Difference between revisions of "806.2 Sediment Control Measures"
(Per Env'l, a complete rewrite of EPG 806 to clarify land disturbance requirements and policies necessary to comply with MoDOT's current land disturbance practices.)
Revision as of 11:07, 7 August 2018
Sediment control is the practice of controlling eroded soil in disturbed areas by deploying best management practices (BMPs) to capture and retain the eroded material before it leaves the project site. It is MoDOT’s intention to establish final stabilization practices as soon as possible, but sediment control BMPs must still be deployed to provide sediment control until vegetative cover or final building materials have been established to prevent sediments from leaving MoDOT project sites.
- 1 806.2.1 Sediment Basin
- 2 806.2.2 Sediment Trap
- 3 806.2.3 Ditch Checks
- 4 806.2.4 Silt Fence
- 5 806.2.5 Rock/Mesh Sediment Control Fence
- 6 806.2.6 Inlet Controls
- 7 806.2.7 Mulch Berms
- 8 806.2.8 Type C Berms
- 9 806.2.9 Straw Bales
806.2.1 Sediment Basin
Sediment basins are constructed to trap and store water and sediment that may not be caught by upgrade erosion and sediment control measures. The basins consist of an excavation with defined side slopes and rock riprap placed in inlet and outlet areas.
A temporary sediment basin is an excavated or dammed storage area that is used for short-term erosion and sediment control purposes. They are constructed with available grading equipment at locations shown on the contract plans to control sediment discharge until more permanent BMPs are installed and the site is stabilized. In some cases a temporary sediment basin can be placed in a location that enables it to continue to be used as a permanent sediment and drainage control basin.
If the sediment basin is to be permanent, its slopes shall be stabilized with rock riprap or equivalent.
806.2.1.1 Design Considerations
The location of sediment basins will be shown on the plans. Each basin will be of sufficient size to contain a volume of a local 2-year, 24-hour storm event as determined by using the National Oceanic and Atmospheric Administration’s National Weather Service Atlas 14. Sediment basins are required (unless infeasible due to site constraints) when large disturbed areas (>10 acres) concentrate flow to one discharge point, but they should be considered for any disturbed area, 5 acres or larger, which drains to one discharge point.
Where the use of a sediment basin of sufficient size as described above is impractical, other similarly effective BMPs that will provide equal water quality protection shall be provided, such as sediment traps, must be employed to minimize sediment loss from MoDOT right of way.
806.2.1.2 Construction Considerations
Sediment basins should be installed at the time of clearing and grubbing, and will normally remain in service until all disturbed areas draining into the structure have been satisfactorily stabilized.The area where a sediment basin is to be constructed shall be cleared of vegetation to enable removal of sediment. The inlets of these sediment basins shall be constructed with a wide cross section and minimum grade to prevent turbulence and allow deposition of the soil particles. Sediment basins shall always have stabilized outlets to discharge water from the surface of the basin. The stabilized outlets typically consist of one, or a combination of the following: rock, a riser pipe, or a surface skimmer (e.g., Faircloth Skimmer®). As a general rule, basins should be designed and constructed twice (minimum) as long as wide in order to maximize time of concentration within the structure. To add additional sediment removal capability to basins, baffles can be designed within the basin to slow storm water flow and increase treatment time within the basin. Basically, the longer the water takes to get from the inlet of the basin to its outlet, the more effective the treatment and the better the water quality at the outfall. Sediment basins shall normally remain in service until all disturbed areas draining into the structure have been satisfactorily stabilized. When use of temporary sediment basins is to be discontinued, all excavations are to be backfilled and properly compacted, fill material removed, and the existing ground restored to its natural or intended condition.
Where the use of a sediment basin of sufficient size as described above is impractical it should be documented in the SWPPP and other similarly effective BMPs must be employed to minimize sediment loss from MoDOT right of way. These similarly effective BMPs or BMP systems could include, but are not limited to sediment traps, ditch checks, Type C berms, etc., and the use of appropriate erosion control items to cover up exposed soil. An explanation for selecting these similarly effective BMPs instead of a basin will be documented in the project SWPPP.
