Understanding ErosionErosion 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 ControlThere 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 and Permit RequirementsTo 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 Name||Performance Requirement||Purpose||Applicability|
|Sheet and Rill Erosion||Maximum allowable cumulative soil loss is 7.5 tons/acre/year||Minimize soil loss and prevent water quality and aquatic habitat degradation||All sites requiring an erosion control plan|
|Gully and Streambank Erosion||Prevent gully and streambank erosion||Minimize soil loss and prevent water quality and aquatic habitat degradation||All sites requiring an erosion control plan|
|Stable Outlet||Provide stable outlet capable of carrying design flows at a non-erosive velocity||Prevent downstream erosion||All sites requiring an erosion control plan|
- Detailed, written description of how the site will be developed to ensure appropriate practices are being proposed and can implemented with the proposed construction schedule.
- Plan drawing of site to show the location of property lines, lot dimensions, limits of disturbed area, limits of impervious area, land cover type, natural and artificial water features, 100-yr flood plain boundaries, delineated wetland boundaries, and locations of proposed erosion controls.
- Direction of runoff flow to determine effects of stormwater quantity and quality.
- Watershed size for each drainage area to determine how much of the area to be developed is affected by other drainage flowing through the construction site; to design culvert sizes and drainage channels; to determine the sediment produced by the site under construction.
- Provisions to prevent mud tracking off site including the tracking pad design (length, depth, etc.).
- Provisions to prevent the delivery of sediment to stormwater conveyance systems to ensure capacity is not compromised and sediment is not transported off-site to receiving waters.
- Universal Soil Loss Equation spreadsheet(s) to show the 7.5 ton/acre/year soil loss standard is being met.
- Provision for sequential steps mitigating erosive effect of land disturbing activities including a series and schedule of practice installation to mitigate the increase in runoff and the pollutants it carries.
- Time schedules for completion and installation of all elements of the erosion control plan to calculate the amount of sediment that will leave the site and to select the site practice(s).
- Fertilizer and seeding rates and recommendations to illustrate how the disturbed areas will be returned to stable conditions.
- Itemized estimated cost (including labor) of erosion control practices to determine the applicability of a financial security document.
- Design discharge for ditches and structural measures to accommodate a 10-year, 24-hour storm and safely pass the 100-year, 24-hour storm event.
- Cross sections of, as well as profiles within, road ditches to ensure non-erosive velocities.
- Culvert sizes to maintain water quantity control to pre-development conditions and ensure that the time of concentration of runoff does not affect existing structures.
- Runoff velocities to illustrate that they are not erosive and to ensure all slopes are stable.
- Proof of a stable outlet to ensure that stormwater is being discharged from the site at a non-erosive velocity.
- Copy of preliminary review letter, permits, or approvals by other agencies to ensure applicable permits have been applied for.
- Any other information necessary to reasonably determine the location, nature and condition of any physical or environmental features of the site.
- Acknowledgment that any proposed changes to the erosion control plan will be submitted for approval prior to implementation to ensure the plan and site stay in compliance.
