ENVIRONMENTAL ENGINEERING
Flexible and permeable river protection The increasing incidence and severity of extreme flood events, combined with rising stormwater velocities, constantly threatens to undermine riverbank protection systems. Environmentally engineered countermeasures are an effective response, says Louis Cheyne, managing director, Gabion Baskets. By Alastair Currie
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iverbanks and riverbeds are typically composed of soft underlying materials, like clayey soils and black turf, that are especially prone to erosion. In addition to localised damage, this can also contribute to downstream impacts like excessive sedimentation build-up in dams. “In developing the best river erosion system design, the goal is to work with the hydraulic forces rather than against them,” Cheyne explains. “In this respect, gabion systems provide the best approach, since they are essentially wire-framed structures filled with rocks that together enable some degree of permeability. The extent of this will be dependent on the site. For example, in mass gravity gabion retaining wall structures, erosion protection blankets are often included to minimise fines loss.” The double-twisted hexagonal woven steel mesh used to construct gabion baskets and mattresses is designed to flex, which is important for longer-term integrity. Selecting a Class A galvanised wire specification is equally important to counter abrasion and corrosion.
Galfan and PVC In especially demanding conditions, Galfan coated wire is recommended where high
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IMIESA January 2021
Construction works in progress to repair and upgrade an existing gabion mattress system designed to protect a pedestrian bridge crossing a strongly flowing river in the DRC
levels of pollution are present. This is an increasingly common factor within urban river systems exposed to industrial waste and untreated effluent. Galfan is composed of a higher-grade steel coated with a zinc-aluminum layer. The use of Galfan is equally recommended for fast-flowing, turbulent river courses. Here, higher levels of oxygenation occur as the rapidly flowing water swirls around gabion structures. In the case of both Class A and Galfan wire, a PVC coating can also be applied to achieve additional protection. Typically, the woven mesh used for baskets and mattresses is 2.7 mm and 2.2 mm respectively.
However, some clients specify a 2.7 mm wire thickness for both products for added overall system durability. “Thanks to the simplicity of gabion construction techniques, you can place or ‘float’ the baskets and mattresses directly on to the riverbed,” Cheyne continues. “This would obviously not be the case for a precast or in situ concrete structure, which will always need a reinforced foundation, ideally placed on the bedrock. As an alternative approach, we have seen concrete retaining block wall structures built on top of gabion baskets, founded on an apron mattress in the river, which then proves optimal.”
