Revetment
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Revetments are structures placed on banks or cliffs in such a way as to absorb the energy of incoming water or explosives. They are usually built to preserve the existing uses of the shoreline and to protect the slope, as defense against bombs or artillery, or to secure an area from stored explosives.
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[edit] Fortifications
According to the U.S. National Park Service, and referring mostly to their employment in the American Civil War, a revetment is defined as a "retaining wall constructed to support the interior slope of a parapet. Made of logs, wood planks, fence rails, fascines, gabions, hurdles, sods, or stones, the revetment provided additional protection from enemy fire, and, most importantly, kept the interior slope nearly vertical. Stone revetments commonly survive. A few log revetments have been preserved due to high resin pine or cypress and porous sandy soils. After an entrenchment was abandoned, many log or rail revetments were scavenged for other uses, causing the interior slope to slump more quickly. An interior slope will appear more vertical if the parapet eroded with the revetment still in place."[1]
[edit] Sea Revetments
Like seawalls, revetments armor and protect the land behind them. They may be either watertight, covering the slope completely, or porous, to allow water to filter through after the wave energy has been dissipated.
Most revetments do not significantly interfere with transport of littoral drift. They do not redirect wave energy to vulnerable unprotected areas, although beaches in front of steep revetments are prone to erosion. Materials eroded from the slope before construction of a revetment may have nourished a neighboring area, however. Accelerated erosion there after the revetment is built can be controlled with a beach-building or beach-protecting structure such as a groyne or a breakwater.
As already mentioned, Revetments are structures which protect the land from the erosive powers of wave action. They can be made from timber, rock armourstone or precast concrete. They prevent wave action reaching the base of the cliff.
Rock or rock armour has become increasingly important in coastal defence works over the last 20 years or so. The rock used will typically be a very hard rock that is exceedingly resistant to erosion. When rock is placed on the beach it is arranged so that large void spaces are left. When waves hit the rock they are only partially reflected, and can flow round the rocks, expending their energy less quickly which reduces scour. Another advantage of using rock is that if the rocks are moved during heavy storms it is a simple procedure to replace them in the correct position. Rock armour is often used to protect the base of seawalls and prevents direct attack and damage of the wall. Additionally the rock provides support for the wall, improving its stability. Rock armour can also be used on its own either as a longitudinal defence (like a sea wall) known as Rip Rap or to construct groynes. Importantly, because the rock needs to be resistant it is often very different to the material found naturally at a location. Consequently rock armour may look out of place and is not suitable for use in all locations.
Timber Revetments were first constructed as cheaper alternative to a solid seawall. Timber Revetments are constructed from wooden piles driven into the beach and connected by a sloping lattice of timber planks. Either a concrete apron or steel sheet piles are used to provide foundations for the structure and these are intended to remain below beach level. Timber Revetments are designed so that they will break the force of the wave without reflecting the energy which might cause scouring of the beach. A Timber Revetment reduces the wave energy available to attack the cliffs but it does not prevent erosion from taking place. In addition, sediment, which can be sand, shingle or pebbles, is carried through or over the revetment structure by an incoming wave. However, the sediment cannot easily return seaward once the force of the wave has been dissipated. This action results in a build up of material behind the revetment and the increased depth of material protects the foot of the cliff from being attacked by the sea.
Timber Revetments are built at the top of the beach to absorb wave energy during high tide conditions because this is the only time waves can reach the base of the cliffs. Unfortunately, if beach levels in front of a Timber Revetment drop then the steel piles or apron can become exposed. They then tend to act as a simple seawall (although the are not designed to do this) and reflect a large proportion of the wave energy, which can cause further erosion of the beach and ultimately the potential undermining and collapse of the Revetment.
[edit] Freshwater Revetments
Many revetments are used to line the banks of freshwater rivers, lakes, and man-made reservoirs, especially to prevent damage during periods of floods or heavy seasonal rains.
[edit] External links
[edit] Fortifications
[edit] River and Levee Management
[edit] References
- ^ Military Earthworks Terms. Retrieved on February 27, 2007.