When the depth of sediment reaches 1/2 of the depth of the structure in any part of the pool, all accumulation shall be removed. Discharges from the basin shall not cause scouring of the receiving area. Removed accumulated sediment and excavated material removed during construction of the sediment basin shall be disposed of in locations where sediment will not again erode into the construction areas or into natural waterways.
806.2.2 Sediment Trap
A sediment trap is a temporary sediment collection structure constructed of rock or other non-earthen material used to detain runoff so that sediments are allowed to drop out. The trap may also be excavated in lieu of rock construction.
806.2.2.1 Design Considerations
The location of sediment traps will be shown on the plans. The length and height of the sediment trap depends on the volume of water that flows through the drainage structure and the width of the drainage channel. Sediment traps will be utilized at every outfall and may be used downgrade of drainage structures to control sediment. Sediment traps are not appropriate where impounded sediment and gravel could accumulate inside of the culvert. Estimated quantities for each trap located on the project will be shown to the nearest cubic yard. See Standard Plan 806.10 for sediment trap details.
Sediment traps are not typically appropriate in streams that are regulated by the US Army Corps of Engineers under Section 404 of the Clean Water Act. However, certain construction within the regulated channel may necessitate their use. The design of a sediment trap in this situation must be approved by the Design Division's Environmental and Historic Preservation section prior to inclusion in the plans.
806.2.2.2 Construction Considerations
Sediment traps need to be in place prior to clearing and grubbing operations and will remain in place until the site has achieved final stabilization.
Sediment traps will be constructed of rock or other non-erodible material sufficient to impound water in accordance with Standard Plan 806.10 and Standard Specification 806.60. Estimated quantities for each trap located on the project will be shown to the nearest cubic yard. Sediment traps may be dewatered through a single riser pipe, over a stabilized spillway (rock-lined, lined with erosion control blanket or turf reinforcement matting, vegetated), or, where applicable, allowed to filter through the interstices of a constructed rock barrier.
Maintenance of the trap must be completed once the sediment deposits accumulate to ½ the height of the trap. In situations where long-term maintenance issues are absent, and permanent vegetation has established, sediment traps may be left in place as a permanent structure as long as there is no threat to the natural or human environment.
806.2.3 Ditch Checks
Ditch checks are temporary obstructions placed in a drainage way used to control erosion and sedimentation by reducing storm water velocities. Sediment deposits will be captured by the checks during low flow conditions.
806.2.3.1 Design Considerations
There are two types of ditch checks that can be used: rock and alternate.
Rock ditch checks are the predominant ditch check to be used. Because of their size, rock checks are to be placed outside the clear zone. They are specified for larger drainage areas and ditch slopes 10 percent or less and where expected ditch flow volumes and velocities are high, or in locations where the project is in proximity to streams or other sensitive areas.
Alternate ditch checks include, but are not limited to, triangular silt checks, filter socks/logs (but not straw wattles), and sandbags. Alternate ditch checks have a minimum effective height of 9 inches, as measured in the field.
Alternate ditch checks are specified where drainage areas are fairly small (3 acres or less), ditch slopes are 4 percent or less and expected ditch flow volumes and velocities are small. For scenarios that exceed the criteria established above, a combination of rock ditch checks and erosion control blankets are utilized.
Standard Plan 806.10 (Sheet 3 of 7) shows the spacing for ditch checks. The estimate of the required number of ditch checks is based on an effective height of 9 or 18 inches.
If the total number of alternate ditch checks needed on a project is minimal, it is advisable to just specify all ditch checks as rock for simplicity of contract administration. The last two ditch checks, in any ditch check system should be one rock ditch check followed by a sediment trap.
806.2.3.2 Construction Considerations
(Important: Straw wattles, straw bales and geotextile silt fence are no longer acceptable as a ditch check BMPs.)
Ditch checks shall be placed and constructed according to the contract plans. As soon as practical, once a ditch/conveyance is capable of conveying water, ditch checks should be constructed to provide sufficient protection until the conveyance is complete and the remaining BMPs can be installed. Once disturbance activities have ceased on any part of the project and will not resume for a period of 14 days ditch checks shall be constructed to provide interim stabilization. Once interim stabilization efforts begin they shall be completed within 14 days. Ditch checks shall remain in place until final stabilization is achieved.
