Erosion Mgmt Factors

Erosion Management Factors

The Universal Soil Loss Equation gives us an easy way to predict the effectiveness of erosion control products.

Erosion Control on land, as opposed to Stream and Channel Erosion (see the Stream and Channel Section), can be predicted based on standard designs. For each geographic location there are a number of factors that affect erosion. The Universal Soil Loss Equation is a way of calculating the effect of Erosion Control practices on the expected soil loss for a particular area. The Universal Soil Loss Equation is defined as: A=R x K x LS x C x P

Where:

 

  • A = The annual soil loss due to erosion.
  • R = Rainfall factor for a particular location.
  • K = The erodibility factor for the soil type.
  • LS = The length and slope factor (gradient).
  • C = Cover management factors.
  • P = Support practice factors (structures).

For each geographic area there is a characteristic combination of rainfall factors (R), erodibility factors (K), and slope gradient factors (LS) that we cannot control. These factors represent the erosion potential for that particular site. For a more complete discussion of the Universal Soil Loss Equation please contact your representative. The equation is only shown here to demonstrate that C and P factors control the loss of soil independent of the erosive potential of the site.

The factors that we can control are the C factors and P factors of a site. These are the management factors of a site. The C, or the Cover Management Factors include such techniques as ground cover, vegetation, and pavements. Layfield provides Erosion Control Blankets (ECB’s), Turf Reinforcement Mats (TRM’s), Erosion Control Geotextiles, Geo Cells, and Flexible Forms for Concrete as Cfactor materials.

If erosion is already taking place then we need to stop the silt and sediment from running off the construction site. P, or Support Practice factors, trap sediment that is suspended in flowing water. Layfield provides Silt and Sediment control products such as Silt Fence, Silt Dikes, Silt Bags, Catch Basin Filters, and Floating Silt Curtain for P factor use. Note that the erosion has already taken place with a P factor tool, while C factor tools prevent erosion from starting.

 

Table 1 Erosion Management Factors
Erosion Control Treatment C Factor P Factor
Bare Soil 1.00 1.00
Disked Bare Soil 1.00 0.90
Sediment Basin/Trap 1.00 0.50
Straw Bale Barrier 1.00 0.80
Silt Fence Barrier 1.00 0.50
Asphalt/Concrete Pavement 0.10 1.00
Competant Gravel Layer 0.05 1.00
Established Native Grass (100% coverage) 0.03 1.00
Sod Grass 0.01 1.00
Agricultural Crop 0.45 1.00
Erosion Control Blankets 0.002 to 0.003 1.00
Turf Reinforcement Mats 0.002 to 0.003 1.00

Bare soil has a P and C factor of 1.0 which has no effect in the equation of preventing erosion. Each erosion management treatment has a measurable effect on erosion. Table 1 shows how different C and P factors affect the measured soil loss of a site.

If we look at Table 1 we can see that C factor management (cover) techniques are more effective than P factor (support practice) techniques. The use of C factor techniques can reduce the soil loss on a given site to less than 10% of that of bare soil. Vegetation provides a standard C factor of between 1 to 3% (0.01 to 0.03) while products such as ECB’s can provide C factors as low as 0.002.

P factors are much less effective than C factors. P factors also let the soil move to another location before being trapped. The soil erodes and moves to the P factor trap before being stopped. P factor techniques, such as silt fences, will trap "50%" of the silt that arrives at the fence. For each silt fence placed in the path of the water carrying silt "50%" of the silt will be retained. A number of P factor devices can be combined to prevent the run-off of silt from the site. Three silt fences in series would only allow 0.5 x 0.5 x 0.5 = 0.125 of the erodible soil to leave the site (about 13%). By combining more than one P factor technique an effective silt control plan can be devised for sites where cover factors are not practical (sites under construction for example).

Layfield Plastics recommends that P factor techniques should be used for construction and for temporary silt control measures. For permanent Erosion Control a C factor Technique should be employed.

 

Performance of Erosion Control Products

Undisturbed soil is not usually as prone to erosion as various natural vegetative covers prevent loss of soil. Once soil is distrubed however, the risk of erosion is high. During construction it is often not practical to cover all the open soil to prevent erosion. During construction, P factor techniques such as silt fences, sediment basins, and straw bales are used to prevent loss of soil. Once the construction project is completed and the soil has been re-graded, it is important to cover the soil as soon a possible to prevent costly erosion. The C factor techniques available from Layfield include Erosion Control Blankets (ECB’s), Turf Reinforcement Mats (TRM’s), Erosion Control Geotextiles, Geo Cells, and Flexible Forms for Concrete.The products available from Layfield Plastics have clearly defined P and C factors.

P Factor Products

Layfield's primary P factor materials are the silt fence and the triangular silt dike. Silt fences and dikes are used for controlling silt run-off from a site so that off-site areas are not contaminated. Layfield manufactures silt fence in many different styles to suit your requirements. Other P factor techniques include Silt Bags, Catch Basin Filters, Floating Silt Curtains, and sediment basin. Sediment basins can benefit from the use of a geomembrane liner in permeable soils. Please see the section on Geomembranes for the economical RPE® lining material which is ideal for sediment basins.

C Factor Products

Layfield has a number of products available that provide excellent coverage of C factor management practices. Erosion Control Blankets (ECB's) and Turf Reinforcement Mats are designed to retain soil in place until vegetation is established. Permanent vegetation will provide the long term coverage but in the short term a revegetation mat can prevent erosion until vegetation is established. ECB's and TRM's have C factors around 0.002 which means that over 99% of the soil is retained when installed in accordance with installation instructions. There are a number of varieties of ECB's and TRM's available for most slope gradients and lengths.

Geotextiles used in Erosion Control would normally be used with a gravel topping layer. A gravel layer would provide a C factor of about 0.05.

Geo-Cell is a way of retaining gravels and sands in place on slopes and channels. With a gravel layer, a Geo-Cell would be expected to have a C factor of 0.05.

Flexible Fabric Concrete Forms (the uniform section mat and the high strength block type mats) normally provide a C factor of 0.10. Filter point mats may have higher C factor ratings however data is not available.

Geomembranes make excellent temporary cover materials during construction on steep slopes and in excavations. The Geomembrane keeps precipitation from contacting the soil and causing erosion and provides a C factor of up to 0.10. Layfield can fabricate large panels of RPE®Geomembrane materials that make excellent temporary erosion control covers. More information on RPE® geomembranes can be found in the Geomembranes section.

Erosion Control is one of the most varied uses of geosynthetics. Other products may be available. Please contact your Layfield representative for details of products stocked in your area.

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