Vr = Calculated volume of runoff from the 1-year 24-hour design storm for the entire contributory area with the maximum area of disturbance characterized as bare soil.
Vs = Is the required active storage volume determined using Figure 2.
b. The active storage volume may be calculated based on routing the 1-year, 24-hour storm provided the principal outlet requirements stipulated in section V.D.2 are maintained. This method will require the use of a model.
Note: Both these methods require iterative calculations.
Shape – The length to width ratio of the flow path shall be maximized with a goal of 3:1 or greater. The flow path is considered the general direction of water flow within the basin including the treatment surface area and any forebay.
C. Embankments – Earthen embankments shall be designed to address potential risk and structural integrity issues such as seepage and saturation. All constructed earthen embankments shall meet the following criteria.
1. The base of the embankment shall be stripped of all vegetation, stumps, topsoil and other organic matter.
2. Side slopes shall be 3:1 or flatter. The minimum embankment top width shall be adequate to provide structural stability. Where applicable the top width shall be wide enough to provide maintenance access.
3. There shall be a core trench or key-way along the embankment.
Any pipes extending through the embankment shall be bedded and backfilled with equivalent soils used to construct the embankment. The bedding and backfill shall be compacted in lifts and to the same standard as the original embankment. Excavation through a completed embankment shall have a minimum side slope of 1:1 or flatter.
Measures shall be taken to minimize seepage along any conduit buried in the embankment.
D. Outlet – Sediment basins shall have both a principal outlet and an overflow spillway.
1. Timing – Outlets must be constructed in conjunction with the remainder of the basin and must be constructed prior to the basin receiving runoff. Sediment basins are ineffective until the outlet is constructed.
2. Principal Water Quality Outlet – The principal water quality outlet shall be designed to pass the 1-year 24-hour storm without use of the overflow spillway or other outlet structures. The maximum outflow (qo) from the principal water quality outlet shall be less than or equal to the q
o used in Equation 1 (V.B.1). If the sediment basin is to serve as a permanent stormwater basin, the principal outlet structure can be modified (i.e. removable plates) to meet flow requirements encountered during and after construction; separate outlet structures do not need to be constructed.
Note: Local ordinances may require control of larger storm events such as the 2-year 24 hour storms. In these cases, additional or compound outlets maybe required.
3. Overflow (Emergency) Spillway – An overflow spillway shall be provided consisting of an open channel constructed adjacent to the embankment and built over a stabilized area. The spillway shall be designed to carry the peak rate of runoff expected from a 10-year, 24-hour design storm or one commensurate with the degree of hazard, less any reduction due to flow in the principal outlet. The top of the embankment shall be at least one foot above the design high water level and a minimum of 1 foot above the invert of the overflow spillway. The overflow spillway shall be protected from erosion. Flow from the overflow spillway shall be directed away from the embankment.
4. Outlet Protection – All outlet designs shall incorporate preventive measures for ice damage, trash accumulation, and erosion at the outfall. For orifices less than 8-inches in diameter, or equivalent, additional measures to prevent clogging are required.
E. Inlet Protection – Inlets shall be designed to prevent scour and reduce velocities during peak flows. Possible design options include flow diffusion, plunge pools, directional berms, baffles, or other energy dissipation structures.
F. Location – Temporary sediment basins should be located to provide access for cleanout and disposal of trapped sediment.
G. Removal – Temporary sediment basins shall be removed after the contributing drainage area has been stabilized. Complete final grading and restoration according to the site plans. If standing water needs to be removed it shall be done in accordance with WDNR Conservation Practice Standard Dewatering (1061).
VI. Considerations
A. When constructing a sediment basin that will also serve as the long-term stormwater detention pond, build the sediment basin to the larger of the two sizes required either for stormwater control or erosion control. In addition, when sizing the outlet structure first design the outlet for the long-term stormwater management requirements then check to satisfy the flow requirements for sediment control during construction. If additional flow restriction is needed consider use of a temporary restriction plates or other measures to avoid having to construct separate outlet structures for the sediment basin and stormwater basin.
B. Over-excavation beyond the required depth in the sediment storage area of the sediment basin may allow for less frequent maintenance. Addition of other measures in the contributing drainage area may reduce sediment accumulation and associated maintenance requirements.
C. The use of a sediment forebay can extend the useful life of the main sediment storage area by trapping the majority of sediment in the forebay area. Separation of the forebay from the rest of the basin requires construction of a submerged shelf (if wet) or a stone or stabilized earthen embankment. The forebay should have a surface area equal to at least 12% of the total basin area.
D. In addition to soil stability issues, interior slopes of sediment basins should be selected based on safety issues commensurate with the degree of hazard.
VII. Plans and Specifications
A. Plans and specifications for installing sediment basins shall be in keeping with this standard and shall describe the requirements for applying the practice to achieve its intended purpose.
1. Location of sediment basin
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)
