Erosion - Mining and the Environment: From Ore to Metal - page 782

The permeability of a soil proi le also affects soil erodibility, since greater ini ltration
reduces runoff. The presence of organic matter in soil can also reduce erodibility, probably
due to the absorptive and possibly binding capability of organic matter (Day 2000).

Slope Length and Gradient (LS)
The combined slope length and slope gradient factor, LS, is the ratio of soil loss per unit
area on a site to the corresponding loss from a 20 metre-long experimental plot with a 9%
slope (Day 2000; Goldman et al . 1986). As slope length increases, total soil loss and soil loss
per unit area increase due to the progressive accumulation of runoff in the down-slope
direction. Slope gradient determines l ow velocity. Slopes greater than 5 to 10% will usu-
ally require special controls to prevent water erosion. Terraces, strip cropping, contour
ploughing, and similar techniques are often applied to retard overland l ow. The effective-
ness of such erosion-control practices depends on the local conditions. For example, con-
touring is far more effective in low-rainfall areas than the high-rainfall areas commonly
encountered in the tropics. The value of LS can have a wider variation than any other
factor in the USLE (Day 2000).

Slopes greater than 5 to 10%
will usually require special
controls to prevent water
erosion.

Vegetative Ground Cover and Cover Management (C)
The 'vegetative ground cover and cover management factor' in the USLE equation is an
empirical factor that represents the effects of the type and extent of vegetation at a site, as
well as soil cover management, such as the use of protective mulch cover. Clearly, highly
erodible conditions exist during site preparation, construction and mining periods when
the soil is bare and highly disturbed. Day (2000) reports a value for C of 0.01 for an undis-
turbed area with native vegetation, and a value of 1.0 for bare ground. This suggests that
clearing an undisturbed area can increase erosion by a factor of 100. Exposed cut and i ll
slopes and the slope of mine waste placements are expected to erode unless protected, par-
ticularly in the tropics.

Exposed cut and fi ll slopes
and the slope of mine waste
placements are expected to
erode unless protected.

Support Practices (P)
Support practices account for control measures that reduce the erosion potential of runoff
by their inl uence on drainage and on hydraulic forces exerted by runoff on soil. The value
for P rel ects the effects of practices on both the amount and velocity of water runoff. For
example, higher P values apply to sites where the graded surface is smooth and uniform,
while lower P values are applicable for prepared surfaces that are rough and irregular
(Day 2000). This is because increased surface roughness reduces surface water l ow velo-
city and, thereby, reduces erosion.
Practices that control the runoff from adjacent areas are essential in erosion control.
Such runoff to disturbed land is sometimes termed run-on, and can create major problems.
Unless run-on resulting from rainfall on adjacent tributary slopes is properly controlled,
severe erosion of exposed soil will occur. When run-on to disturbed areas is concentrated at
distinct locations rather than as sheet l ow, erosion due to run-on often outweighs erosion
due to direct rainfall on the disturbed area itself.

Unless run-on resulting from
rainfall on adjacent tributary
slopes is properly controlled,
severe erosion of exposed soil
will occur.

Assessing the Impact of Water Erosion

The USLE is not rocket science: the factors are intuitive, and the equation is simple alge-
bra. However, soil loss equations such as USLE are based on empirical factors that in turn
depend on numerous and often unknown parameters. The USLE equation, originally devel-
oped for croplands, was revised in 1997 to account for soil losses from lands disturbed by