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.
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
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.
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 Construction Stormwater General Permit, must be identified in the site’s Stormwater Pollution Prevention Plan.
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.
The Construction Stormwater General Permit requires sediment control practices to be established on all downgradient perimeters and located upgradient of any buffer zones. 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 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.
The permit requires permittee(s) to 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. Permittees must complete any short‐term activity that requires removal of sediment control practices as quickly as possible and must re‐install sediment control practices before the next precipitation event even if the short‐term activity is not complete.
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 general, perimeter controls are high-maintenance BMPs, particularly after rain events. The following table 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
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|Flow attenuation||Little or no design benefit|
|Runoff volume reduction||Little or no design benefit|
|Erosion prevention||Little or no design benefit|
|Sediment control||Primary design benefit|
|Nutrient loading||Secondary design benefit|
|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|
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.
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.
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 following describes the components and installation of the silt fence:
Fiber logs, fiber rolls, biorolls, or wattles fabricated from a variety of materials (e.g., straw, wood fiber, compost, coconut, etc.) can perform effectively as perimeter control BMPs for relatively flatter sites or smaller drainage areas. They are also effective when used with mulch or erosion control blankets on steeper slopes to break up slope lengths, intercept and spread out downslope flows, promote vegetative growth, and generally prevent slope erosion.
Design guidelines for using fiber logs as perimeter controls include:
Perimeter control berms consisting of stabilized soil, rock, rock bags, mulch, compost, grubbed roots and brush, and other material can also serve to reduce sediment discharges offsite, through ponding, filtration, or redirection of runoff to a sediment trap or other treatment unit. These systems are a very efficient method for sediment removal, but may require frequent maintenance as they are prone to clogging from mud and soil. In addition, installation may be labor intensive, they are easily damaged by equipment, and removal can be difficult. Design considerations include the general guidelines listed at the beginning of this section, as well as those listed below.
Perimeter control ditches function like soil berms – they redirect downslope sheet flow from the disturbed area perimeter to a sediment trap, settling pond, rock berm, or other treatment unit designed to settle or filter soil from site runoff. Design considerations include the following.
Permit requirements specify that all construction sites must remain in compliance throughout winter, even if no construction is occurring. Properly functioning sediment controls must remain in place during minor thaws and during the large spring snowmelt to prevent transport of sediment to surface waters. The best way to ensure proper functioning of perimeter controls throughout winter is to have all sediment controls installed prior to the first freeze. For winter construction, materials such as compost berms, logs and rolls, fiber rolls, rock bags, and rock filters can be installed over snow cover to provide new sediment control. All BMPs must be inspected and maintained immediately following intermittent snow melt or rainfall that occurs in winter months.
MnDOT Standard Plan 5-297.405 provides standard detail for perimeter control BMPs utilized for temporary sediment control (effective date: 8/6/2014). BMPs covered include “Flotation Silt Curtain”, “Sediment Control Logs”, “Filter Berms”, “Bale Barriers”, and several “Silt Fence” plans (including Types TB, HI, MS, and PA).
The MnDOT Standard Specifications for Construction (2016 edition) outline specific requirements for various types of perimeter control BMPs, including selection and materials specifications. Applicable specifications include 2573 (Storm Water Management), 3149 (Granular Material), 3874 (Filter Berm), 3882 (Mulch Material), 3886 (Silt Fence), 3887 (Flotation Silt Curtain), 3890 (Compost), 3893 (Sandbags), and 3897 (Sediment Control Log).
For all perimeter control BMPs, initial inspections should ensure that controls are intercepting all upslope runoff from disturbed areas. Check upslope areas to determine if there are opportunities to reduce the volume of runoff being handled by perimeter controls, reduce the amount of sediment in the runoff (i.e., by installing diversions above the disturbed area), or immediately stabilize idle bare areas and/or portions of the site that are at final grade. Where perimeter controls redirect flows to sediment traps, ponds, or other treatment BMPs, ensure that berms/ditches are stable, flow paths are unobscured, and that the treatment BMP is able to handle inflows appropriately and are functioning properly. If it is determined through inspection that the selected perimeter control BMPs are ineffective, they must be upgraded to a method that is effective at keeping sediment on the site.
MnDOT’s workmanship and rework schedule (2016; version under development at the time of manual update) identifies common deficiencies for various types of perimeter control BMPs and corrective actions for these deficiencies. Once complete, the full, final version of this table will replace Table 2573-1 in MnDOT Standard Specifications for Construction (2016 edition).
Temporary Sediment Control - Corrective Actions
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|Item||Corrective Action Required if||Corrective Action|
|Floating silt curtain||
|Sediment control logs||
The subsections below provide additional inspection guidelines for each of the BMPs addressed in the section above.
The maintenance guidelines below respond directly to the inspection findings that result from observing field conditions listed in the previous section. For best results, complete maintenance tasks immediately, before conditions worsen and consume more time and resources to resolve. Below are some key requirements from the 2018 MPCA Construction Stormwater General Permit.
Other maintenance guidelines include:
The following table summarizes estimated BMP costs based on MnDOT data summarizing average bid prices for awarded projects in 2014.
|Bid Item||Item Description||Units||Average Price|
|2573.502/00010||Silt Fence, Type HI||LF||$2.58|
|2573.502/00020||Silt Fence, Type PA||LF||$7.00|
|2573.502/00030||Silt Fence, Type SD||LF||$19.87|
|2573.502/00040||Silt Fence, Type MS||LF||$1.99|
|2573.502/00050||Silt Fence, Type TB||LF||$10.24|
|2573.505/00010||Flotation Silt Curtain Type Still Water||LF||$14.99|
|2573.505/00020||Flotation Silt Curtain Type Moving Water||LF||$39.57|
|2573.515/00013||Filter Berm Type 3||LF||$54.49|
|2573.515/00015||Filter Berm Type 5||LF||$22.95|
|2573.533/00011||Sediment Control Log Type Straw||LF||$2.90|
|2573.533/00012||Sediment Control Log Type Wood Fiber||LF||$4.29|
|2573.533/00013||Sediment Control Log Type Coir||LF||$13.25|
|2573.533/00015||Sediment Control Log Type Compost||LF||$2.58|
|2573.533/00016||Sediment Control Log Type Rock||LF||$5.00|
|2573.533/00017||Sediment Control Log Type Blanket System||LF||$2.52|
LF=linear foot; SF=square foot
Except where more stringent requirements are presented in this guidance, perimeter controls shall comply with MnDOT and other state requirements. Primary design references include:
The following is a list of additional resources that are not specific to Minnesota: