Berms are compacted or vegetated structures designed to slow, pond, or filter runoff; divert runoff on a construction site to a sediment trap/basin; and/or ensure clean upland runoff does not move into disturbed areas. They may also be called filter berms, diversion berms, or earth berm barriers, depending on their composition and use. Berms can be constructed from soil, rock, wood chips, and compost, depending on the construction site characteristics (discussed below). They serve as a temporary perimeter control BMP where the installation of silt fence is not feasible. They may be used in combination with a silt fence to minimize under-draining of the fence and increase sheet flow runoff filtering.
Berms prevent off-site sedimentation by diverting runoff to a sediment trapping device and can also be used to divert clean water from entering a disturbed area. They can also trap sediment by ponding and settling out sheet flow runoff, or by filtering sediment as runoff passes through permeable berms. Stabilized earth/soil berms are constructed from compacted soil, compost, mulch/wood chips, aggregate, or other filtering materials.
The size, soil condition, and topography of the site must be considered before selecting, sizing, and installing a stabilized berm. They should be installed before upslope construction activities begin, particularly any activities causing soil disturbance. They should be maintained until the contributing area is fully stabilized and vegetation is established. Soil berms should be stabilized immediately with vegetation, erosion control blankets, or similar measures to prevent the berm from eroding and becoming a source of sediment in runoff. image
Berms are effective for treating smaller, flatter drainage areas. Berms can be used on larger sites by installing multiple berms to partition the drainage area so that individual structures are not overwhelmed by incoming flow. Berm effectiveness is reduced when slopes exceed 10 percent. Soils used to construct berms should be free of roots, large rock, vegetation, organic matter, and other non-soil material. Soils with sand content exceeding 70 percent may not be effective for berms experiencing high velocity flows. Berms constructed of wood chips, compost, or mulch are recommended for flatter areas (i.e., < 6%) while berms constructed of earthen materials are more suitable for steeper slopes.
Berms are well suited for redirecting sheet flow to treatment areas such as temporary sediment traps and can be installed along a slope to intercept sheet flow. Earth/soil berms are generally not recommended for areas receiving concentrated flow unless they are armored with erosion control blankets or turf reinforcement matting. However, they are appropriate downgradient from areas of disturbed earth and across or at the bottom of a slope, provided that sufficient space is provided to disperse and handle the accumulated runoff and resultant flow. They can also be installed along roadways and other areas to facilitate diversion of runoff and at the tops of slopes as an erosion prevention measure.
Stabilized earth and soil berms are cited for use for several construction activities in the MPCA Construction Stormwater General Permit (2018), specifically for practices that are susceptible to clogging from excess sediment:
Diversion berms are highly effective for moving sediment-laden flows to traps, basins, or other structures/areas where sediment can be removed via settling or filtration. When used as a sheet flow barrier, berms can be effective in sediment removal, but may need frequent maintenance as they are prone to clogging from mud and soil. They are somewhat effective for flow attenuation by slowing down flow, and for removing nutrients and metals associated with sediment. Berms are typically ineffective for managing floating or dissolved pollutants.
Expected performance for stabilized earth/soil berms.
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|Flow attenuation||Secondary design benefit|
|Runoff volume reduction||Little or no design benefit|
|Soil erosion||Primary 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|
Berms should be integrated into an erosion prevention and sediment control system that takes the entire site into consideration. Prior to any earth disturbing activities, berms should be installed where appropriate on downgradient limits-of-disturbance, and upgradient of buffer zones or disturbed zones (i.e., where used to divert clean upland runoff around areas to be graded). They should be integrated with other BMPs, such as minimization of disturbed areas, minimization of the length of exposure time, or adequate seeding and mulching. If downgradient berms become overloaded with sediment and runoff volumes, additional upgradient controls (e.g., silt fencing, temporary vegetative stabilization, sediment traps) may be necessary. Like other perimeter control BMPs, stabilized earth/soil berms must remain in place until the construction site is fully stabilized.
For best results, plan and schedule project activities to minimize maintenance needs, ideally through timely grading operations and stabilization of disturbed areas with seeding, mulching, or other ground covers. Timing of berm 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 stabilized berms reinstalled immediately after the activity is finished, or before the next precipitation event, whichever comes first.
Berm design is dependent on their intended use, i.e., as a flow diversion device or as a flow interceptor and small sediment barrier/trap. Berms can be constructed from material stockpiles on site. For example, shredded wood mulch or wood chips can be derived from the clearing and grubbing phase of the project. Rock can be used to construct temporary berms, and site development work can be adapted to include contoured berms during mass grading. Topsoil is often stockpiled at the construction site until needed for final grading; this can be a valuable resource for constructing a stabilized berm until it is needed elsewhere. The following design recommendations apply to berms:
Recommended berm shape, base width, height, and side slopes depend on the berm material and are summarized in the table below.
