7.70 SUGGESTED EROSION AND SEDIMENT CONTROL DEVICES FOR SPECIFIC SITUATIONS
The best way to control erosion is to minimize the disturbed areas and to get the disturbed areas stabilized as soon as possible. Stabilizing seeding should be the first option. This requires planning and coordination to get “things” finished and seeded. When seeding is done late in the season, the growth may not be enough to provide adequate protection to the soils. Thus, mulching should be applied along with the seeding. Also, there are many cases during the construction projects that special care or devices need to be utilized. Following are some situations/information and solutions that should be used as guidelines during construction:
Typically, soils high in silt, low in clay, and low in organic material are the most erodible. Sediment control devices must be appropriate for the types of soil throughout the project. The inspector/engineer needs to review the soil sheets and soil report for the project.
The steeper and longer the ditch, the faster and stronger the flow would be. Again, the ditches should be seeded. They may also require Special Ditch Control, Turf Reinforced Mat (TRM), or rock. A Permanent Erosion Control Matrix used by designers is available as guidance to help determining the type of material appropriate for different ditch grades and lengths. It is available at https://iowadot.gov/design/dmanual/01E-06/EC-104.pdf.
Nevertheless, silt fence ditch checks or another type of ditch check are still needed. The frequency and the distance between the checks must be determined.
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The following guidelines are recommended for silt fence ditch checks: |
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Ditch Grade |
Approximate Spacing (ft.) |
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≤ 0.5% |
315 |
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> 0.5% to ≤1% |
155 |
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> 1% to ≤1.5% |
100 |
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> 1.5% to ≤ 2% |
75 |
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> 2% to ≤ 2.5% |
60 |
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> 2.5% to ≤ 3% |
50 |
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> 3% to ≤ 3.5% |
45 |
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> 3.5% to ≤ 4% |
40 |
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> 4% to ≤ 5% |
35 |
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> 5% to ≤ 5.5% |
30 |
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> 5.5% to < 6% |
25 |
If a silt fence ditch check has been
overwhelmed and blown out twice, rock check dams should be installed.
Rock check dams should be installed 15 feet beyond the edge of travel way and
not installed in the median.
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The following guidelines are recommended for rock check dams: |
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Ditch Grade |
Approximate Spacing (ft.) |
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≤ 0.5% |
400 |
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> 0.5% to ≤1% |
200 |
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> 1% to ≤1.5% |
130 |
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> 1.5% to ≤ 2% |
100 |
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> 2% to ≤ 2.5% |
80 |
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> 2.5% to ≤ 3% |
65 |
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> 3% to ≤ 3.5% |
55 |
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> 3.5% to ≤ 4% |
50 |
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> 4% to ≤ 4.5% |
45 |
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> 4.5% to ≤ 5% |
40 |
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> 5% to ≤ 5.5% |
35 |
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> 5.5% to ≤ 6.5% |
30 |
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> 6.5% to ≤ 8% |
25 |
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> 8% to ≤ 10% |
20 |
In addition to these guidelines, the length of the ditch must be considered, especially when the ditch grade is 3% or more.
For ditches with grades steeper than 4.0% and longer than 1,000 feet, a rock flume is more appropriate than a rock ditch check. The rock flume, 30 to 40 feet in length, is constructed by placing erosion stone or Class E revetment at the lower end of the ditch. Additional rock flumes should be placed as needed for steeper ditch grades.
Drainage Ways/Ponds/Lakes/Wetlands, etc.
Silt needs to be prevented from leaving our project! Silt basins, riprap dams, silt fences, seeding, and available ROW should be looked at, especially when the construction work is close to drainage ways/ponds/lakes/wetlands.
ROW or at least temporary easement must be available to install the necessary erosion control devices near these “sensitive” areas. Depending on the situation, a silt basin or even a temporary sediment control basin should be created to allow the silt to settle. The overflow location of the basin should be armored with riprap to make sure that the water energy is minimal before leaving the ROW. Finally, a silt fence or multiple rows of silt fence or rock checks should be placed to catch the silt from escaping the project.
As the fills and/or the bridge berms get higher, the potential for erosion increases significantly. Without some seeding or mulching, there will be gullies on the slopes. Seeding or some sort of stabilization must be done as the fill is going up.
When the backslope is not too high (less than 10 feet) and the drainage area above the backslope is small, the potential for erosion on the backslope is low. However, when the backslope is high and the drainage area onto the backslope is large, something must be done. For example, a small intercepting ditch next to the top of the backslope could be utilized to carry the water along the top of the backslope down to the ditch to avoid the erosion on the backslope. Sometimes the runoff will be concentrated at one location. If this is the case, a letdown flume should be placed to minimize or prevent erosion.
When a sandy backslope is encountered, topsoil should be used to dress the slope. A wood excelsior mat or straw mat would be the next choice for slope protection.
Since culverts are at the lowest spots, this is
where the water will drain off. Most of the time, the water will be drained
into a creek or drainage way. Silt fences or logs must be placed on both sides
of the box culvert wing from the end of the wing at least around the toe and along
the top of the box to catch the silt from the area right next to and above the
culverts. Seeding has to be done as soon as possible. This is also a
good location for slope protection.
