Mount Roland

DEGRADED RIVERBANKS REPAIRED

Rip-rap

Rip-rap is 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.

The picture below shows the same stretch of river after stabilisation of the riverbank with rock.

Groynes

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.

Positioning 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.

This picture shows a series of groynes placed on a strecth of the Minnow river.

Care needs 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.

Woody debris

The 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. The logs 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.