This section of the report seeks to explain the mechanism and factors which influence soil erosion. Though look technical but it imperative that engineers have a detailed understanding of the mechanisms of erosion before they can effective apply management strategies. Wrong application of principleS will cause serious long term environmental complications.
Mechanism of soil erosion
Soil erosion goes through three main processes or mechanisms. These includes Detachment, Transport and Deposition. These mechanisms depend on the soil characteristics,Soil’s Erodibility, Erosivity, land use, slope and land cover.
Universal soil loss equation
The severity of an erosion is dependent on the combination of a number of factors. This is summarized in a model known as the universal soil loss equation(USLE).The USLE relates the rate of erosion from an exposed area (A) to the erosive power of the rain (R), the soil erodibility (K), the land slope and length (LS), the degree of soil cover (C), and conservation practices (P):
A = R x K x LS x C x P
The important aspect of this equation to note is the linear relationship between the equation’s parameters. As any parameter is changed, the resulting erosion yield is similarly changed. Though other conventional methods and models are very relevant tools in erosion estimation but most of the discussions in these paper will directly or indirectly be focused on the components of the universal soil equation to address the mechanism of soil erosion. The extent of the erosion determines the feasibility of the control technique to be used.
Soil Detachment
Soil detachment , as the first stage plays an instrumental role in the erosion process. Various researchers have given definition to soil detachment.
Soil detachment, defined as the soil particles being separated from the soil matrix at a particular location on the soil surface by erosive agents (Wang et al., 2014a; Zhang et al., 2003), is a key process affecting soil erosion since it determines the amount of sediment that is potentially transferred to surface water bodies. Soil detachment rate is expressed as the sediment amount detached per unit area per unit time (Zhang et al., 2009a). With increase in sediment concentration in flowing water, more energy is used for sediment transport, which causes a decrease in soil detachment rate (Lei et al., 2002; Zhang et al.,) Soil detachment capacity is a key parameter in many process-based erosion models such as the Water Erosion Prediction Project (WEPP) model (Nearing et al., 1989).
Soil detachment capacity by overland flow is influenced by various factors such as flow hydraulics, soil properties, root systems, tillage operations, and land use (Knapen et al., 2007a; Scherer et al., 2012). For a given soil, flow hydraulics (e.g. discharge, slope gradient, flow depth, and velocity) control the process of detachment (Govers, 1992; Zhang et al., 2003). Soil detachment capacity increases with flow discharge and slope gradient and is more sensitive to discharge than slope gradient. Shear stress and stream power are commonly used to simulate erosion processes in process-based models (Nearing et al., 1991). However, some studies indicate that stream power is better than shear stress to predict soil detachment capacity (Cao et al., 2009; Zhang et al., 2003).
Soil erodibility and soil characteristics
Erodibility represents the soil’s response to rainfall and runoff erosivity. Using such a definition, soil erodibility is not a measurable single parameter because it includes all the soil characteristics (both static and dynamic ones) that control a range of sub-processes affecting soil erosion. Hence, the soil characteristics that control the soil’s behaviour with respect to soil–water interaction (i.e. infiltration rate, permeability, water retention forces, porosity, exchangeable ions, primary particle sizes and mineralogy, aggregate size and stability, soil organic matter) are all important, as they interact, directly and indirectly, with rainfall to produce ponding water in soil surface depressions and surface runoff. Erodibility varies with soil texture, aggregate stability, SOM contents and hydraulic properties of the soilThe stability of the soil mass is therefore depended on the clay minerals present. Illite and smectite more readily form aggregates but the more open lattice structure of these minerals and the greater swelling and shrinkage which occur on wetting and drying render the aggregates less stable than those formed from kaolinite.
Effects of rainfall intensity and slope gradient on erosion Characteristics
Soil erosion induced by rainfall has been demonstrated to be an important driving force of ecosystem degradation (Han et al. 2011; Zhu 2012), which is complex phenomenon of detachment and transportation of soil particles by raindrop/splash and surface flow scouring that can transport particles away from original position (Sirjani and Mah-moodabadi, 2012). Therein, detachment, transportation, and deposition are the three main physical processes involving interactions among rainfall characteristics, overland flow and, soil properties (Defersha et al.2011; Shi et al.2012;Sirjani and Mahmoodabadi2012), in which detachment rate agented by detaching soil particles from the soil surface is always controlled by the acting force of raindrop (Shih and Yang, 2009), transportation rate agented by transferring the sediment away from the soil matrix is depended on the transfer force of a thin layer of overland flow produced by rainfall (Ali et al., 2012), and then the deposition rate is governed primarily by the resistance of the underlying surface condition (Han et al., 2011). Regarding those parameters that the soil erosion depended, rainfall intensity and slope gradient are the two dominant factors that control the hydrologic response and have been extensively studied via numerical simulation, experiments, and analytical solutions, and are still the hot topics of the soil erosion research (Assouline and Ben-Hur2006; Han et al., 2011; Donjadee and Chinnarasri, 2012;Ran et al). Hudson (observed that in simplest terms steep land is more vulnerable to water erosion than flat land for reasons that erosive forces, splash, scour and transport, all have a greater effect on steep slopes. Soil erosion generally is a function of slope attributes.
In Summary, an in-depth understanding of the mechanism and the interaction between the various factors presents a great tool for engineers in the planing and implementation of soil erosion strategies.