Key factors affecting soil erosivity
A crucial component in developing an ESC plan is assessing the erosion risk which should inform the level of erosion and sediment control to be implemented at various phases throughout the site development. Whether erosion risk is assessed using the Revised Universal Soil Loss Equation (RUSLE) or some other means, a key input and consideration will be related to soil erosivity (K factor). Principally, erosion will be driven by water erosion and/or wind erosion, with both processes related to the detachment and transportation of soil from the land by the action of water (rainfall and runoff) or wind.
In terms of wind erosion, the potential for soil erodibility varies based on soil texture, aggregate size and aggregate density. Given ESCP’s are primarily focussed on managing water erosion, the following discussion relates to soil erosivity within this context.
Soil erodibility is influenced by soil texture and the strength of bonds between soil particles (known as aggregate stability). The soil texture is related to the infiltration rate, which when rainfall occurs influences the runoff rate and hence erosion. In other words, soils with low infiltration rates accelerate erosion because of high runoff rates. In addition to this, a soil with low infiltration and weak bonding between soil particles will be very susceptible to erosion (Hazelton & Murphy, 2007).
The most erodible soils are those with poor aggregate stability and a high portion of soil particles which can be easily transported by surface runoff. Soils which fit into this description include the following:
Soils high in silt and fine sand and low levels of organic matter (typically negligible organic matter content in subsoils)
Self-mulching clays
Dispersible clay soils (typically sodic soils)
Those soils when combined with low infiltration such as on construction sites where soils have been heavily compacted (by trafficking or to meet engineering specifications) result in very high soil erodibility.
Typically, soil erosivity values (K factors) are published based on soil texture, however consideration should also be given to the other related properties discussed above. The suggested or default K factors presented within published sources are based on mid-points of texture classes, with adjustment potentially required in instances of dispersive soils.
Remember, soil erosivity is only one component of an erosion hazard assessment, the others being rainfall erosivity, slope length, surface cover and land management practice.