Mine Rehabilitation Handbook - Mining and Blasting

5.3.2 Water Erosion
(a)
Not Recommended
(b) (c) (d)
PLANTING A SHORT
WINDBREAK IN FRONT
OF GAP
FORMING AN
ANGLED WINDBREAK
Recommended
Figure 5.4
Erosion by water is caused mostly by raindrop splash and surface runoff
from intense rainfall events. Even in arid and semi arid areas, high
intensity low frequency rainfall events can cause rapid runoff and
extensive soil erosion. The important factors influencing runoff
characteristics are rainfall, area of disturbance, catchment area, slope
and profiles of channels (angle, length and cross section etc), soil
characteristics and land-use. Properly planned, the rehabilitation of a
mine site can, to some extent, manage all of these variables excepting
rainfall.
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ANGLING THE SIDE OF
THE WINDBREAK
• Minimising Area of Disturbance.
Clearing of vegetation should be limited to that absolutely necessary
for the safe operation of the mine (including fire management).
Minimising the area cleared will reduce costs both for clearing and
site rehabilitation. Provisions that will assist in minimising the area
cleared include:
- preparing detailed site and mine development plans (refer 3.3);
- restricting progressive clearing ahead of the pit to that required for
no more than 6 months production;
- clearly identifying, on the ground, areas designated for clearing;
- training mobile equipment operators on the need to identify the
exact limit of area to be cleared prior to commencement;
- close supervision of mobile equipment during clearing
operations; and
- including penalty clauses for employees and contractors for
damage to areas not designated for clearing.
• Restricting Entry of Runoff to the Site.
Construction of diversion channels or holding structures such as
banks, drains or dams will effectively limit the entry of water on to the
site. This will reduce the potential for soil erosion on the site, but
may, by concentrating runoff, create off-site problems at discharge
points. When planning diversion structures, consider the following:
- Are the structures permanent or temporary? All structures will
need to be designed to accommodate anticipated peak flows.
Consider the consequences of structure failure, from operational,
safety and environmental viewpoints. Always seek help when
designing permanent or major temporary structures.
- Information needed to properly design major erosion control
structures will include rainfall frequency and duration curves,
catchment size and runoff coefficient. Where field data is lacking,
statistical approximates can be derived - seek advice.
- A critical storm return period will need to be nominated as part of
design criteria. Design parameters will depend on the purpose of
the structure and its anticipated life.
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