Rock checks are to be constructed of 4 in to 12 in size rock, full ditch width wide, and 18” min. effective height over the middle of the ditch. The standard design of a rock ditch is a 4 ft. wide base, with 2:1 side slopes, and 2 ft. tall with the middle 6 in. lower than the sides (18 in effective height over the middle). Dimensions may be modified based on individual project needs for higher flow rates. In areas of clay soils, where additional filtration may be needed, the upgrade face of the check can be capped with smaller stone, filter fabric or another approved filtering media. In some cases, it may also be necessary to place a section of ECB or geotextile beneath the rock ditch check and extending downgrade of the structure to prevent the rock from settling into the soil beneath and/or protect from downstream scour within the ditch line.
Alternate ditch checks have an effective height of at least 9 in. as measured in the field and should be installed in accordance with the manufacturer’s recommendations. The SWPPP has a list of acceptable alternate ditch checks or others can be approved by the engineer. Each type of ditch check (particularly the tubular/cylindrical/triangular products) will have specific directions for installation. In all cases care shall be exercised so as to install the device according to manufacturer specifications. Effectiveness may be compromised if not installed correctly.
Ditch checks shall be checked for condition and sediment accumulation after each runoff event. Accumulated sediment shall be removed from the check when the sediment height is no more than half that of the check. Sediment removal will include removal and disposition in a location where it will not erode into construction areas or watercourses. Inspections shall be made to ensure that the center of the check is lower than the edges.
Ditch checks shall remain in place until final stabilization has been achieved. Upon removal, the contractor should be directed to remove and dispose of any excess sit accumulations, grade and dress the area, and stabilize all bare areas to the satisfaction of the engineer. As a general rule for rock ditch checks, once the area has reached final stabilization, any collected sediment should be removed and rock ditch checks can be graded out within the ditch line, serving a similar purpose as a liner. In rare cases, rock ditch checks may remain in place permanently, and resultant accumulated sediment shall be allowed to develop vegetative cover as a permanent feature of the right of way. Similarly, biodegradable structures and their accumulated sediment may be allowed to remain in place if the engineer determines that removal will destabilize the ditch. In cases of compost, mulch, etc. filled checks, the wooden stakes should be pulled and the biodegradable netting cut to encourage more rapid degradation. If the netting is non-biodegradable, the netting shall be cut and removed along with the stakes, but the biodegradable filling may be left to decompose.
806.2.4 Silt Fence
Silt fence is a temporary sediment control measure used to capture suspended particles from sheet flow along the edge of the right of way where runoff attempts to leave the project onto an adjacent property or into an adjacent body of water or wetland. Silt fence must never be used in concentrated flow to cross a ditch, stream or drainage channel; and in no case installed downgrade from a pipe or used as a culvert protection.
806.2.4.1 Design Considerations
Silt fence is to be provided as a perimeter control on all land disturbance projects. The plans shall include silt fence in areas where runoff will exit the right of way or along the toe of fill slopes where sediment could enter and adjacent body of water or wetland. Silt fence is not limited to a linear installation. Silt fence should be designed to be installed on the contour when possible, perpendicular to sheet flow, to prevent overtopping or overloading at single points. If silt fence is run down a grade, not perpendicular to sheet flow, J-hooks should be designed to be installed into the silt fence system to dissipate energy and capture runoff so as not to undermine the fence or overwhelm the system at a low point. J-hooks should be installed toe to top, similar to ditch checks, with the tail of the downgrade J-hook terminating behind the leading edge of the previous (see Standard Plan 806.10).
806.2.4.2 Construction Considerations
There are several construction requirements for silt fences. Fence construction shall be adequate to handle the stress from hydraulic and sediment loading. Where possible, silt fencing should be installed in existing vegetation, outside of, or at the edge of project clearing limits, so that a buffer of undisturbed soil and vegetation remains on both sides of the fence. Fence construction shall be adequate to handle the stress from hydraulic and sediment loading. Geotextile at the bottom of the fence shall be entrenched. The trench shall be backfilled and the soil compacted over the geotextile. When two sections of geotextile silt fence come together or if a new run must be started, the fence shall be overlapped as indicated on the standard drawings.
Post spacing shall not exceed 5 feet. Posts shall be driven a minimum of 24 inches into the ground. Where rock is encountered, posts shall be installed in a manner approved by the engineer. Closer spacing, greater embedment depth and/or wider posts shall be used as necessary in low areas and soft or swampy ground to ensure adequate resistance to applied loads.
