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| + | #REDIRECT[[Detached breakwaters]] |
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| − | Breakwaters aim to protect a coast or activities along the [[coastline]] (e.g. ports, ship wharf) from wave action. Different types of breakwaters exist, from which [[detached breakwater]]s are the most common type of breakwater. This article provides a general introduction of breakwaters and describes submerged or low-crested breakwaters, floating breakwaters and special type breakwaters. The characteristics and application of [[detached breakwater|detached breakwaters]] are described in other articles: [[Detached breakwaters]], [[Applicability of detached breakwaters]] and [[Detached shore parallel breakwaters]].
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| − | ==Introduction==
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| − | Breakwater schemes have many variables, which determine the impact on the shoreline. The variable parameters are outlined in the following:
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| − | *Emerged, submerged or floating
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| − | *Distance from shoreline and location relative to the [[surf zone]]
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| − | *Length and orientation
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| − | *Single or segmented
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| − | *Special shapes
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| − | There are further descriptions in the article [[Detached breakwaters]] for combinations of all these parameters.
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| − | A shoreline management breakwater serves two purposes:
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| − | *To provide shelter from the [[waves]]
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| − | *Through this shelter, to manipulate the littoral transport conditions and thereby to trap some sand
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| − | Some breakwaters, offshore breakwaters, are constructed in deep water as a special type of port, providing shelter for a piled berth connected to the coast by a piled jetty. This kind of port is normally used when the slope of the coastal profile is very gentle, which makes the construction of a traditional port very costly. The philosophy is that the distance to the shoreline behind this kind of port will minimise the impact on the shoreline. However, in practice, it is very difficult to construct such a zero impact port.
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| − | The remainder of this article discusses different types of breakwaters: submerged, floating and other modified breakwaters.
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| − | ==Submerged or low-crested breakwaters==
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| − | Submerged or low-crested breakwaters function by provoking wave-breaking and by allowing some wave transmission so that a milder wave climate is obtained in lee of the submerged structure, although it is not as mild as if the structure was emerged. The [[sediment]] transport capacity behind the breakwater will also decrease, which means that sand will accumulate in a manner similar to an emerged structure with a slightly smaller length. The wave-breaking and overtopping also mean mass transport of water over the structure. There is a close relationship between, on one hand, wave transmission and mass transport over the structure and reduction in the sand transport and, on the other hand, [[crest]] [[freeboard]], [[crest]] width and wave height and steepness.
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| − | ===Reasons for selecting a submerged/low breakwater===
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| − | *The visual impact of a submerged/low structure is less damaging
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| − | *A submerged or low-crested structure is less expensive
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| − | *The impact on the transport climate and on the [[accretion|sand accumulation]] is smoother
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| − | *The overtopping water generates good water circulation behind the breakwater
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| − | *Submerged breakwaters are very similar to natural reefs. They attract fish and are therefore popular among fishermen
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| − | ===Disadvantages of submerged/low crested breakwater===
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| − | *A submerged structure can be dangerous for small craft navigation
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| − | *The overtopping water initiates local currents, which can be dangerous for swimmers
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| − | *A submerged or low-crested structure provides only partial attenuation of the wave action as well as partial shore protection and coast protection
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| − | *The efficiency of a submerged structure with respect to the attenuation of both waves and littoral transport and with respect to shore protection very much depends on the [[crest]] [[freeboard]] of the design. If there is considerable tide and storm surge at the location in the design situation, even a submerged or low crested structure will end up being rather high relative to the normal water-level. This means that one of the advantages of a low-crested structure, namely that it is not visible most of the time, cannot be obtained
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| − | *The design is very difficult and challenging because the proper function of a submerged or low-crested structure depends on both water-level and wave conditions as well as on the specific structure.
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| − | ==Floating breakwaters==
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| − | [[Image:transmission rationame.jpg|300px|thumb|Fig. 1. Rough relation the between the transmission coefficient Ht/Hi and the ratio L/w between the [[wavelength]] L and the width of the floating structure w.]]
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| − | Floating breakwaters work by dissipating and reflecting part of the wave energy. No surplus water is brought into the sheltered area in this situation. Floating breakwaters are normally used as [[piers and trestles|piers]] in marinas, but they are also used as protective structures for marinas in semi-protected areas. They are especially suited for areas where the tidal range is high, as they follow the water-level. Floating breakwaters are seldom used as shoreline management structures because they are not suitable for installation in the open sea.
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| − | The wave transmission coefficient H<sub>t</sub>/H<sub>i</sub>, i.e. the ratio between the height of the transmitted wave and the height of the incoming wave, depends very much on the ratio L/w between the [[wavelength]] L and the width of the floating structure w. As a rule-of-thumb the transmission varies between H<sub>t</sub>/H<sub>i</sub> = 0.3 for L/w = 3 and H<sub>t</sub>/H<sub>i</sub> = 0.9 – 1.0 for L/w = 8, see Fig. 1. below.
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| − | Consider the example of a pontoon width of w = 3 m, and a requirement of a wave transmission of min. H<sub>t</sub>/H<sub>i</sub> = 0.5. In this case the [[wavelength]] should be smaller than L < 4.2w = 12.6 m, which corresponds to an approximate wave period of T = 2.9 seconds. Floating breakwaters can only be used in waters of very limited [[fetch]].
