Erosion - Mining and the Environment: From Ore to Metal - page 781

FIGURE 20.2
Factors Controlling Water Erosion

Soil Loss R K LS C P
R Rainfall frequency, intensity, storm
kinetic energy, etc.
K Particle size, organic matter, per-
meability, rock fragment, etc.
LS Length, average gradient, geom-
etry, etc.
C Vegetation, mulch, surface rough-
ness, cover management, etc.
P Supporting mechanical practices
such as diversion ditch, silt fences, etc.

Run-on

Rainfall (R)

Runoff

less than 30 minutes are common in the tropics, and no erosion controls exist to completely
eliminate soil loss in such circumstances. Analyses of data indicate that when factors other
than rainfall are held constant, soil loss is directly proportional to rainfall intensity.

Soil Erodibility (K)
Soil erodibility, K, is an empirical measure of the ability of rainfall and runoff to detach
and transport soil particles. It correlates with inherent physical soil properties, such as par-
ticle size, organic matter content, soil plasticity and cohesion, the dispersiveness of clay
soils, and the presence of any particle-cementing agents. The two main factors are particle
size and plasticity (Day 2000). Soil plasticity refers to the ability of i ne-grained silt and
clay soils to be rolled and moulded without breaking apart.
Cohesive, highly plastic clay soils are usually resistant to detachment from the soil
matrix and soil erodibility is low. However, some types of clay, known as dispersive clay,
may have high plasticity, but are more susceptible to erosion because of del occulation
of clay particles. Del occulation occurs when the repulsive forces between clay particles
exceed the attractive forces in the presence of water, forming colloidal suspensions. It is
related to the chemical composition of the clay, and various tests can be used to identify
dispersive clays, such as the laboratory pin hole test.
In contrast to non-dispersive cohesive clay, soils with a relatively high content of cohe-
sionless particles, such as silt and sand, are often highly susceptible to erosion. Silt-size par-
ticles are the most susceptible, as they are non-plastic and easily detached, producing high
erosion rates and large volumes of runoff. However, as the particle size of cohesionless soils
increases, erosion resistance increases due to the increased weight of the individual parti-
cles. The scale of cohesionless soil particle sizes ranges from silt, for which the individual
particles are indistinguishable to the naked eye, through i ne to course sand, then gravel,
cobbles and, i nally, boulders.

Soils with a relatively high
content of cohesionless particles,
such as silt and sand, are often
highly susceptible to erosion.