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Background: |
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Salix
were approached by the consulting geotechnical engineers to provide
design proposals to stabilise over 800 metres of steep riverbank
composed of highly erodible colliery shale. Initial proposals looked at
the extensive use of large blockstone to stabilise the bank. However,
the client wanted to explore the possibility of using greener and more
cost effective bioengineering methods that would provide underlying
engineering stability whilst providing an ecological resource and
improving the visual landscape.
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| Site Details: Bank gradient: 1 in 1.5 Slope Length: 12 metres Water Depth (1:100): 4m Velocity (1:100): 4.2m/s Discharge (1:100): 80 cumecs |
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| Design: | |||||||||
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Salix were invited to submit a “design and construct” solution for protecting the riverbanks from erosion. The two key criteria were that any solution could be shown to be stable under the expected velocities and that the ecological landscape value of the site were enhanced. The toe of the bank was protected using a combination of rock rolls and live willow bundles (faggots). |
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Previous experience and large scale trials in Austria had shown that these revetments could withstand the design velocities and provide a stable tow which would not be undercut. The upper bank was protected using two high performance erosion control mats; North American Green P550 and C350 (www.nagreen.com). Independent research and thousands of applications across the world have shown these products to be able to withstand extremely high flow velocities (7.6m/s – P550 and 6m/s C350) when vegetated. Even prior to vegetation establishment the “composite” fibre layer provides exceptional erosion control performance (3.8m/s – P550 and 3.2m/s – C350). The long slope (12m) required specialist equipment to work at the toe of the bank. Salix’ long reach13 tonne excavator with 13m reach was used to grade and install the toe revetments without having to enter the river itself. |
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| Construction and Results: | |||||||||
Construction commenced in January 2005 and was completed in May of that year. The slopes were profiled and prepared using Salix’ long reach excavator. A reclamation mix was applied at a rate of 25gm/m2 along with an NPK fertilizer. The erosion control mats were laid directly on top of the seeded slopes and pinned into place offering erosion protection from the day of installation. The rock rolls and willow faggots were installed over the erosion control matting at the toe of the slope. Native willow cutting and locally occurring riverside grasses and rushes were planted in the marginal areas. |
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The rate of vegetation germination under the North American Green erosion control mats took everybody by surprise with grass having germinated in less than one week. The willow is all starting to ‘strike’ and the marginal grasses have rooted. |
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| River Ebbw 1 year after completion: Note the blockstone (bottom left of photo) that would have been used without the availability of these Salix techniques. | |||||||||
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