ENVIRONMENTAL ENGINEERING
Gabions and riverbank protection The versatility, permeability and resilience of gabion systems make their application well suited for a range of interventions within riverine and wetland environments. However, every site has its own unique requirements that need to be factored into the design, says Louis Cheyne, managing director of Gabion Baskets. By Alastair Currie
T
he recent floods that have swept across South Africa have highlighted the need for riverbank protection, particularly in urban areas where the speed of stormwater run-off on hard surfaces exacerbates erosion. “Mass gravity river walls are among the most common applications for gabion systems when it comes to erosion protection and maximising the structural stability of embankments, which are frequently bordered by roads and buildings. There are many different configuration options to choose from to suit the hydrological conditions and terrain,” Cheyne explains. “A key factor in the design is determining the maximum expected scour depth to protect the toe of the submerged structure from soil loss and potential undermining. That’s often not fully understood by designers and installers, resulting in premature failure and wall collapse over time, especially following major storm events.”
A gabion stilling basin weir installed within a Gauteng wetland
Allowable water velocities
Gabions versus mattresses The two main systems are box gabions and gabion mattresses. The latter are flat structures extensively used in river courses over flat or sloped areas in need of protection against soil loss or scour.
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IMIESA July 2022
Weir construction in progress
Depending on the circumstances, gabion walls can be founded directly on the riverbed, resting on a geotextile sheet. “We’ve also seen walls founded on a loose stone platform in the river or resting on sausage gabions designed to follow the contours of the riverbed. Where a mattress foundation is employed, this should extend out from the front face to the point where maximum scour is anticipated,” Cheyne explains. Depending on the site, these walls can either have a stepped-back design or a vertical front face, and in both cases the base width is typically 55% to 60% of the height of the structure. Gabion Baskets manufactures a complete range of gabion products and supports its solutions with integrated systems that include geotextiles and biodegradable blankets. The latter serve a key role on backfilled slopes forming part of the overall retaining system. “In cases of severe erosion, large sections of the original riverbank might have been washed away. The only solution then is to import material to reshape the embankment that the gabion wall is designed to support. This is a cost-effective approach, since the backfill
