Breakwater: Are Structures That [PDF]

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BREAKWATER Are structures that near the coasts as part of coastal management or to protect an anchorage from the effects of both weather and longshore drift.



A. PURPOSES 1. Reduce intensity of wave action in inshore waters 2. Provide safe harbourage 3. May be designed to protect gently sloping beach to reduce coastal erosion.



B. TYPES 1. Rubble Mound



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Use of structural voids to dissipate the wave energy. Used for protection of harbours and beaches against wave action. Also used for protection of navigation channels and beaches against sediment transportation Componensts Core - The purpose of the core is to prevent wave transmission into harbour; hence the core material is not too coarse. Toe - Toe functions as the foundation for the armour layer. It also may catch armour units displaced from armour layer; The Toe is normally constructed of large stones of quarry run or the most coarse filter layer. Filter Layer - The objective of the filter layer (under layer) is to prevent the core material from being washed out through armour layer. It is built of quarry run. Armour layer -The purpose of armour layer is to protect the core from direct wave attacks by the dissipation of wave energy. Armour layers are built of large rocks or larger concrete armour units. Superstructure -The superstructure is used either in order to reduce the crest elevation or to reduce wave overtopping, or as a roadway for traffic or pipelines. The superstructure is usually constructed of concrete



2. Caisson



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Typically have vertical sides and are usually erected where it is desirable to berth one or more vessels on the inner face of the breakwater. They use the mass of the caisson and the fill within it to resist the overturning forces applied by waves hitting them. They are relatively expensive to construct in shallow water, but in deeper sites they can offer a significant saving over revetment breakwaters. An additional rubble mound is sometimes placed in front of the vertical structure in order to absorb wave energy and thus reduce wave reflection and horizontal wave pressure on the vertical wall.



3. Wave Absorbing Caisson



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Similar to caisson but more sophisticated concept, including various types of perforation in the front wall. Such structures have been used successfully in the offshore oil-industry, but also on coastal projects requiring rather low-crested structures



4. Wave attenuator  The wave attenuator ultimately works by reflecting the wave energy and dissipating the kinetic energy from the incident wave.  The wave induced flow passes the structure and waves break over the top if the structure.  If the incident wave period is close to the resonant period of the break water mooring system, the wave dissipation is increased.  Types of wave attenator Box Breakwater - They are made from a concrete exterior with either a hollow or polystyrene interior. The design of this attenuator is limited by a 25 ft width and 5 ft draft.



Advantages



Disadvantages



-50 year design life -Allows pedestrian access for fishing and temporary boat mooring -Simple shape to build -Proven performance in moderate wave climate



-High cost maintenance if damaged may need towing to dry dock -Connectors can be a problem if not adequately designed.



Mat Breakwater - Mat breakwaters are made out recycled tires. Maze, Goodyear and Wave-Guard tires have been constructed and tested. The tires interlock and float on the oceans surface to reduce wave height.



Advantages



Disadvantages



-Low cost -Easily removed -Constructed with unskilled labour -Lower anchor load that box breakwaters



-Lack of buoyancy (foam is usually needs to be added) -Design life is only 15-20 years -Only effective in mild waves -Unappealing aesthetic



Tethered Breakwater - Made out of plastic buoys in order to reduce the height of the wave. It work in a similar way to the mat breakwaters but is made from plastic bouts rather than tires. Advantages



Disadvantages



-Easily removed -Low anchorage needed -Good aesthetic



-More expensive than mat breakwater -Can't be used by pedestrians



Deadweight Anchors - Deadweight anchors are commonly used to secure floating wave attenuators. This system is usually composed of a concrete block that rests on the lake bottom and is heavy enough that it will not slide. The weight needed for the deadweight anchors for a floating wave attenuator is determined by the mooring-line loads and coefficient of soil static friction. This type of anchorage system allows the wave attenuator to be taken out during the winter months and to not be affected by ice heave forces.