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

The 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. 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

Weir construction in progress

reduces the overall volume of gabions required. Placing biodegradable blankets together with planted seedlings on these slopes also greatly reduces soil loss and future erosion,” Cheyne explains.

Wired by design

For riverine environments, the most suitable approach is to employ Class A galvanised doubletwisted hexagonal woven steel wire mesh to manufacture gabion box and mattress systems. In more corrosive conditions, a higher-specification Galfan wire coating is recommended, with the added protection of a 0.5 mm PVC sleeve to provide a design life exceeding 50 years.

Woven mesh, in various diameters and aperture specifications, has a high tensile as well as compressive strength when filled with suitable non-weathered rock. There’s also a degree of flex, which is an important feature within rivers to cope with varying flow velocities and flotsam impacts. A thicker-diameter wire is recommended in fasterflowing river conditions.

Depending on the rock fill, gabions typically have a 35% void ratio. This has the advantage of achieving a degree of permeability, which is controlled by the geotextile layers behind, below and within the gabions and mattresses themselves.

Packing gabions correctly is essential, as is selecting the right rock size for the river conditions. Without geotextile layers, and where the rocks are too small, the water flow will progressively result in gabions and mattresses losing materials and becoming structurally compromised. It’s also not uncommon for mattresses to lose their covering lid where the lacing wire selected is too thin and therefore unsuitable. Using non-galvanised wire is certain to cause this, as corrosion rapidly sets in and weakens its integrity.

The correct rock specification is equally important. Softer materials like sandstone, for example, will break down in the water.

Gabion Baskets supplies two main mattress aperture/wire diameter configurations to meet standard and more heavy-duty applications. The standard offering has a 60 mm x 80 mm aperture and a 2.2 mm diameter wire specification, with the more robust version measuring 80 mm x 100 mm and manufactured using 2.7 mm diameter wire.

Before construction begins on any site, an expert hydrological study should be undertaken. The findings will have a direct bearing on how thick the river mattress needs to be.

Importance of weirs

“Equally important is the installation of weirs at strategic points in the river to control longitudinal velocities, and here again box gabions and mattresses are perfectly matched to build these structures,” Cheyne continues, adding that

Examples of riverbank protection structures. Every design will be influenced by factors such as the terrain, riverbed contours and water velocities

Structure with deep foundation

Gabion structure built on apron

Sturcture built on loose stones

Sturcture built on cylindrical gabions

A longitudinal cross section of a typical gabion weir design

well-positioned weirs can reduce the length of a river wall, passing on cost savings.

“What some designers forget though are the gabion wingwall extensions required to prevent the river outflanking the weir during floods. A 100 mm concrete cap on the upstream weir notch is also required to protect the gabions from debris.”

Another popular weir application is the installation of catch walls forming part of a gully rehabilitation programme where severe erosion has occurred. Mattresses are also ideal for slowing down stormwater flows on the downstream ends of culverts.

As Cheyne points out, where the hydraulic jump is greater than 0.5 m, a concrete capping layer should be placed on top of the mattress at the point of impact to protect it from damage.

Correct installation crucial

Even with a first-class design, structures can still fail due to poor installation technique. In this respect, Gabion Baskets provides training and project management services, passing on decades of experience.

A completed stilling basin structure with wing wall extensions to reduce the risk of outflanking during floods A river embankment wall constructed to make way for a housing development

A classic example is a weir system installed for a wetland project completed in Brakpan, Gauteng. The contractor on-site had constructed a series of weirs with stilling basins without factoring in the natural flow of the water course. They ended up facing in the wrong angle.

Gabion Baskets was approached to provide design and installation advice. The company also sent one of its trainers to assist with the reconstruction. Since all the materials were in situ, it was simply a case of unpacking and repacking the gabions in their correct position.

“In expert hands, gabion systems can be used to construct just about any retaining system, from mining tip walls to bridge abutments supporting old-school Bailey bridges,” adds Cheyne.

“Within rivers and wetlands, they’re the most natural and perfect choice for combatting erosion and preserving these environments in a way that is unrivalled by alternatives like concrete structures,” Cheyne concludes.