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.

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 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.

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.