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 Practices||Applicable Standard||Site Applicability||Maintenance Requirement||Environmental Concerns||Special Consideration|
|Construction Scheduling||Sheet and Rill Erosion||Widely Applicable||Low||None||Can greatly reduce erosion from a site|
|Deep Tilling||Sheet and Rill Erosion||Widely applicable on sites where heavy grading has occurred||Very Low||None||Should be timed after grading has occurred; Buried Utilities|
|Mulching||Sheet and Rill Erosion||Widely Applicable||Moderate||Limited effectiveness on steep slopes depending on the type of mulch||Must be reapplied/replaced frequently and crimped|
|Polymer Application||Sheet and Rill Erosion||Applicable on sites that are not actively being graded||Moderate||Risk of adverse impacts if over applied||Must be re-applied if site is disturbed after initial application|
|Permanent Seeding||Sheet and Rill Erosion||Widely Applicable||Moderate; Low once established||Possible erosion during establishment; fertilizer runoff||Must match seed mix with the time of year and site conditions; Requires > 3" of topsoil|
|Temporary Seeding||Sheet and Rill Erosion||Widely Applicable||Moderate; Low once established||Possible erosion during establishment; fertilizer runoff||Effective for a maximum of 1 year; Requires > 3" of topsoil|
|Sod||Sheet and Rill Erosion||Widely Applicable||Low after establishment||Fertilizer runoff, Overwatering||May need to be staked on steep slopes & channels; Proper selection of species; Requires > 3" of prepared topsoil|
|Surface Roughening||Sheet and Rill Erosion||Widely Applicable||Moderate||Erosion may increase if not done on the contour of the slope||Need a specially selected tracked or wheeled vehicle|
Table 2: Structural erosion control practices (adapted from Massachusetts Stormwater Technical Handbook, 1997)
|Non-Structural Practices||Applicable Standard||Site Applicability||Maintenance Requirement||Environmental Concerns||Special Consideration|
|Vegetated Buffer Strip||Sheet and Rill Erosion||Applicable when already installed||Low||None||Sufficient/suitable land area; Must be used in conjunction with other practices|
|Permanent Diversion||Sheet and Rill Erosion||Widely Applicable||Moderate||Possible erosion of diversion structure if diverted runoff carries a large sediment load||Must be carefully designed to prevent property damage|
|Temporary Diversion||Sheet and Rill Erosion||Widely Applicable||Moderate||Possible erosion of diversion structure if diverted runoff carries a large sediment load||May need frequent repair/replacement; Channel must be stabilized on slopes with a grade of >2%|
|Erosion Matting||Sheet and Rill Erosion||Widely applicable on low to moderate slopes||Low||Limited effectiveness on steep slopes||Proper installation|
|Gabion||Sheet and Rill Erosion; Gully Erosion||Applicable to vegetated ditches and swales||Low||Does not remove smaller suspended particles||Usually used in conjunction with other practices|
|Sediment Basin||Sheet and Rill Erosion||Applicable on sites with a drainage area of||Low||Maximum sediment removal capacity of 60-80%; Does not remove fine silts and clays||May need frequent repair/replacement; Sufficient/ suitable land area; Proper design and construction|
|Sediment Trap||Sheet and Rill Erosion||Applicable on sites with a drainage area of||Low||Maximum sediment removal capacity of 60-80%; Does not remove fine silts and clays||May need frequent repair/replacement; Sufficient/ suitable land area; Proper design and construction|
|Silt Fence||Sheet and Rill Erosion||Widely Applicable||High||Sediment transport; High rates of failure if not properly installed and maintained; Disposal||Longevity, proper installation|
|Slope Drain||Sheet and Rill Erosion; Gully Erosion||Applicable on sites that are vulnerable to convey runoff downslope||Moderate||Possible erosion around inlet & outlet||Pipe size|
|Stone Check Dam||Sheet and Rill Erosion; Gully Erosion||Applicable to vegetated ditches and swales, Drainage areas||Low to Moderate||Does not remove smaller suspended solids|
|Stone Tracking Pad||Sheet and Rill Erosion||Widely Applicable||Low to High||None||Cost effective; Must use >3" clear stone|
|Stone Weeper||Sheet and Rill Erosion; Gully Erosion||Applicable to vegetated ditches and swales, Drainage areas||Low to Moderate||Does not remove smaller suspended particles|
|Inlet Protection||Sheet and Rill Erosion||Widely Applicable||Moderate to High||Ineffective for large storm events; Limited effectiveness with large sediment loads||May need frequent cleaned/replacement; used in conjunction with other practices|
The 7.5 Tons/Acre/Year StandardResearch has shown the average soil loss on uncontrolled construction sites is approximately 30 tons/acre/year. By limiting soil loss from construction sites to 7.5 tons/acre/year, an average reduction of at least 75% from uncontrolled conditions will be achieved. 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 7.5 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 From Construction SitesThe Dane County Land and Water Resources Department has developed an Excel worksheet that calculates soil loss from construction sites. This spreadsheet uses the Universal Soil Loss Equation (USLE) to determine whether the combination of proposed erosion control practices will limit soil loss from sheet and rill erosion to 7.5 tons/acre/year or less. The USLE, its variables, and an example calculation can be found on the Universal Soil Loss Equation and Spreadsheet page. Note that the USLE 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 ErosionGully 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.