 86 and the CN for the forested area is 55 resulting in a composite CN of 80. Using TR-55, the runoff volume calculated for the 1-year 24-hour design storm is 0.7 inches (0.6 acre-feet for the entire 10-acre site). The time of concentration was calculated as 0.4 hours resulting in a peak flow of 6 cfs.
Step 2: Begin sizing sediment basin using Equation 1. The technical standard lists silt loam under particle class 2 with a settling velocity of 7.3*10-5 ft/sec. We are also going to assume a starting surface area of 0.25 acres (10,890 ft2). An alternative approach is to assume an outflow velocity.
SA = 1.2 * (qout / v
s)
Solve for qout: 10,980 ft2 = 1.2 * (qout / 7.3*10-5 ft/sec)
qout = 0.67 cfs
Step 3: Using Figure 2: Approximate Detention Basin Routing for Type II Storms determines the volume of storage (VS) needed.
qout = 0.67 cfs (calculated in Step 2)
qin = 6.0 cfs (peak flow calculated using TR-55 in Step 1)
VR = 0.6 acre-feet (volume of runoff calculated using TR-55 in Step 1)
qout / qin = 0.67 cfs / 6.0 cfs = 0.11. Using Figure 2 with a q
out / qin = 0.11, the VS/VR is determined to be 0.54. Therefore the VS = 0.54 * 0.6 acre-feet = 0.324 acre-feet (14,113 ft3)
Step 4: Check configuration: Calculate maximum head on outlet using surface area and volume.
SA = 10,890 ft2 and a VS = 14,113 ft3 we get a depth (H) of 1.29 feet = 14,113 ft
3 / 10,890 ft2
Step 5: Size Outlet: Assuming an orifice type outlet calculate the size needed to meet the qout calculated in Step 1 and the H calculated in Step 4.
Using the orifice equation: qout = C*A*(2gH)1/2 with C=0.6 (coefficient) , A = Area = ft2, g = 32.2, and H = hydraulic head expressed in feet.
qout = 0.6*A*(2*32.2* H)1/2 so 0.66 = 0.6*A*(2*32.2*1.29)1/2 therefore A = .12 ft2
An area of 0.12 ft2 corresponds to an orifice outlet of 4.7 inches in diameter.
Step 6: Iteration: While the above solution works, the sediment basin has not been optimally sized and we have an orifice diameter that is not a standard pipe size. An iterative approach can be used to reduce the surface area of the sediment basin and obtain a more common orifice diameter. We can assume a 4-inch orifice since it is close to diameter calculated in Step 5 and we can start with the depth we calculated in Step 4. The iterations below each represent Steps 2 through 5.
Iteration 1:
qout = 0.43 (H) 1/2 = 0.43 (1.29) 1/2 = 0.48 cfs which is less than the 0.66 cfs calculated in Step 1. Therefore, we can go back to Step 1 and repeat the sizing procedure and downsize the sediment basin.
SA = 1.2 * (qout / vs) = 1.2 * (0.48 cfs / 7.3*10-5 ft/sec) = 7,890 ft2
Using Figure 2:
qout = 0.48 cfs
qin = 6.0 cfs (peak flow calculated using TR-55 in Step 1)
VR = 0.6 acre-feet (volume of runoff calculated using TR-55 in Step 1)
qout / qin = 0.48 cfs / 6.0 cfs = 0.08. Using Figure 2 with a qout / qin = 0.08, the VS/VR is determined to be 0.62. Therefore the VS = 0.62 * 0.6 acre-feet = 0.372 acre-feet (16,204 ft3)
SA = 7,890 ft2 and a VS = 16,204 ft3 we get a depth (H) of 2.05 feet = 16,204 ft
3 / 7,890 ft2
qout = 0.43 (H) 1/2 = 0.43 (2.05) 1/2 = 0.61 cfs which is more than the 0.48 cfs we used so iterate.
Iteration 2:
SA = 1.2 * (qout / v
s) = 1.2 * (0.61 cfs / 7.3*10-5 ft/sec) = 10,027 ft2
Using Figure 2:
qout = 0.61 cfs
qin = 6.0 cfs (peak flow calculated using TR-55 in Step 1)
VR = 0.6 acre-feet (volume of runoff calculated using TR-55 in Step 1)
qout / qin = 0.61 cfs / 6.0 cfs = 0.10 Using Figure 2 with a q
out / qin = 0.10, the VS/VR is determined to be 0.54. Therefore the VS = 0.54 * 0.6 acre-feet = 0.324 acre-feet (14,113 ft3)
SA = 10,027 ft2 and a VS = 14,113 ft3 we get a depth (H) of 1.41 feet = 14,113 ft
3 / 10,027 ft2
qout = 0.43 (H) 1/2 = 0.43 (1.41) 1/2 = 0.51 cfs which is less than the 0.61 cfs we used so we are OK or we can iterate again until we have qout that are almost identical.
After Iteration 2, we have a sediment basin with a SA = 10,027 ft
2 and a VS = 14,113 ft3. We have a principal water quality outlet consisting of a 4-inch orifice. This design meets the water quality requirements of the technical standard.
Sediment Trap
1063 (09/05)
Wisconsin Department of Natural Resources
Conservation Practice Standard
I. Definition
A temporary1 sediment control device formed by excavation and/or embankment to intercept sediment-laden runoff and to retain the sediment.
II. Purposes
To detain sediment-laden runoff from disturbed areas for sufficient time to allow the majority of the sediment to settle out.
III. Conditions Where Practice Applies
Sediment traps are utilized in areas of concentrated flow or points of discharge during construction activities. Sediment traps shall be constructed at locations accessible for clean out. Sediment traps are designed to be in place until the contributory drainage area has been stabilized. The contributory drainage area shall be a maximum of 5 acres. For concentrated flow areas smaller than one acre, ditch checks may be installed; refer to WDNR conservation practice standard Ditch Check (1062). For larger drainage areas and/or for sediment basins requiring an engineered outlet structure refer to WDNR conservation practice standard Sediment Basin (1064) or Wet Detention Basin (1001).
IV. Federal, State, and Local Laws
Users of this standard shall be aware of applicable federal, state, and local laws, rules, regulations, or permit requirements governing the use and placement of sediment traps. 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.