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| − | Floating breakwaters thus cannot be used as [[shoreline]] management structures at moderately exposed and exposed locations. For more information on different types of floating breakwaters, see the article [[floating breakwaters]].
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| − | ==Modified breakwaters and headlands==
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| − | ===Background===
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| − | [[Image:artifical headland.jpg|300px|thumb|Fig. 2. Optimisation of coastal breakwater to artificial headland, applicable for moderately exposed to exposed coasts for small angles of incidence.]]
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| − | The supporting structures for coastal restoration schemes have, in the preceding discussions, mainly been traditional groynes and breakwaters. Various unwanted effects associated with these structures have also been highlighted. This subsection discusses possible modifications to the layout of traditional structures to minimise these unwanted effects, see also Fig. 2.
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| − | The philosophy behind the optimisation of the traditional, coast-parallel breakwater into an artificial headland (Fig. 2.) is as follows:
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| − | #To improve the bypass and to minimise offshore loss as well as lee side [[erosion]] | |
| − | #To eliminate dangerous rip [[currents]] as well as lee areas, which may otherwise trap debris
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| − | #To enhance the aesthetic appearance and to gain some useful land.
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| − | ===Functional characteristics===
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| − | The initial current patterns for these structures are illustrated in Fig. 3. These current patterns only show the initial situation without [[Beach nourishment|nourishment]] so they are not fully applicable in evaluating the conditions after initial fill or trapping of sand has taken place. However, they are useful as indicators of how the different schemes will function. The following characteristic conditions are important.
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| − | {|
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| − | |[[Image:current paterns A.jpg|300px|thumb|Figure 3a and 3b. Current patterns for: a) A coast-parallel breakwater, b) A curved optimised breakwater]]
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| − | |[[Image:current paterns B.jpg|300px|thumb|Figure 3c and 3d: Current patterns for: c) A Shore-connected smooth breakwater and d) An artificial headland.]]
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| − | |}
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| − | ===Important characteristics===
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| − | *Comparing a and b: The curved breakwater guides a larger amount of [[longshore current]] (and the [[littoral transport]]) around the structure on the offshore side, but turning the flow slightly towards the [[shore]], it provides better bypass and less lee side [[erosion]]. The eddy in the lee area downstream of the structure is smaller for the curved structure, but it is still there. The scour hole at the lee end is evaluated to be smaller.
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| − | *Comparing b and c: The breakwater connected to the shore provides a very smooth passage for the longshore current (and the littoral transport) around the structure, thereby providing optimal bypass and less lee side erosion. The eddy at the downstream side no longer exists, which improves the safety for swimmers and reduces the trapping of seaweed and debris. The internal current pattern of the area sheltered by the structure is not relevant because this area will be filled with sand. This sandy area will become very attractive for recreational activities.
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| − | *Comparing c and d: The headland acts more or less like the breakwater connected to the shore; the only difference is a smoother transition of the longshore current (and the littoral transport) at the upstream end of the structure. This is due to the smooth transition between the coast and the structure in the case of the headland. The transition will be even more smooth if sand fill is introduced upstream of the headland. There is no eddy at the downstream side, which improves the safety for swimmers and reduces the trapping of seaweed and debris. If the reclaimed area is sufficiently elevated, it can be used for permanent recreational installations.
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| − | The headland can also be partly submerged, whereby it will act as a headland continuing into a reef. Careful design will make this type of headland appear almost natural.
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| − | ===Applicability===
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| − | The curved breakwater, the breakwater connected to the shore and the headland are useful substitutes for traditional [[groynes]] and breakwaters on coastal types 1 and 2. The curved breakwater can be used for all types of coasts, where traditional breakwaters can be used, see above. Neither the breakwater connected to the shore nor the headland can be used as replacements for traditional segmented breakwaters with small gaps and pocket beaches.
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| − | ==See also==
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| − | * [[Floating breakwaters]]
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| − | Articles on detached breakwaters:
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| − | * [[Detached breakwaters]]
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| − | * [[Detached shore parallel breakwaters]]
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| − | * [[Applicability of detached breakwaters]]
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| − | See also:
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| − | * [[Theme 5#Coastal protection techniques|Coastal protection techniques]]
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| − | * [[Protection against coastal erosion]]
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| − | ==References==
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| − | :Mangor, Karsten. 2004. “Shoreline Management Guidelines”. DHI Water and Environment, 294pg.
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| − | {{author
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| − | |AuthorID=13331
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| − | |AuthorFullName=Mangor, Karsten
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| − | |AuthorName=Karsten}}
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| − | [[Category:Theme 5]]
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| − | [[Category:Shoreline management]]
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| − | [[Category:Protection of coastal and marine zones]]
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| − | [[Category:Coastal defence]]
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| − | [[Category:Coastal flooding]]
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| − | [[Category:Coastal erosion]]
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| − | [[Category:Coastal flooding management]]
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| − | [[Category:Coastal erosion management]]
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| − | [[category:Coastal structures]]
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