Groyne
A groyne (groin in the United States) is a rigid hydraulic structure built out from the shore (in coastal engineering) or from the bank (in rivers) and interrupts the flow of water and sediment. Groynes serve a multitude of functions.
In coastal engineering
Groynes are one of the most common and effective methods of coastal defense against transportation of sediment. Groynes are structures which run perpendicular to the shoreline. Groynes extend from the upper foreshore or beach down towards the sea, usually at right angles to the sea. Groynes are usually made of wood, concrete, or piles of large rocks. Groynes are generally constructed in groups called groyne fields.
The length, elevation, and spacing between groynes should be determined according to local wave energy and beach slope. Groynes that are too long or too high tend to accelerate downdrift erosion because they trap too much sand. Groynes that are too short, too low, or too permeable are ineffective because they trap too little sand. Flanking may occur if a groyne does not extend far enough landward.
Groynes are extremely cost-effective, requiring little maintenance, and are one of the most common coastal defence structures. However, groynes are increasingly viewed as making the coastline look unnatural and ugly, and are opposed by many people. Groynes are very often used in tandem with seawalls to provide a very effective means of coastal defence.
How groynes work
Basically the idea of a groyne is to prevent longshore drift. Groynes are very useful and are the cheapest and most effective way of protecting a cliff and the beach behind it. The purpose of a groyne is to create and maintain a wide beach on its updrift side, primarily for recreation, but also for protection to the land behind. First, groynes act as a barrier to physically stop sediment transport (sand) in the direction of longshore transport through the system. This causes a build-up of the beach on the groyne's updrift side.
However, this is often accompanied by accelerated erosion of the downdrift beach, known as terminal groyne syndrome, as it occurs after the terminal groyne, which receives little or no sand via longshore transport. (It is important to realize that groynes do not add any new sand to the beach, but merely retain some of the existing sand on the updrift side of the groin.) If a groyne is correctly designed, then the amount of material it can hold will be limited, and excess sediment will be free to move on through the system. However, if a groyne is too large it may trap all the sediment reaching it and this can cause severe beach erosion problems on the down-drift side.
In rivers
Groynes (often referred to as "spur dikes" or "wing dykes"), are often constructed (nearly) perpendicular to the river banks, beginning at the riverbank with a root and ending at the regulation line with a head. They serve to maintain a desired channel for the purpose of preventing ice jamming, improved navigation and erosion control over lateral erosion, that would form Meanders. Groynes have a major impact on the river morphology; generally speaking, they cause autonomous degradation of the river. The areas between the groynes are referred to as groyne fields. [1]
Types of groynes
Various types of groynes can be distinguished according to their construction, action on stream flow and appearance. Przedwojski et al. (1995)[2], claim that the following is necessary for a full description of groynes:
Classification according to the method and materials of construction
Groynes may be permeable, allowing the water to flow through at reduced velocities, or impermeable, blocking and deflecting the current. Permeable groynes are fabricated from piles, bamboo or timbers, whereas impermeable groynes (also called solid groynes or rock armour groynes) are constructed using rock, gravel, gabions, etc.
Classification according to submergence stage
Groynes may be designed either as submerged or as non-submerged under normal conditions. Which of the two types will be used is dictated by the design conditions. Usually impermeable groynes are designed to be non-submerged, since flow over the top of solid groynes may cause severe erosion along the shanks. Submerged groynes, on the other hand, may be designed permeable, depending on the degree of flow disturbance that is needed.
Classification according to the action on the stream flow
Groynes may be classified as attracting, deflecting or repelling. Attracting groynes point downstream, they serve to attract the stream flow toward themselves and do not repel the flow toward the opposite bank. Deflecting groynes are generally short ones and used for local protection. They serve to change the direction of flow without repelling it. Repelling groynes point upstream. They serve to force the flow away from themselves.
Classification according to their appearance in planview
Groynes may be built with different planview shapes. Examples are straight groynes, T head, L head, hockey stick, inverted hockey stick groynes, straight groynes with pier head, wing, or tail groynes.
Gallery
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Groyne on the East coast of England -
Timber groynes in Bournemouth, England -
Timber groyne in Bournemouth, England -
Groynes in Sitges
See also
References
- ^ Yossef, Mohamed F.M. (2005). Morphodynamics of Rivers with Groynes. DUP Science. ISBN 90-407-2607-8.
{{cite book}}: Unknown parameter|month=ignored (help) - ^ PRZEDWOJSKI, B. (1995). River training techniques- fundamentals, design and application. A.A. Balkema.
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