Erosion - Mining and the Environment: From Ore to Metal - page 779

Where eroded Earth material is transported and deposited downstream or down-wind
of the erosion site, such deposits may: block streams and raise river bed levels increasing
the risk of l ooding and interfering with navigation; block drains and irrigation channels;
cause siltation of reservoirs resulting in a potential loss of scarce irrigation and drinking
water resources; and cause silting of stream-beds, thereby destroying the habitat for many
invertebrates and generally degrading aquatic habitats. If eroded particles contain metals,
or other contaminants subsequently dissolve, adverse impacts on the downstream aquatic
environment can be exacerbated. Increased turbidity may turn a clear stream into muddy
water with signii cant negative visual impacts, leading to adverse perceptions in local com-
munities, whether or not there is signii cant contamination.
The deposition of eroded material on land areas can negatively impact l ora, fauna and
ecosystems, as well as agricultural land. The impact on agricultural land may also be positive,
depending on the nature of the deposits. Increased fertility in deposition areas is possible.
Subsurface erosion also occurs, although less commonly. The movement of water under-
ground results in seepage forces that may dislodge soil particles from within or under-
neath an Earth dam, road embankment or constructed slope. The progressive erosion of
particles in an area of concentrated leakage extending into the soil body is known as pip-
ing. There are several causes of piping, such as: (1) poor construction control, resulting
in inadequate compaction within the soil embankment or slope; (2) leakage through soil
tension cracks or cracked pipes that result from dam or embankment settlement; (3) leak-
age under dams where there is a natural variation in foundation soils; or (4) the presence
of dispersive soils that disaggregate when saturated. The design and construction of dams,
slopes and embankments is complex and requires the guidance of experienced geotechni-
cal engineers. In addition, adequate supervision during construction and ongoing moni-
toring and maintenance are needed in order to ensure adequate stability.
The remainder of this chapter will concentrate on ground surface water erosion and
wind erosion, including a description of the physical processes involved, and the principles
and techniques for effective erosion management. While water erosion is one of the most
devastating perpetual disruptive forces in mining both from the standpoint of its physical
impact and as a carrier of unwanted materials, wind erosion can also present a formidable
management challenge. Wind erosion is commonly associated with most Earth-moving
activities, and signii cant fugitive dust emissions occur as a direct consequence. Airborne
particles can cause nuisance and potentially negative health effects for communities down-
wind of the mine. In fact, fugitive dust is one of the most common sources of complaints
from communities located near surface mining operations.

Fugitive dust is one of the most
common sources of complaints
from communities located near
surface mining operations.

20.1 SURFACE WATER EROSION

Surface water erosion can result from the impact of raindrops, as well as from soil parti-
cles being picked up and carried by water l owing across the ground surface. Such sur-
face l ow can be classii ed according to the concentration and depth of l ow ( Figure 20.1 ).
If surface water erosion occurs uniformly as a consequence of overland l ow across slopes, it
is termed sheet erosion, and results from slopewash that moves as a thin and relatively uni-
form i lm. With time the l ow may concentrate into pathways, resulting in erosion paths
classii ed with increasing pathway depth as rill erosion, gully erosion and stream-channel
erosion.
Surface water erosion is an ongoing natural phenomenon. It can be accelerated or
intensii ed if either the natural slope angle is increased, if the vegetative ground cover is
removed or the surface is disturbed. Mining activities do all these.