Sediment control practices - Perimeter controls for disturbed areas
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Contents
Perimeter Controls for Disturbed Areas
Definition
Perimeter controls for disturbed areas are temporary sediment barriers that intercept and remove soil and debris from sheet flow runoff on construction sites. Removal mechanisms include ponding the runoff to allow for settling, and physically filtering sediment as it passes through a sediment barrier (e.g., silt fence, rock/soil berm, gravel bags, fiber log, etc.). Some designs (e.g., berms, ditches) use perimeter controls to collect and convey sheet flow to larger treatment areas, such as sediment traps, where sediment removal can occur through multiple processes.
Purpose and Function
Perimeter controls intercept sheet flow from slopes and remove sediment and other contaminants through ponding, settling, and physical filtration, effectively preventing contaminants from leaving the site and entering surface waters. Concentrated flows within the site typically require treatment via source controls, sediment traps, settling ponds, or other methods prior to reaching the site perimeter. Ditch check dams are also used for small concentrated flows.
Although silt fences are one of the most commonly used methods of perimeter control, other perimeter control BMPs are available that can be equally or more effective depending on site conditions. Typical perimeter control methods include:
- Silt fence (super duty, machine sliced, hand installed, preassembled)
- Ditch checks (fiber rolls/biorolls, filter/rock/compost log, etc.)
- Sediment traps or berms (rock, soil, compost, etc.)
- Perimeter control ditches
- Other (sand bags, rock logs, snow berm, etc.)
In most applications, treatment occurs at or along the perimeter control device, e.g., silt fence, rock berm, and fiber log. Other applications may involve the use of perimeter control berms or ditches to direct sheet flows to a treatment area, such as a sediment trap, sediment basin, or other BMP. Perimeter controls are generally recognized as the last line of defense in a system of erosion prevention and sediment control, and are usually placed near the downgradient borders of disturbed areas and soil stockpiles. They are also used to separate work zones from adjacent waterbodies.
Applicability
Perimeter controls are universally used to ensure that sediment and other contaminants are contained on the construction site. The wide variety of perimeter control materials, configurations, and uses make them a standard practice on nearly all construction sites where clearing, grading, excavation, or fill activities occur. The location and type of perimeter control BMPs, along with other sediment control BMPs required by the Permit, must be identified in the site’s Stormwater Pollution Prevention Plan.
Site Applicability
Perimeter controls are useful on nearly every site as part of a system of BMPs designed to prevent erosion and control sediment transport to offsite areas. The selection, installation, operation, and maintenance of a particular perimeter control or group of controls is based on site considerations: slopes, soils, site size, proximity of waterbodies, and other factors. In general, super duty silt fence and rock berms provide the most protection, and are typically indicated below steeper, longer slopes under construction during the wet season. Shorter, flatter slopes under construction during dry or frozen periods can be protected with lighter duty controls (e.g., fiber rolls). The use of silt fences as a sediment barrier is not recommended in areas of concentrated flow, such as ditches. In these cases, soil berms, silt dikes, straw wattles, excelsior logs, rock check dams, or other BMPs suitable for concentrated flow should be used.
Permit Applicability
Section IV.C.2 of the 2013 MPCA Construction Stormwater General Permit states that “sediment control practices must be established on all downgradient perimeters and be located upgradient of any buffer zones” (for information on buffer zones required for temporary BMPs during construction, see Part IV.C.9; for buffer zones required as permanent BMPs, see Appendix A, Part C.3). “The perimeter sediment control practice must be in place before any upgradient land‐disturbing activities begin. These practices shall remain in place until Final Stabilization has been established”.
In addition, the permit notes that “(a) floating silt curtain placed in the water is not a sediment control BMP to satisfy perimeter control requirements in (Part IV.C.2) except when working on a shoreline and below the waterline. In those cases, a floating silt curtain can be used as a perimeter control practice if the floating silt curtain is installed as close to shore as possible. Immediately after the short term construction activity (e.g., installation of rip rap along the shoreline) in that area is complete, an upland perimeter control practice must be installed if exposed soils still drain to the surface water.”
Section IV.C.3 is also applicable to perimeter control BMPs, and states: Permittee(s) “shall re-install all sediment control practices that have been adjusted or removed to accommodate short‐term activities such as clearing or grubbing, or passage of vehicles, immediately after the short‐term activity has been completed. The Permittee(s) shall complete any short‐term activity that requires removal of sediment control practices as quickly as possible. The Permittee(s) must re‐install sediment control practices before the next precipitation event even if the short‐term activity is not complete.”
Effectiveness
Perimeter control BMPs are effective at preventing offsite sediment discharges in sheet flow when used with an appropriate upgradient system of erosion prevention and sediment control BMPs (e.g., upslope diversions, timely seeding and mulching) that minimize both the area and time of exposed soil. In some cases, full stabilization and management of disturbed upgradient areas can remove the need for extensive perimeter controls. In general, perimeter controls are high-maintenance BMPs, particularly after rain events. Table 4 summarizes expected performance for an array of typical water quantity and quality target constituents for properly selected, installed, and maintained perimeter controls.
