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

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 Polymers, 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.

To minimize erosion from construction sites and protect the county’s lakes and streams from sediment pollution, it's required that plans for all construction sites include practices that meet the standards in the table below.

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.

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 80%, compared to uncontrolled conditions. By analyzing the USDA Natural Resources Conservation Service’s definition of particle size distribution for Plano Silt loam (Figure 2), 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 80% during construction, the 5 micron (µm) particle will need to be trapped.

Figure 2: 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 3). 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.

Figure 3: Particle settling time in 2 feet of water

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 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.