Riverbank erosion along the Castlebar River channel forced the closure of a section of the Great Western Greenway in Co. Mayo. Rivus designed a cost effective and creative nature-based solution that restored the riverbank, secured the Greenway, contributed to local biodiversity and sequestered carbon.
The Problem
During the winter of 2015/16, a 150 metre section of the Great Western Greenway collapsed outside Castlebar, Co. Mayo, and had to be closed to the public. The Greenway runs adjacent to the Castlebar River. Erosion of the riverbank had undermined the road, causing it to give way. Due to the serious health and safety concerns that the collapse presented, use of the Greenway was effectively stopped, affecting people who enjoyed the amenity and local businesses that relied on it. What's more, with each subsequent flood, the problem was getting worse as the river continued to erode the embankment supporting the Greenway.
Willow panels and geocoir are used to stabilise the riverbank in a tiered revetment
The Rivus Solution
Having been told of the problem by the Mayo County Council engineer, we sent our technical design team to assess the damage. Rivus staff had previously worked with Mayo County Council on other sections of the Greenway during the construction stage and had stabilised eroding riverbanks using natural and low-cost ecological engineering solutions.
To solve this problem, we identified the specific reasons why the river had eroded the banks, undertook a topographical survey and presented a design and construction methodology report to Mayo County Council. The design incorporated a combination of ‘hard’ and ‘soft’ engineering solutions to stabilise the embankment. It had three main elements:
1) Place an anti-erosion toe in the bed of the river made from locally-sourced stone (rip-rap),
2) Develop a 'living' retaining wall for the Greenway made of panels of willow rods (spilings), and
3) Reinstate the road and fencing back to its original specifications.
We were commissioned by Mayo County Council to undertake the works in April 2016.
The tiered willow revetments help to stabilise the bank
The Result
The high tensile strength of the willow wall and natural 'geocoir' lining, along with the tiered riverbank design, ensured that the Greenway was able to bear high loads upon completion of the works, and it reopened immediately after the workmen departed.
Its strength has only increased in time as the root systems of the willow grow into the riverbank. The branches and leaves of the willow that lie along the river channel meanwhile shield the bank itself from high river flows by slowing the water down and absorbing its energy, rather than deflecting it into the bed of the river and causing further erosion.
Since this nature-based solution was installed 2 years ago, the Greenway has remained open to the public with no further closures and no further damage. It's also important to note that this type of structure has the capacity to provide both shelter and food to over 100 species of native insects, birds, reptiles and mammals and can sequester around 500kg (0.50t) CO2 every year.
This shows the riverbank eight weeks after the works were completed - a haven for biodiversity!
FOR MORE INFORMATION ABOUT THIS PROJECT, CONTACT ALAN SULLIVAN.