Revetments

This article focuses mainly on revetments of pattern-placed stones, which are widely used to protect dikes, dams, dunes and sloping backshores from wave attack. This article is rather short because several aspects of revetments are discussed in other articles, in particular:

Stability of rubble mound breakwaters and shore revetments deals with the design and stability of rubble-mound revetments

Seawalls and revetments deals with the morphological impact of revetments, toe scour and criteria for application

Wave overtopping deals with wave run-up and overtopping of revetments

Overtopping resistant dikes deals with grass revetments

Method

Revetments can be an exposed structure as well as a buried structure.

Exposed revetments

In coastal protection practice, revetments are usually made as sloping structures and are often constructed as permeable rubble mound structures using natural stones or concrete blocks. Rubble mound armoring enhances wave energy absorption and minimizes reflection and wave run-up.

However, revetments can also consist of different kinds of pattern-placed natural blocks (e.g. basalt columns), concrete slabs, some of them permeable and interlocking. In this way their functionality is increased in terms of absorption and strength.

Net mesh stone-filled mattresses, such as gabions, are also used; however, they are only recommended for use at fairly protected locations.

Revetments can also consist of sand-filled geotextile fabric bags, mattresses and tubes, see Sand-filled geosystems in coastal engineering. Such structures must be protected against UV-light to avoid weathering of the fabric. Sand-bagging is often used as emergency protection. Geotextile fabric revetments are fragile against mechanical impact and vandalism, and their appearance is not natural.

Fig. 1. Examples of revetments.

Buried revetments

Fig. 2. An example of an emergency revetment constructed in concrete blocks, the revetment will later be buried into an artificial dune. (Danish Coastal Authority^[1])

A buried revetment can be constructed as part of a soft protection, e.g. as a hard emergency protection built into a strengthened dune which acts as shore protection and/or sea defence.

Functional characteristics

Revetments are often constructed on coasts where erosion or storm damage threatens the landward slope, dune, dike or backshore. A revetment fixes the landward boundary of the active coastal profile, but it does not remove the cause of erosion in the coastal profile. If sediment losses continue, the beach in front of the revetment may gradually become lower and narrower. This effect is especially important where the coast would otherwise retreat landward.

Because revetments are usually sloping and permeable, they generally reflect less wave energy than smooth vertical seawalls and may cause less local scour and turbulence. However, like seawalls, they reduce the release of sediment from the protected section and can therefore have an adverse effect on the sediment budget of adjacent shorelines.

Stability of pattern-placed revetments

Fig. 3. Schematic cross-section of a pattern-placed revetment on an earthen embankment. The toe is buried below the beach surface to reduce the risk of undermining.

Pattern-placed revetments usually consist of a top layer of placed concrete blocks, basalt columns or natural stones, supported by a granular or geotextile filter on the underlying soil, dike core or embankment body (Fig. 3). The top layer resists direct wave attack, while the filter prevents washout of the subsoil and allows drainage. Special attention is needed at the toe, crest and lateral transitions, where local erosion or loss of support can initiate damage.

The placed elements can be broadly classified as blocks or columns. Blocks are usually more regular and tightly arranged, whereas columns are more irregular. The open spaces between columns are typically filled with granular joint filling to enhance clamping. Columns generally have a larger height-to-width ratio than blocks, which is beneficial for their stability (Fig. 4).

Fig. 4. Column revetment of Marazion sea wall, Cornwall, United Kingdom. Credit: Charles Gould. Licensed under Creative Commons.

The hydraulic stability of a pattern-placed revetment depends on the magnitude of the hydraulic loading, in particular wave height and wavelength, on the slope of the revetment, and on the thickness, density and interlocking of the top layer. Important damage mechanisms include deformation of the revetment surface and washout of joint filling. Deformation can occur when filter material migrates downslope or is locally redistributed, causing variations in the thickness and support provided by the filter layer. This process can be promoted when wave impact slightly lifts the columns from the filter layer. Washout of joint filling can be caused by repeated wave loading that removes granular material from between the columns. The resulting reduction in joint filling weakens the clamping and interlocking of the columns, so that they can be lifted more easily from the filter layer. Loss of joint filling can also facilitate filter migration or washout of the underlying soil, depending on the filter structure. Complete removal of a single column has only a limited immediate effect on hydraulic stability if no other structural damage is present.^[2]

Other relevant damage mechanisms and vulnerable locations include:

damage near the wave-impact zone;
toe instability or loss of toe support;
failures at transitions with adjacent types of protection cover;
local settlement or deformation caused by loss of support beneath the top layer.

The stability of rubble mound revetments is discussed in the article Stability of rubble mound breakwaters and shore revetments.

Applicability

A revetment mainly protects a slope, dune foot, dike, dam or backshore against erosion by wave action, storm surge and currents. By itself it usually does not provide the crest height needed for flood protection, but when applied to a dike or dam it can be an essential component of the flood-defense system. A revetment is therefore a passive coastal protection measure, used at locations exposed to erosion or as a supplement to seawalls, dikes or dams at locations exposed to both erosion and flooding. Revetments are used on all types of coasts.

Pattern-placed revetments are suitable where a fixed sloping structure must be protected against wave attack, especially on dikes, dams, embankments and engineered backshores. They are less suitable where large morphological adjustment is expected, where the toe is frequently undermined, where the foundation is weak, or where beach recreation and natural landward migration are management priorities.

Rubble revetments and similar structures have a permeable and fairly steep slope; normally a 1:2 slope is used. This slope is suitable neither for recreational use nor for the landing or hauling of small fishing boats. Consequently, this kind of structure should not be used at locations, where the beach is used for recreation or fishing activities. For such locations, other types of protection measures must be considered, but if a revetment is required, a more gently sloping structure with a smooth surface is recommended.

Where ecological value is an objective, the surface roughness, joint structure, material choice and presence of pools or crevices can be adapted to provide more habitat, although this does not remove the broader morphological effects of fixing the shoreline, see Ecological enhancement of coastal protection structures.

References

↑ Danish Coastal Authority, 1998. "Menneske, Hav, Kyst og Sand". (in Danish), (Man, Sea Coast and Sand in English). Kystinspektoratet 1973-1998.
↑ van der Vegt, N., Klerk, W.J., Peters, D.J., van Gent, M.R.A. and Hofland, B. 2024. Estimating the effect of assumed initial damage to the hydraulic stability of pattern-placed revetments on dikes using finite element modeling. Coastal Engineering 189, 104484

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