The integrity of silt fences must be maintained until disturbed areas have achieved final stabilization. Regular inspections shall be done to insure silt fence is in proper working order. Fence should be inspected for broken of loose stakes, holes in the geotextile fencing, and any undermining or scouring of the system. Where construction activities have changed the natural contour and drainage runoff, silt fence must be inspected for proper location and effectiveness. Where deficiencies exist, additional silt fences shall be installed.
Sediment deposits shall be removed and disposed of when the deposit approaches one-half the height of the fence or sooner. If required by heavy sediment loading, a second silt fence shall be installed. Secondary fence installations do not relieve the obligation to maintain the first installation. Any BMP installed must be maintained in good working order.
The silt fence shall remain in place until final stabilization has been achieved. Upon removal, the contractor shall be directed to remove and dispose of any excess silt accumulations, grade and dress the area, and establish vegetation on all bare areas.
At the time of installation, silt fencing is to be installed in permanent grass, outside of the clearing limits so that a buffer of undisturbed soil remains on both sides of the fence. Perimeter silt fence is not installed across a drainage ditch, stream or water channel.
806.2.5 Rock/Mesh Sediment Control Fence
Rock/mesh sediment control fences are used in high sheet flow volume runoff areas where traditional silt fence will not function as designed. Wire mesh, T-posts, and grade 4 or grade 5 rock for drainage, in accordance with Sec 1009, comprise the system to impound runoff and allow solids to drop out.
806.2.5.1 Design Considerations
Rock/mesh sediment control fence provides a superior level of protection when areas of higher storm water velocity flows are present along a sites perimeter. Use this device in lieu of other silt fence applications at the toe of fill sections, especially along streams and wetlands and in other areas where there is insufficient right of way to construct better impoundment devices, such as sediment basins or sediment traps. As with silt fence applications, the sediment control fence should be placed perpendicular to storm water flow, allowing the water to pass either over or through the rock/mesh sediment control fence, never around it.
806.2.5.2 Construction Consideration
This device is constructed using a 4 ft. wire mesh (hardware cloth – 24 gauge, ¼ in. openings) folded in half to form a 90° angle. This mesh is then wired to, and supported by 5 ft. metal “T” posts spaced 3 ft. apart and driven approximately 2 ft. into the ground. Lastly, a layer of grade 4 or grade 5 aggregate for drainage (Sec 1009) is placed against the mesh, with a minimum height of 12 in., but preferably 18 inches. (Refer to Standard Plan 806.10.)
The sediment control fence should be placed perpendicular to storm water flow, allowing the water to pass either over or through the rock/mesh sediment control fence, never around it.
Rock/mesh sediment control fences shall be inspected for structural damage, undercutting, sediment buildup, or lack of drainage due to sediment clogged stone. Sediment deposits shall be removed and disposed of when the deposit approaches 1/2 the height of the fence or sooner. Accumulated sediment removed from the fence shall be disposed of in locations where sediment will not erode into construction areas or into waters of the state. If the filter stone (aggregate for drainage) becomes sediment-clogged and no longer serves as a filter, it may be appropriate to replace it with new stone.
The rock/mesh sediment control fence shall remain in place until areas that drain to the fencing are stabilized. Upon removal, the contractor shall be directed to remove and dispose of any excess sediment accumulations, grade and dress the area, and establish vegetation on all bare areas. If the engineer determines that sediment control fence shall remain in place for a period of time after the job is closed out, arrangements will be made for the contractor or MoDOT Maintenance personnel to remove the fence once the area is sufficiently stabilized.
806.2.6 Inlet Controls
Storm drain (culvert, drop or curb) inlet protection measures prevent soil and debris from entering storm drain inlets.
806.2.6.1 Design Considerations
Inlet protection shall be provided for all existing and new inlets where land disturbance operations are planned. Vegetative buffers and/or sediment control BMPs such as silt fence or waddles/socks should be utilized behind curb sections to prevent sediment from overtopping the curb and entering the inlet.
