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Construction BMPs
Construction BMPs
2.0 Fundamental Erosion and Sediment Control Principles
2.1 Erosion
Soil erosion can generally be defined as
the removal of soil by wind and water.
Although soil erosion is a natural
process, accelerated soil erosion occurs
on construction sites due to activities
that disturb the natural soil and
vegetation.
Water erosion has five primary
mechanisms: raindrop erosion, sheet
erosion, rill erosion, gully erosion, and
channel erosion. Raindrops dislodge
soil particles, making them more
susceptible to movement by overland
water flow. Shallow surface flows on
soil rarely move as a uniform sheet for
more than several feet before
concentrating in surface irregularities,
known as rills. As the flow changes
Photograph 7-2. Erosion is a common occurrence during
construction activities, which can result in sediment movement off
site and deposition in waterways when not properly managed.
(Photo courtesy of Douglas County)
from a shallow sheet to a deeper rill flow, the flow velocity and shear stresses increase, which detach and
transport soil particles. This action begins to cut into the soil mantle and form small channels. Rills are
small, well-defined channels that are only a few inches deep. Gullies occur as the flows in rills come
together into larger channels. The major difference between rill and gully erosion is size. Rills caused by
erosion can be smoothed out by standard surface treatments such as harrowing. Gully erosion, however,
typically requires heavy equipment to regrade and stabilize the land surface.
Wind erosion occurs when winds of sufficient velocity create movement of soil particles. The potential
for wind erosion is dependent upon soil cover, soil particle size, wind velocity, duration of wind and
unsheltered distance.
Erodibility of soils is affected by multiple factors including physical soil characteristics, slope steepness,
slope lengths, vegetative cover, and rainfall characteristics. Physical properties of soils such as particle
size, cohesiveness, and density affect erodibility. Loose silt and sand-sized particles typically are more
susceptible to erosion than "sticky" clay soils. Rocky soils are less susceptible to wind erosion, but are
often found on steep slopes that are subject to water erosion. Most of the soils in Colorado are
susceptible to wind or water erosion, or both. When surface vegetative cover and soil structure are
disturbed during construction, the soil is more susceptible to erosion. Vegetation plays a critical role in
controlling erosion. Roots bind soil together and the leaves or blades of grass reduce raindrop impact
forces on the soil. Grass, tree litter and other ground cover not only intercept precipitation and allow
infiltration, but also reduce runoff velocity and shear stress at the surface. Vegetation reduces wind
velocity at the ground surface, and provides a rougher surface that can trap particles moving along the
ground. Once vegetation is removed, soils become more susceptible to erosion.
7-2 Urban Drainage and Flood Control District November 2010
Urban Storm Drainage Criteria Manual Volume 3