Expected performance for perimeter control
Link to this table
| Water Quantity | |
|---|---|
| Flow attenuation | Little or no design benefit |
| Runoff volume reduction | Little or no design benefit |
| Water Quality | |
| Erosion prevention | Little or no design benefit |
| Sediment control | Primary design benefit |
| Nutrient loading | Secondary design benefit |
| Pollutant removal | |
| Total suspended solids | Primary design benefit |
| Total phosphorus | Secondary design benefit |
| Heavy metals | Secondary design benefit |
| Floatables | Little or no design benefit |
| Oil and grease | Little or no design benefit |
Planning Considerations
Perimeter controls should be integrated into an erosion prevention and sediment control system, taking the entire site into consideration. Prior to clearing, grubbing, grading, and other earth disturbing activities, perimeter controls should be installed on all down gradient limits-of-disturbance, and upgradient of buffer zones. Integrate perimeter controls with other BMPs, such as upslope diversions of runoff around disturbed areas, minimization of disturbed areas, minimization of the length of exposure time, adequate seeding and mulching, etc. If downgradient perimeter controls become overloaded with sediment and runoff volumes, additional upgradient controls may be necessary. Perimeter controls must remain in place until final stabilization has been established.
As noted above, perimeter controls typically require a high degree of maintenance. For best results, plan and schedule project activities to minimize maintenance requirements, ideally through timely grading operations and stabilization of disturbed areas with seeding, mulching, or other ground covers. Timing of perimeter control installation may be adjusted to accommodate short term activities, such as clearing and grubbing, and passage of vehicles. Such activities must be completed as quickly as possible and the perimeter control BMPs reinstalled immediately after the activity is finished, or before the next precipitation event, whichever comes first.
Design and Construction
Perimeter control design is driven by the size of the site and drainage area, slope steepness, soil type(s), proximity of waterbodies, and other factors. The design of a site’s perimeter control system should anticipate ponding that will occur upslope of the controls, and provide sufficient storage and deposition areas. Stabilized outlets are also required to prevent flows from overtopping the controls at undesired locations. The subsections below contain specific design information for the various types of materials that may be used to construct perimeter control BMPs. For all of the subsections below, the following design considerations apply:
- Specify that perimeter controls be installed downslope from disturbed areas and soil stockpiles, so that they intercept all sediment-laden flows.
- Perimeter controls are not needed upslope of disturbed areas, except when used to divert flows away from disturbed areas (i.e., diversion berms and ditches – not silt fencing).
- Where possible, leave room between the perimeter control BMP and the disturbed area for any equipment that will be used to remove sediment from or otherwise service the BMP.
- Install initial perimeter control BMPs before clearing, grubbing, grading, and other earth-disturbing activities occur.
- Keep perimeter controls in place until all upslope areas are fully stabilized.
- For larger sites and/or those with steeper slopes (> 2% slopes, > ½ acre), perimeter controls are placed on the topographic contour, with the ends turned up to prevent bypasses.
- Where the perimeter control approach includes directing sheet flow to a separate BMP for treatment, design must include stabilization of concentrated flow areas (e.g., ditches, berms) to prevent erosion caused by moving water. Where installed adjacent to waterbodies, this approach typically involves pumping the redirected flow to an upslope location for treatment.
- Additional perimeter controls may need to be added, or controls moved to different locations on a site as conditions change.
Silt Fence
Silt fencing is the perimeter control BMP used on most construction sites, and is moderately to very effective if sited, installed, and maintained properly. Silt fencing is intended to slow velocities and temporarily detain incoming runoff so that sediment is removed by settling and filtration – i.e., as muddy water passes through the fabric. Design and installation considerations for silt fencing includes the information below.
- The most common types of silt fence installations include:
- Super duty (consisting of a concrete or water filled jersey barrier with geotextile)
- Heavy duty (used where extra strength is required, such as near water bodies, on steep slopes, and in areas with unstable or highly erodible soils; typically has metal posts and is machine sliced)
- Preassembled (light-duty; posts are pre-attached to geotextile)
- Do not use in areas of concentrated flow – silt fencing is for sheet flow only.
- For predominantly sandy soils, fabrics with Standard Sieve opening sizes of 10 to 30 are acceptable. For mostly silty soils, use fabric with 30 to 60 Standard Sieve openings. For clayey soils, specify silt fence fabric with Standard Sieve size openings of 60 to 100 or higher. Use the higher range values (i.e., smaller openings), for silt fences along surface waters and buffer areas.
- Silt fences should be installed on the contour (as opposed to up and down a hill).
- Since silt fence is not designed to withstand high water levels, locate them so that only shallow pools can form – i.e., no more than half the height of the silt fence.
- Spacing of wood or metal posts should not exceed 6 feet and should be installed on the downhill side of the fabric.
- Silt fence serves no function along ridges or near drainage divides where there is little movement of water. Confining or diverting runoff unnecessarily with a sediment fence may create erosion and sedimentation problems that would not otherwise occur.
The following describes the components and installation of the silt fence:
- Anchoring of the silt fence is critical. The bottom 6 to 12 inches of the fabric must be buried and compacted to prevent bypasses.
- Burial of the bottom of the fabric can be via trench and backfill, or via the slicing method.
- Posts can be 2x2 inch hardwood, metal T-posts, or round metal posts. Steel posts are used typically for heavy duty applications, while standard applications may use wooden posts.
- Posts should be driven at least 1.5 to 2 feet into the ground, to support runoff and sediment loads.
- Use J-hooks to intercept and trap flows that may run along silt fence sections.
- Low fence line corners and dips should be reinforced with rock or other berms/checks.
- Silt fence ends should be turned upslope to capture runoff and prevent bypasses.
- Lightly pull fabric taut during installation – avoid excessive sagging between posts.
- Fabric can be attached to posts with staples, plastic ties, wire, or clips.