In case of water discharging strongly from the pipe onto an adjoining property/farm/pond, etc., especially in the no-ditch situation, many things may be needed. First, the ROW has to be available. Second, one or two riprap dams or a splash basin may be needed to slow down the water. Finally, one or two silt fences adjacent to the ROW line to catch silt would be beneficial.
At the end of the ditch, water is going to drain off the highway project. Depending on the situation, temporary sediment control basins, riprap dams, rock checks, rock flumes, silt basins, and silt fences should be looked at. For example, the slope of the ditch may be steep and carrying a large volume of water. The soil for the project is very sandy. In this case, a riprap dam may be needed near the end of the ditch. Also, where a steeper slope occurs, a rock flume may be added. In contrast, if the project is in a flat area with mainly cohesive soil, maybe a silt fence ditch check is all that is required.
The slope of letdown pipes is very steep. Thus, the velocity of the water at the outlet of the pipe is very high. This water will cause severe erosion. In this case, a silt retention basin at the top allowing water into the pipe slowly should be considered. Also, a layer of riprap with engineering fabric underneath or even a flume of riprap will need to be installed to dissipate the water velocity or energy.
Sometimes the outlet of the letdown pipe is right next to a creek. Because the water flow rate in the creek can be very high in the spring, the pipe must be protected. This means that riprap must be installed on both sides of the pipe and maybe even on the top to prevent scouring around the culvert and uplifting of the pipe.
Drainage from the median and ditches can cause erosion on the berm face. Rock flumes sometimes are needed to minimize the erosion on the berm face.
The flow rate at this location may be high. Turbulent flow may cause erosion. Silt may enter the storm sewers or farm tiles. In order to minimize erosion, preventive actions should have been taken at other places so that the water flow rate and siltation are minimal. Silt fences or another type of perimeter control should be placed around the drop inlets or intakes.
When the deck is wide and high above the ground, the water dropping on the ground from the deck drain can cause erosion. Erosion stone or riprap may have to be placed under the deck drain to absorb the energy from the water impact.
Sometimes the borrow is located on a hill. After the topsoil is stripped and excavation begins, water runoff from the borrow could be high. Silt fences may not be able to hold the water. A small dike along the low side of the borrow would minimize the potential of the silt leaving the borrow.
Terraces would also be helpful.
A V-Ditch is not the most desirable shape for a ditch; but, due to site conditions, they exist. Some seeding and mulching along with special ditch control should be done. In addition, ditch checks should be placed in the ditch to slow down the water velocity.
Area of Super Elevated Curve
Due to the increased slope of super-elevated curves, surface water in the curve will flow to a concentrated area. During the grading project, erosion problems may be minimal. However, after the pavement is completed, the velocity of the water draining off the pavement will increase significantly. A letdown flume may be needed to be constructed on the “low” side of the super-elevated curve.
Silt Curtains
Silt curtains are included as bid items with projects when there will be bridge or streambank construction activities in or beside a water body (such as a river, lake, creek, or stream). Typically, only Floating Silt Curtain (Hanging) will be included as a bid item. However, there are rare times when the Location and Environment Bureau will specify the use of Floating Silt Curtain (Containment) bid item.
The main purpose of the silt curtain is to isolate the work area from the water body, thus allowing sediment to settle out of suspension. A silt curtain alone should not be considered as adequate perimeter control. Other sediment controls, such as silt fence or perimeter and slope sediment control devices, should be installed upland.
Silt curtains should not be installed across a river or stream. Refer to EC-202 for installation configurations.
For creeks, streams or rivers, the Design Bureau typically estimates the silt curtain bid quantity as the length of the water body from ROW to ROW plus any easements. This quantity is doubled if working on both sides of the creek, stream or river. The contractor and inspector/engineer should review the contractor’s plan for disturbed areas prior to installing a silt curtain in order to determine the needed length to contain the disturbed zone.
The inclusion of a Floating Silt Curtain (Hanging) bid item does not mean the silt curtain has to be installed. Rather, the use of the silt curtain is at the discretion of the inspector/engineer.
Additionally, it may also be determined that installing a rock berm along the water’s edge will be a more effective form of sediment control.
Temporary Stream Diversions
A temporary stream diversion is to be included as a bid item for projects involving installation or extension of box culverts or precast box culverts 6 feet by 6 feet or larger and arch pipe culverts 102 inches by 62 inches or larger. For smaller box culverts and arch pipe culverts, the contractor is to determine the means and methods of managing the water, complying with Specification 1107.18, and this work will be incidental to the project.
Temporary stream diversions were developed as a best management practice when working in areas where flows are low enough and/or the watershed is small enough to allow normal base flows to be handled practically in a small diversion channel or pipe.
This practice allows the contractor several methods of diverting the flow around the work area – through use of a diversion channel or pipe or hose.
Even if the creek or stream may be dry or there is a minimal flow, the temporary stream diversion dikes and energy dissipation material should be in place in the event of rain.
The energy dissipation material (i.e. revetment) is included in the bid price of the temporary stream diversion. However, upland sediment control measures are not included in the bid price and should be paid for separately.
Refer to Standard Specifications Section 2418 and Standard Road Plan EW-402 for more information.