Recommended dimensions for stabilized berms. Source: MnDOT 2018 Standard Specifications
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|Berm Type/Material||Recommended shape||Recommended base width||Recommended height||Recommended side slopes|
|Type 1 - compost||Trapezoidal||5 feet||2 feet (minimum)||2:1|
|Type 2 - slash mulch||Trapezoidal||5 feet||2 feet (minimum)||2:1|
|Type 3 - rock weeper system||Trapezoidal||8 feet||2 feet (maximum)||2:1|
|Type 4 - topsoil||Trapezoidal||7 feet||2 feet (minimum)||2:1|
|Type 5 - rock||Trapezoidal||5 feet||2 feet (maximum)||2:1|
Permissible velocities for prevention of erosion of the berm are based on soil type and are summarized in the table below. These values reflect the maximum velocity of runoff that should be intercepted by a berm before erosion is likely to occur.
Maximum runoff velocity (feet per second) for a berm based on soil type. These values reflect the maximum velocity of runoff that should be intercepted by a berm before erosion is likely to occur.
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|Soil Type||Permissible Velocity (fps)|
|Sand, silt, sandy loam, and silty, loam||1.5||1.5||2.0||3.0|
|Sandy clay, lam, and sandy clay, loam||2.0||2.5||3.0||4.0|
Berm material selection, design, siting, and installation is dependent on its use, either as a flow diversion device or as a flow interceptor (e.g., flow attenuator, sediment removal BMP). General construction recommendations include the following.
The MnDOT Standard Specifications for Construction (2018 Edition) includes several specifications relevant to stabilized earth/soil berms. MnDOT Specification 2573.3 Part E (page 513) discusses installation of filter berms used as a perimeter control, with specific guidance provided based on construction material, as follows:
Specification 2573.3 Part P.1 covers maintenance of filter berms, stating: “Expand, enlarge, or augment the filter berm with additional erosion and sediment control practices if concentrated flows bypass or breach the berm or to maintain the dimensions of the berm.” Inspection and maintenance of stabilized earth/soil berms is discussed below.
Specification 2573.5 Part A discusses basis of payment for filter berms. The specification states: “The contract pay item for Topsoil filter berm includes seed and mulch. The contract pay item for rock or rock weeper filter berm when converted from temporary to permanent conditions includes the cost to modify the berm dimensions.”
Specification 2573.5 Part B.1 covers temporary seeding as a stabilization measure for soil berms, stating: “Perform temporary seeding in addition to temporary mulching on graded areas with topsoil and unable to receive permanent seeding or slopes and topsoil berms left idle for longer than 21 days. Use cover crop and mid-term stabilization seed mixtures as shown in 3876, “Seed”, Table 3876-1 for temporary seeding. Prepare the soil in accordance with 2574.3.A and 2574.3.B, except for stockpile and berms where no soil preparation is needed.”
Specification 3874 is specifically dedicated to filter berms and outlines specific design requirements. The specification includes the following guidance:
The control of runoff is a key element of sediment control on a construction site, and berms can be an effective measure for managing sediment from construction runoff if they are properly installed and maintained. Frequent inspection and maintenance are essential to the proper performance of this BMP. As noted above, Specification 2573.3 Part P.2 discusses maintenance requirements for berms, stating: “Expand, enlarge, or augment the filter berm with additional erosion and sediment control practices if concentrated flows bypass or breach the berm or to maintain the dimensions of the berm.” Inspect berms weekly, before rain events, and after rain events, particularly events of 0.5 inch or greater, to ensure they are functioning. If not, complete the following tasks as needed.
The following table summarizes estimated BMP costs based on MnDOT data summarizing average bid prices for awarded projects in 2017.
Unit costs related to stabilized earth/soil berms
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|Bid Item||Item Description||Units||Average Price|
|2573515/00011||Filter berm type 1 (compost)||linear feet||$28.00|
|2573515/00013||Filter berm type 3 (rock weeper system)||linear feet||$26.97|
|2573515/00014||Filter berm type 4 (topsoil)||linear feet||$2.30|
|2573515/00015||Filter berm type 5 (rock)||linear feet||$48.02|
|2105604/00003||Grade protection||square yard||$23.50|
|2574550/00012||Compost grade 2||cubic yard||$50.22|
|2575513/00050||Mulch material type 5 (slash mulch)||cubic yard||$45.00|
|2105604/00034||Geotextile fabric type IV||square yard||$2.31|
|2451511/00030||Coarse filter aggregate (CV)||cubic yard||$52.78|
|2511501/00011||Random riprap class I||cubic yard||$64.10|
|2511501/00012||Random riprap class II||cubic yard||$74.82|
Except where more stringent requirements are presented in this guidance, BMPs 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: