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Erosion Control

Understanding Erosion

Erosion occurs when soil particles are detached from the land surface and carried down slope by moving water. Figure 1 illustrates how this process commonly occurs. First, a raindrop’s velocity increases as it approaches the soil surface. This velocity, plus the drop’s weight, provides sufficient energy at impact to detach soil particles. Once detached, upslope soil particles are carried by runoff until the flow spreads out, the gradient decreases and energy dissipates. As the flow loses the energy needed to suspend particles, it deposits the particles as sediment.

Dane County construction sites are highly susceptible to erosion for several reasons. First, soil is easily detached from the land surface because vegetation and the surface layer of organic soil are stripped. Second, heavy machinery compacts the soil causing it to lose infiltration capacity, which increases the volume of water that becomes runoff and the potential to wash soil downhill. Third, since there is no vegetation to spread runoff into sheet flow, it is more likely to concentrate and cause gully erosion. Fourth, Dane County soils are comprised of large amounts of silt, which is easily detached from the other soil particles.

Figure 1: Conceptual model of erosion

Erosion Control

There are a variety of strategies for minimizing soil loss from construction sites. These include preventing soil detachment, diverting runoff around disturbed areas, and trapping sediment carried by runoff before it leaves the site. The most important strategy for controlling construction site erosion is preventing soil particle detachment through soil stabilization. Vegetation should be re-established as soon as possible after land is disturbed. In the meantime, other erosion control practices, such as polymer application, erosion matting, and mulching must be in place. A second line of defense is to prevent runoff from contacting detached soil particles by diverting runoff around disturbed areas. Diversions minimize the opportunity for runoff to entrain detached soil particles and carry them offsite. Finally, when soil particles are detached and carried by runoff, practices that slow and/or trap sediment must be installed to prevent suspended sediment from leaving the site and entering water bodies. Figure 2 illustrates how erosion prevention, diversion, and inlet protection can be integrated to minimize erosion.

Figure 2: Construction site erosion control practices (adapted from Association of Bay Area Governments)

Dane County Erosion Control Standards

To minimize erosion from construction sites and protect the county’s lakes and streams from sediment pollution, the Erosion Control and Stormwater Management Ordinance requires that plans for all construction sites include practices that meet the standards in the table below.

Standard NamePerformance RequirementPurposeApplicability
Sheet and Rill ErosionLimit cumulative sediment deposition to 5 tons/acre/yearMinimize soil loss and prevent water quality and aquatic habitat degradationAll sites requiring an erosion control plan
Gully and Streambank ErosionPrevent gully and streambank erosionMinimize soil loss and prevent water quality and aquatic habitat degradationAll sites requiring an erosion control plan
Stable OutletProvide stable outlet capable of carrying design flows at a non-erosive velocityPrevent downstream erosionAll sites requiring an erosion control plan
In order to assist in meeting the standards and requirement set forth by the ordinance, Tables 1 and 2 list non-structural and structural practices that could be used to achieve the performance standards. The table briefly describes where practices should be used along with maintenance requirements, environmental concerns and any special considerations for the practices.

Table 1: Non-structural erosion control practices (adapted from Massachusetts Stormwater Technical Handbook, 1997)

Non-Structural PracticesApplicable StandardSite ApplicabilityMaintenance RequirementEnvironmental ConcernsSpecial Consideration
Construction SchedulingSheet and Rill ErosionWidely ApplicableLowNoneCan greatly reduce erosion from a site
Deep TillingSheet and Rill ErosionWidely applicable on sites where heavy grading has occurredVery LowNoneShould be timed after grading has occurred; Buried Utilities
MulchingSheet and Rill ErosionWidely ApplicableModerateLimited effectiveness on steep slopes depending on the type of mulchMust be reapplied/replaced frequently and crimped
Polymer ApplicationSheet and Rill ErosionApplicable on sites that are not actively being gradedModerateRisk of adverse impacts if over appliedMust be re-applied if site is disturbed after initial application
SeedingSheet and Rill ErosionWidely ApplicableModerate; Low once establishedPossible erosion during establishment; fertilizer runoffMust match seed mix with the time of year and site conditions; Requires > 3" of topsoil
SodSheet and Rill ErosionWidely ApplicableLow after establishmentFertilizer runoff, OverwateringMay need to be staked on steep slopes & channels; Proper selection of species; Requires > 3" of prepared topsoil
Surface RougheningSheet and Rill ErosionWidely ApplicableModerateErosion may increase if not done on the contour of the slopeNeed a specially selected tracked or wheeled vehicle

Table 2: Structural erosion control practices (adapted from Massachusetts Stormwater Technical Handbook, 1997)

Non-Structural PracticesApplicable StandardSite ApplicabilityMaintenance RequirementEnvironmental ConcernsSpecial Consideration
Vegetated Buffer StripSheet and Rill ErosionApplicable when already installedLowNoneSufficient/suitable land area; Must be used in conjunction with other practices
Permanent DiversionSheet and Rill ErosionWidely ApplicableModeratePossible erosion of diversion structure if diverted runoff carries a large sediment loadMust be carefully designed to prevent property damage
Temporary DiversionSheet and Rill ErosionWidely ApplicableModeratePossible erosion of diversion structure if diverted runoff carries a large sediment loadMay need frequent repair/replacement; Channel must be stabilized on slopes with a grade of >2%
Erosion MattingSheet and Rill ErosionWidely applicable on low to moderate slopesLowLimited effectiveness on steep slopesProper installation
GabionSheet and Rill Erosion; Gully ErosionApplicable to vegetated ditches and swalesLowDoes not remove smaller suspended particlesUsually used in conjunction with other practices
Sediment BasinSheet and Rill ErosionApplicable on sites with a drainage area of LowMaximum sediment removal capacity of 60-80%; Does not remove fine silts and claysMay need frequent repair/replacement; Sufficient/ suitable land area; Proper design and construction
Sediment TrapSheet and Rill ErosionApplicable on sites with a drainage area of LowMaximum sediment removal capacity of 60-80%; Does not remove fine silts and claysMay need frequent repair/replacement; Sufficient/ suitable land area; Proper design and construction
Silt FenceSheet and Rill ErosionWidely ApplicableHighSediment transport; High rates of failure if not properly installed and maintained; DisposalLongevity, proper installation
Slope DrainSheet and Rill Erosion; Gully ErosionApplicable on sites that are vulnerable to convey runoff downslopeModeratePossible erosion around inlet & outletPipe size
Stone Check DamSheet and Rill Erosion; Gully ErosionApplicable to vegetated ditches and swales, Drainage areas Low to ModerateDoes not remove smaller suspended solids 
Stone Tracking PadSheet and Rill ErosionWidely ApplicableLow to HighNoneCost effective; Must use >3" clear stone
Stone WeeperSheet and Rill Erosion; Gully ErosionApplicable to vegetated ditches and swales, Drainage areas Low to ModerateDoes not remove smaller suspended particles 
Inlet ProtectionSheet and Rill ErosionWidely ApplicableModerate to HighIneffective for large storm events; Limited effectiveness with large sediment loadsMay need frequent cleaned/replacement; used in conjunction with other practices

The 5 Tons/Acre/Year Standard

Research has shown the average soil loss on uncontrolled construction sites is approximately 30 tons/acre/year. Limiting sediment deposition to 5 tons/acre/year (equivalent to 7.5 tons/acre/year soil loss) provides an average reduction of approximately 75%, compared to uncontrolled conditions. By analyzing the USDA Natural Resources Conservation Service’s definition of particle size distribution for Plano Silt loam (Figure 3), a soil that is similar to the majority of Dane County’s soils, it is shown that in order to achieve a trapping efficiency of 75% during construction, the 5 micron (µm) particle will need to be trapped.

Figure 3: Trapping efficiency based on Plano silt loam soil distribution

In Dane County, it is not feasible to trap particles smaller than 5 µm from a cost/benefit and engineering standpoint. In a pond that is two feet deep, trapping the 5 µm particle requires a settling time of 6 hours, which is feasible for pond design. However, the particle settling time increases exponentially with decreasing particle size (Figure 4). For example, trapping the 3 µm instead of the 5 µm particle increases the required settling time from 6 to 24 hours, but only increases the expected trapping efficiency by 5%. Designing a pond with a settling time of 24 hours would be much more costly and require a larger land area. Thus, a soil loss standard lower than 5 tons/acre/year would provide small additional benefit at a very high cost. Dane County’s approach is equivalent to the intent of the performance standards for construction sites in the Department of Natural Resources’ Chapter NR 151, Wis. Adm. Code .

Figure 4: Particle settling time (2 foot depth)

Calculating Soil Loss and Sediment Discharge From Construction Sites

The Wisconsin Department of Natural Resources has developed an Excel spreadsheet that calculates soil loss and sediment discharge from construction sites. This spreadsheet uses the Universal Soil Loss Equation (USLE) to determine whether the combination of proposed erosion control practices will limit sediment discharge from sheet and rill erosion to 5 tons/acre/year. Guidance on how to show compliance with this standard can be found on the Soil Loss and Sediment Discharge page. Note that the spreadsheet estimates soil loss from sheet and rill erosion only. It does not predict soil loss resulting from high channel velocities, gully erosion or streambank erosion.

Gully and Streambank Erosion

Gully erosion is caused by concentrated overland flow of surface water in depressions and drainage ways. The surface water’s erosive force removes topsoil while increasing energy as it moves downslope. Once an unprotected gully begins to form, lateral erosion takes place, widening the gully and undercutting the sides where additional soil is removed. Preventive practices and proper management of gullies are required on construction sites.

Streambank erosion removes soil along the banks and bed of a channel. The erosion is the result of high flow within the stream channel after rain events. The erosive force of the flow causes undercutting of the banks, which deposits large amounts of sediment directly into the stream channel. The sediment is then carried and deposited downstream.