806.2.6.2 Construction Considerations
Temporary inlet protection shall be implemented at existing inlets prior to land disturbance, and new inlets are to be protected as they are put into service. As phases of the project change, inlet controls may need to be modified to ensure proper sediment filtration.
Geotextile silt fence shall not be used as an approved inlet protection. Geotextile may be used as a cover for a constructed wood or steel frame. It recommended for additional support and protection, the frame is wrapped with wire reinforcement prior to applying the geotextile material.
During construction, elevated curb inlets and median inlets, as well as excavations around inlets, may serve as "riser pipes" as long as they are sufficiently higher (approximately 9 in. or more) than the existing grade. Sediment that accumulates at the base of the riser pipe following storm water events shall be removed when it reaches 1/2 of the original height of the riser pipe. Once the desired grade has been achieved and the inlet becomes flush to that grade, subsequent inlet protection is required.
806.2.7 Mulch Berms
Mulch berms may be used for perimeter protection and are an acceptable alternative for geotextile and other silt fence applications.
806.2.7.1 Design Considerations
There is currently not a bid item for mulch berms. The reuse of cleared trees and brush as a mulch berm are at the request of the contractor and as approved by the engineer. Where large amounts of clearing and grubbing exist, notes should be added to encourage the use of mulch berms.
806.2.7.1 Construction Considerations
When constructed, mulch berms shall be piled to a height of at least two feet, preferably installed in existing vegetation, outside of, or at the edge of project clearing limits, so that a buffer of undisturbed soil and vegetation remains on both sides of the berm. Mulch berms should be installed at the time of clearing and grubbing, and must be maintained for as long as necessary to contain sediment from runoff. Mulch berms should be installed on the contour when possible to prevent overtopping or overloading at single points.
Mulch berms shall be inspected as part of the storm water routine inspection. It is also recommended that casual daily inspections be made during periods of prolonged rainfall. Where deficiencies exist, additional mulch, or another appropriate BMP shall be installed.
806.2.8 Type C Berms
A Type C berm is a rock barrier used to provide sediment control protection between the bridge end spill fill slopes and the stream bank.
806.2.8.1 Design Considerations
Type C berms are specified at the toes of spill slopes around bridge construction operations. It is important that the contract plans show the general presence of a Type C berm so contractors may bid accordingly. However, the actual precise location of the structure will be determined at the time of installation.
806.2.8.2 Construction Considerations
Type C berms shall be placed at the toes of spill slopes around bridge construction operations and will be constructed to the specified dimension (see Standard Plan 806.10). While contract plans may show the general location of the Type C berm, the precise location of the structure can only be determined at the time of installation and shall be field fit to provide maximum stream protection, yet enable the installation of piers, bents and accommodate movement of equipment.
Installation of the berm should be coordinated with other perimeter control BMPs to insure total protection of the area. Gaps between the Type C berm and silt fence, and other BMPs, may allow sediment laden runoff to escape unrestricted. Type C berms must be installed above the regulatory "ordinary high water mark". Type C berms are typically temporary, but may be permanent depending on the ultimate desired use of the right of way beneath the bridge.
The Type C berm must be evaluated as phases of the project change to be sure it is functioning as intended. Modifications may be required if field conditions make the original berm installation ineffective. Type C berms shall be checked for sediment accumulation after each runoff event. Sediment shall be removed when it reaches 1/2 of the original height or before. Sediment removal will include removal and disposition in a location where it will not erode into construction areas or waters of the state.
(Note: Oftentimes temporary stream crossings are used in proximity to Type C berms. These crossings can cause gaps in the berm for equipment passage, which could potentially be a conduit for sediment delivery to the water-body. Use caution when using these two practices in the same location and assure adequate protection of the water-body. Refer to EPG 806.8.6.3.6 Temporary Pipes and Temporary Construction Crossings for more information.)
806.2.9 Straw Bales
Bales of straw are no longer acceptable sediment control BMPs on MoDOT projects and will not be used as such. Straw is acceptable as mulch when applying temporary ground cover or establishing permanent vegetative cover. Straw used as ground cover is required to be embedded or tackified per Section 802 of the Missouri Standard Specification for Highway Construction.
Straw bales are an acceptable practice used to control diamond grinding residue that is discharged onto MoDOT right of way due solely to the short duration of the discharge. See EPG 806.8.6.4.11 Straw Bales.