Hill slope erosion is caused by the direct impact of raindrops on the soil surface, overland (inter-
rill) flows, and small channel flows.425 Overland flow begins as surface depressions are filled and when rain falls faster than water infiltrates into the soil. Although overland flow is often viewed as a sheet of water flowing over the surface, it typically includes numerous shallow, but easily definable channels, called rills. The relative amount of sediment detached and transported by inter-rill flow is small compared to splash and rill erosion. Rills are small enough to be removed by normal tillage operations, but may become too large (gully) to remove with tillage. Rill erosion is substantially more erosive than overland flow and is a function of hill slope length, depth of flow, shear stress, and critical discharge. Rill erosion starts when the eroding force of the flow exceeds the ability of the soil particles to resist detachment. Flow depth and velocity increase substantially where surface irregularities concentrate overland flows into rills. Once rills are established, the concentrated flow develops more detachment force, and the rill formation process is enhanced. Rill development moves upslope as headcuts. Some rills develop rapidly and become more deeply incised. These master rills become longer and deeper than their neighbours. Occasionally flows from adjacent rills break into master rills by eroding the boundary between them. As the rill flow becomes concentrated toward master rills, previously parallel rills develop a recognisable dendritic drainage pattern. As rills coa-
425 Brooks et al, 1991.
lesce, flow concentrations and velocity increase until the more deeply incised rills become gullies.
Wind erosion is greatest on fine soil particles such as silt, clay, and organic materials. This wind-driven sorting increases the proportion of coarse materials in wind-eroded sites. Windblown particles are moved in three ways: (1) saltation, the bouncing of particles across the surface; (2) suspension in wind; and (3) surface creep, the movement of larger particles caused by the pushing action of saltating particles striking larger particles.426 The amount of wind erosion is affected by soil erodibility, surface roughness, climate, unsheltered distance of soil exposed to wind, and vegetation cover. Thus wind erosion is reduced by rougher soil surfaces, lower wind speed at the soil surface, and more plant or litter coverage of the soil surface.
Mass movement is the downward movement of slope-forming materials without the primary assistance of a fluid. It occurs on steep slopes under the influence of gravity, often exacerbated by the weight of water in the soils. Mass movement occurs on steep slopes when deforestation, mining, fire, overgrazing, construction, or cultivation disrupts the landform-climate-vegetation equilibrium by removing the vegetation. Well-vegetated slopes generally move downward much slower than less vegetated slopes.427 Plants, especially woody plants with strong, deep roots, greatly increase soil strength, providing a stabilising effect on the slope. In some cases, the plants also transpire significant quantities of water from the slope, thus reducing the weight that contributes to mass movements.
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