a method of reinforcing river banks to prevent erosion, particularly
where this is caused by water hitting the bank with some force,
or swirling round beside the bank. Rock is generally used, though
sometimes logs will provide a good solution. Even when the gaps
between the rocks are filled with gravel, and seeds or seedlings
have been planted there, it can take up to two years before a bank
stabilised with rip-rap looks natural and attractive.
This is an
example of a degraded riverbank, which had been severly eroded to
the point that the fence that was once on solid ground was left
hanging in the air.
below shows the same stretch of river after stabilisation of the
riverbank with rock.
Where a current
needs to be diverted away from a river bank to prevent erosion,
one or more groynes (a protective structure of stone or concrete,
also spelled groin) may provide a viable option.
a groyne in a river forces the water to travel further, which slows
the water, reducing the eroding effect. As
with all river structures, groynes need to be substantial enough
to withstand the pressure of the highest floods that occur.
shows a series of groynes placed on a strecth of the Minnow river.
to be taken to ensure that constructing a groyne to resolve a current
flow problem at one point in a river does not simply cause another
one further downstream. Some trial and error over a number of years
may be needed before the desired result is achieved.
placement of large logs ('woody debris') in the rivers has been
one of the major techniques we have used to encourage the safe return
of fish and other native aquatic species and to prevent erosion.
trap smaller fragments which promote the growth of plants, again
improving stabilisation of the river banks.
We found that
the best way to ensure that the logs stayed where we put them was
to use an excavator to ram the logs up to two metres into the banks
then to place large stones between the banks and the projecting
logs, filling the gaps with gravel. Vegetation covers the area surprisingly
quickly, leaving an attractive and stable bank.