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.
A. Timing – Sediment traps shall be constructed prior to disturbance of up-slope areas and placed so they function during all phases of construction. Sediment traps shall be placed in locations where runoff from disturbed areas can be diverted into the traps.
B. Sizing Criteria – Properly sized sediment traps are relatively effective at trapping medium and coarse-grained particles. To effectively trap fine-grained particles, the sediment trap must employ a large surface area or polymers. The specific trapping efficiency of a sediment trap varies based on the surface area, depth of dead storage, and the particle size distribution and concentration of sediment entering the device.
1. Surface Area – The minimum surface area of a sediment trap shall be based on the dominant textural class of the soil entering the device. The surface area calculated below represents the surface for the permanent pool area
(if wet) or the surface area for the dead storage. This surface area is measured at the invert of the stone outlet (see Figure 1).
a. For coarse textured soils (loamy sand, sandy loam, and sand):
As (coarse) = 625 * Adr
b. For medium textured soils (loams, silt loams, and silt):
As (medium) = 1560 * Adr
c. For fine textured soils (sandy clay, silty clay, silty clay loam, clay loam, and clay):
As (fine) = 5300 * Adr
For the equations above:
As = surface area of storage volume in square feet
Adr = contributory drainage area in acres
Note: The equations above were derived using a representative particle distribution for detached sediment for each textural class.
Sediment traps designed based on this standard will achieve 80% reduction of suspended solids for the drainage area.
d. The surface area of sediment traps used in areas with fine to medium sized soils can be reduced when used in conjunction with water applied polymers. When employing polymers, size the surface area for controlling fine particles using the criteria for medium soils (V.B.1.b.) and when controlling medium sized particles use the sizing equation contained in (V.B.1.a.) for coarse soils. See WDNR Conservation Practice Standard Sediment Control Water Application of Polymers (1051) for criteria governing the proper use and selection of polymers.
2. Depth – The depth of the sediment trap measured from the sediment trap bottom to the invert of the stone outlet, shall be at least three feet to minimize re-suspension and provide storage for sediment.
3. Shape – The sediment trap shall have a length to width ratio of at least 2:1. The position of the outlet to the inlet shall be as such to minimize short-circuiting of the water flow path.
4. Side Slopes – Side slopes shall be no steeper than 2:1.
Note: A sediment trap sized with the surface area equations above, a three-foot depth, and 2:1 side slopes will generally result in an 80% sediment reduction. Slopes flatter than 2:1 will require larger surface areas to provide adequate storage.
C. Embankment – Embankments of temporary sediment traps shall not exceed five feet in height measured from the downstream toe of the embankment to the top of the embankment. Construct embankments with a minimum top width of four feet, and side slopes of 2:1 or flatter. Earthen embankments shall be compacted. Where sediment traps are employed as a perimeter control, the embankments shall have stabilization practices place prior to receiving runoff.
D. Outlet – Sediment traps shall be constructed with both a principal and emergency spillway. The stone outlet of a sediment trap shall consist of a stone section of embankment (stone outlet) located at the discharge point. The stone outlet section provides a means of dewatering the basin back to the top of the permanent storage between storm events, and also serves as a non-erosive emergency spillway for larger flow events.
1. Outlet Size – The size of the outlet shall depend on the contributory drainage area and desired outflow. The length of the stone outlet / weir outlet can be calculated based on the size of the drainage area found in Table 1. Refer to section IX References for the equation used to calculate flow through a stone outlet or gabion.
The emergency spillway (top of the weir) shall be sized to adequately pass the 10-year 24-hour storm without over topping the sediment trap. The crest of the spillway shall be at least one foot below the top of the embankment. The, minimum weir lengths provided in Table 1 are adequate to pass the 10 year event.
Note: The weir length has little effect on overall treatment efficiency provided the sizing criteria in Section V.B. is adhered to. The stone outlet shall have a minimum top width of 2 feet and a maximum side-slope of 2:1. Discharge from the sediment basin shall be safely conveyed to a stormwater facility, drainage way, or waterbody. The discharge velocity shall be below the velocity to initiate scour unless appropriate stabilization methods are employed.
2. Stone Size – Stone shall consist of angular well graded 3 to 6 inch clear washed stone.
3. Keyway Trench – The stone outlet shall be protected from undercutting by excavating a keyway trench across the stone foundation and up the sides to the height of the outlet. See Figure 1. Underlying with geotextile fabric is optional.
E. Provide access for cleanout and disposal of trapped sediment.
VI Considerations
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.
