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34-459: Plymouth Breakwater is a 1,560-metre (1,710 yd) stone breakwater protecting Plymouth Sound and the anchorages near Plymouth , Devon , England . It is 13 metres (43 ft) wide at the top and the base is 65 metres (213 ft). It lies in about 10 metres (33 ft) of water. Around 4 million tons of rock were used in its construction in 1812 at the then-colossal cost of £1.5 million (equivalent to £126 million today). In 1806, as

68-434: A jetty or a mole , may be connected to land or freestanding, and may contain a walkway or road for vehicle access. Part of a coastal management system, breakwaters are installed parallel to the shore to minimize erosion . On beaches where longshore drift threatens the erosion of beach material, smaller structures on the beach may be installed, usually perpendicular to the water's edge. Their action on waves and current

102-409: A change in the profile and height. John Rennie died in 1821; his last work in connection with the breakwater was to draw up proposals for a lighthouse. Whidbey continued to work on the breakwater and other engineering projects, including the breakwater's lighthouse, until retirement around 1830; the work was completed by Rennie's sons, George and Sir John . Plymouth Breakwater Lighthouse stands on

136-501: A fixed red light with a white sector indicating an anchorage to the north-east (later the colours were reversed). Since 1854 a second white light has also been displayed, from a window lower down in the tower, indicating a channel between two hazards: the Draystone and the Knap; it used a small, sixth-order dioptric lens. By 1867 a 7 cwt bell had been installed at the lighthouse to serve as

170-420: A fog signal. In 1879 a larger (32 cwt) bell was installed, and the old bell was transferred to Gunfleet Lighthouse . (The new bell had previously been in use at Start Point lighthouse , where a fog siren had replaced it two years earlier). The bell sounded four strokes every minute in foggy weather. The following year, the light was made occulting (being eclipsed for three seconds every half minute). In 1920

204-415: A function of the distance the breakwaters are built from the coast, the direction at which the wave hits the breakwater, and the angle at which the breakwater is built (relative to the coast). Of these three, the angle at which the breakwater is built is most important in the engineered formation of salients. The angle at which the breakwater is built determines the new direction of the waves (after they've hit

238-417: A military band. On completion, management of the light was transferred to Trinity House . A beacon was placed at the eastern end of the breakwater, consisting of a 6-foot (1.8 m) spherical cage on a 17-foot (5.2 m) pole; the cage was designed as a refuge for six shipwrecked sailors. The lighthouse had been provided with a second-order catadioptric lens array by Henry Lepaute of Paris; it showed

272-492: A red sector to the north-east; the subsidiary white light has an isophase characteristic, two seconds on, two seconds off. In 1860, a Royal Commission , established by Lord Palmerston , produced a plan for the defence of Plymouth and other Royal Dockyards . The Breakwater Fort was designed to defend the entrances to Plymouth Sound in conjunction with forts and batteries on either shore. Designed by Captain Siborne, work on

306-473: A signal station, and from 1937, an anti-aircraft training school. It was finally released by the military in 1976. Breakwater (structure) A breakwater is a permanent structure constructed at a coastal area to protect against tides, currents, waves, and storm surges. Breakwaters have been built since antiquity to protect anchorages , helping isolate vessels from marine hazards such as wind-driven waves. A breakwater, also known in some contexts as

340-442: 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. Such a design provides additional protection on the sea side and a quay wall on the inner side of the breakwater, but it can enhance wave overtopping . A similar but more sophisticated concept

374-567: Is a station at which a fog signal exists, but at which there is no lighthouse . A light tower might be appended to the station at a later date, as happened at The Cuckolds Light in Maine . A number of these stations were constructed along the California coast, although few survive in their original form today. Fog signal buildings [REDACTED] Media related to Fog signal buildings at Wikimedia Commons This lighthouse -related article

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408-651: Is a wave-absorbing caisson, 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 (e.g. on an urban promenade where the sea view is an important aspect, as seen in Beirut and Monaco ). In the latter, a project is presently ongoing at the Anse du Portier including 18 wave-absorbing 27 m (89 ft) high caissons. Wave attenuators consist of concrete elements placed horizontally one foot under

442-518: Is designed to absorb the energy of the waves that hit it, either by using mass (e.g. with caissons), or by using a revetment slope (e.g. with rock or concrete armour units). In coastal engineering , a revetment is a land-backed structure whilst a breakwater is a sea-backed structure (i.e. water on both sides). Rubble mound breakwaters use structural voids to dissipate the wave energy. Rubble mound breakwaters consist of piles of stones more or less sorted according to their unit weight: smaller stones for

476-465: Is intended to slow the longshore drift and discourage mobilisation of beach material. In this usage they are more usually referred to as groynes . Breakwaters reduce the intensity of wave action in inshore waters and thereby provide safe harbourage. Breakwaters may also be small structures designed to protect a gently sloping beach to reduce coastal erosion ; they are placed 100–300 feet (30–90 m) offshore in relatively shallow water. An anchorage

