In certain circumstances, steep riverbanks can become quickly eroded by the action of the river against the face of the bank. This can result in excess silt entering the watercourse causing damage to the river habitat. The eroding bank can also be exacerbated by the pressure from stock grazing and lack of trees and other vegetation to bind the riverbank.
In the right circumstance, the re-profiling of an eroding riverbank can slow down erosion and reduce the amount of excess silt entering the watercourse.
Changing the angle of the riverbank to approximately 35° allows the river to rise easily up and over the bank and back down again rather than directly into the face of the bank eroding it more quickly.
Where there is also grazing, stock exclusion fencing should also be considered to give the bank a greater chance to stabilise and to re-vegetate.
The combination of re-profiling and fencing is known as ‘assisted natural recovery’.
Pearls in Peril Re-profiling
A carefully selected steeply eroding riverbank was re-profiled as part of the Pearls in Peril Project. A
Flood Defence Consent and careful mitigation was required to undertake the work.
A digger was used to re-profile 140m of riverbank. The top turves were removed section by section and once the soil was removed by the digger, the turves were immediately replaced and secured with willow stakes. Replacing the turves straightway ensured that they were kept clean and intact and that the bare soil was not exposed for long thereby reducing the risk of it entering the watercourse. Using willow stakes to secure the turves allows the opportunity for the willow to grow and further stabilise the bank.
As part of the re-profile coarse glacio-fluvial cobble was released and used to stone pitch the toe of vulnerable parts of the riverbank. The natural model for the pitching was based on other parts of the bank where cobbles have naturally dropped from the eroding bank to ‘self-heal’ the toe slope. Stone pitching is more sophisticated than ‘rip-rap’ and whilst a machine was used to supply the raw materials to the base of the bank, skilled hand placement was required to place stones along the toe of the bank to mimic the natural accumulation in the other sections of the bank.
Coir mesh was placed in a couple of vulnerable areas to allow time for the grass to grow throw it and stabilise the bank. In addition, post the 2015/16 floods, a couple of areas of the bank were exposed and this has now been willow spiled to protect it from further erosion.
The landowner agreed to a very generous strip of approximately 15m which has also been planted with native trees.
See before and after photos below and also a short timelapse of the work.
|Timelapse of re-profiling||m4v||44.79 MB|