Coastal protection using gabions should be limited to a secondary defence in exposed areas or a primary defence in sheltered or estuary locations.
The protection system can be as a retaining structure or as a revetment. In all cases, only PVC-zinc coated or stainless steel gabions should be considered, as zinc or aluminium-zinc coatings are susceptible to corrosion (however if the structures are temporary then these coatings can be considered).
Due to the imposed forces created by breaking waves revetment slopes should not exceed 1 in 3 and quality of construction is very important. Units must be packed tightly otherwise stone movement within the basket will occur resulting in abrasion of the fabric of the gabion.
Adequate consideration must be made to toe scour, whether it be a retaining structure or a sloping revetment, otherwise failure due to undermining will occur.
Advice should always be sought for each situation.
Gabion river walling is designed as a mass gravity structure, however if the river water levels are variable then the wall should be designed on the basis that the soil is saturated and the possibility of residual hydrostatic pressures as a result of rapid draw down must be considered.
The wall foundation must be protected against under scour by either of the means detailed below. For scour aprons the width of mattress protection in front of the wall should be 1.5 to 2 times the anticipated depth of scour and for embedment solutions the depth should be greater than the anticipated scour depth.
If adequate scour protection is not given failure by rotatation or settlement of the wall will occur.
Weir structures are used where there is a sudden change in river bed levels or on steep gradients in water courses to reduce water velocity. The weir is effectively a dam which allows water to flow over the crest controlled by introducing a notch that can be rectangular or ‘V’ shaped.
The water can flow over the notch to a lower bed level, the weir is designed as a retaining structure with an apron protection to prevent erosion of the bed. In instances where there are high flows over the weir, it may require a stilling pool to be included in the design generally with an additional counter weir. These structures require the hydraulic conditions within the water course to be considered to determine the protection of the river bed on the down stream side. The flow conditions are turbulent and the mattress protection depth will be greater than for normal gabion mattress protection works.
Where a large change in bed level is required over a short distance a series of concurrent drops can be created known as a cascade structure.
In some instances a sloping weir may be necessary which results in increased water velocity. The slope is protected with a mattress or gabion system. Consideration is required for the erosive nature of high water velocities and the down stream river bank and bed protection.
These types of structures generally combine a retaining wall with wing walls and a scour protection apron often form part of a river wall protection scheme where drainage pipes discharge into water courses.
The gabions may also be used only as scour apron protection to concrete outfall or inlet structures to provide erosion protection.
Due to the nature of discharge of the outfalls, consideration of the turbulent flow conditions occurring is required when determining the depth of gabion protection.
The design considerations are similar to those encountered in culverts and around bridge piers.
