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Gully and Streambank Erosion
Most gullies and streams running through farmland show some
signs of erosion. The most devastating erosion occurs during
periods of peak flow following heavy rains, especially if
soils in the upper catchment are saturated. In larger streams,
bank erosion is largely a catchment issue and may require
the involvement of other farmers and land managers to find
collective ways of tackling excessive runoff.
Erosion of small tributaries and drainage lines contributes
to the bulk of sediment in larger streams and can often be
tackled effectively by individual farmers. Any action in the
small waterways high in the catchment will help reduce flooding
and erosion downstream, and help improve water quality.
Gully and streambank erosion are often triggered by particular
land and water management practices. Activities such as the
extensive clearing of land, uncontrolled stock grazing along
watercourses, and the straightening of channels disturb the
equilibrium that exists between the flow regime and channel
structure, leading to instability.
There are three types of soil erosion processes that may contribute
to gully and streambank erosion:
1. Subaerial erosion — this occurs when the bank is exposed
to the weather and is subject to rainsplash, stock trampling
etc.
2. Scour — the direct removal of bank material by flowing
water.
3. Slumping — a mass movement that occurs when bank material
slips or falls into the stream often as a result of waterlogging
of the bank.
Identifying which of these are contributing to an erosion
problem and understanding the process itself, can guide revegetation
to control the problem. Before tackling a problem at a particular
point, it is important to look at the condition of the watercourse
up and down stream and if possible, observe the stream during
a storm.
In many cases the revegetation of unstable banks can provide
relatively cheap long-term stability. Trees on the bank can
reduce waterlogging and the risk of slumping, and the roots
may also bind unstable soils. Grass and shrubs on the top
of the bank and up the slope can protect the bank from direct
scouring by flowing water. Although revegetation is a positive
option for the environment, in some cases it is not enough
to address streambank stability problems. In these instances
it may be appropriate to use complementary engineering solutions.
Buffer
Strips for Nutrient Trapping
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