510-430: Is limited in practice by the natural fracture properties of locally available rock. Shaped concrete armour units (such as Dolos , Xbloc , Tetrapod , etc.) can be provided in up to approximately 40 tonnes (e.g. Jorf Lasfar , Morocco), before they become vulnerable to damage under self weight, wave impact and thermal cracking of the complex shapes during casting/curing. Where the very largest armour units are required for

544-809: Is only safe if ships anchored there are protected from the force of powerful waves by some large structure which they can shelter behind. Natural harbours are formed by such barriers as headlands or reefs . Artificial harbours can be created with the help of breakwaters. Mobile harbours, such as the D-Day Mulberry harbours , were floated into position and acted as breakwaters. Some natural harbours, such as those in Plymouth Sound , Portland Harbour , and Cherbourg , have been enhanced or extended by breakwaters made of rock. Types of breakwaters include vertical wall breakwater, mound breakwater and mound with superstructure or composite breakwater. A breakwater structure

578-562: The Napoleonic Wars impended, Lord St. Vincent commissioned John Rennie and Joseph Whidbey to plan a means of making Plymouth Bay a safe anchorage for the Channel Fleet . These plans may have been taken from ones made by George Matcham (1753 – 3 February 1833). In 1811 came the order to begin construction; Whidbey was appointed Acting Superintending Engineer. This task required great engineering, organizational and political skills, as

612-466: The Newport breakwater. The dissipation of energy and relative calm water created in the lee of the breakwaters often encourage accretion of sediment (as per the design of the breakwater scheme). However, this can lead to excessive salient build up, resulting in tombolo formation, which reduces longshore drift shoreward of the breakwaters. This trapping of sediment can cause adverse effects down-drift of

646-520: The United States Army Corps of Engineers Coastal engineering manual (available for free online) and elsewhere. For detailed design the use of scaled physical hydraulic models remains the most reliable method for predicting real-life behavior of these complex structures. Breakwaters are subject to damage and overtopping in severe storms. Some may also have the effect of creating unique types of waves that attract surfers, such as The Wedge at

680-641: The breakwaters), and in turn the direction that sediment will flow and accumulate over time. The reduced heterogeneity in sea floor landscape introduced by breakwaters can lead to reduced species abundance and diversity in the surrounding ecosystems. As a result of the reduced heterogeneity and decreased depths that breakwaters produce due to sediment build up, the UV exposure and temperature in surrounding waters increase, which may disrupt surrounding ecosystems. There are two main types of offshore breakwater (also called detached breakwater): single and multiple. Single, as

714-403: The breakwaters, leading to beach sediment starvation and increased coastal erosion . This may then lead to further engineering protection being needed down-drift of the breakwater development. Sediment accumulation in the areas surrounding breakwaters can cause flat areas with reduced depths, which changes the topographic landscape of the seabed. Salient formations as a result of breakwaters are

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748-427: The choice depending on tidal range and water depth. They usually consist of large pieces of rock (granite) weighing up to 10–15 tonnes each, or rubble-mound. Their design is influenced by the angle of wave approach and other environmental parameters. Breakwater construction can be either parallel or perpendicular to the coast, depending on the shoreline requirements. Fog signal station A fog signal station

782-461: The collided wave energy and prevent the generation of standing waves. As design wave heights get larger, rubble mound breakwaters require larger armour units to resist the wave forces. These armour units can be formed of concrete or natural rock. The largest standard grading for rock armour units given in CIRIA 683 "The Rock Manual" is 10–15 tonnes. Larger gradings may be available, but the ultimate size

816-461: The core and larger stones as an armour layer protecting the core from wave attack. Rock or concrete armour units on the outside of the structure absorb most of the energy, while gravels or sands prevent the wave energy's continuing through the breakwater core. The slopes of the revetment are typically between 1:1 and 1:2, depending upon the materials used. In shallow water, revetment breakwaters are usually relatively inexpensive. As water depth increases,

850-525: The effect of the incident wave, creates waves in phase opposition to the incident wave downstream from the slabs. A submerged flexible mound breakwater can be employed for wave control in shallow water as an advanced alternative to the conventional rigid submerged designs. Further to the fact that, the construction cost of the submerged flexible mound breakwaters is less than that of the conventional submerged breakwaters, ships and marine organisms can pass them, if being deep enough. These marine structures reduce

884-406: The free surface, positioned along a line parallel to the coast. Wave attenuators have four slabs facing the sea, one vertical slab, and two slabs facing the land; each slab is separated from the next by a space of 200 millimetres (7.9 in). The row of four sea-facing and two land-facing slabs reflects offshore wave by the action of the volume of water located under it which, made to oscillate under

918-564: The lighthouse was converted to run automatically on acetylene gas. Following departure of the resident keepers, the light was monitored from the Trinity House fog signal station at nearby Penlee Point . Oversight of the lighthouse was passed from Trinity House to the Ministry of Defence in 1993. The bell remained in use until 1994, when it was replaced by an electronic fog horn. The main light currently flashes once every ten seconds, white with

952-458: The many strictly technical challenges were complicated by the significant resources devoted to the project, from which various parties evidenced a desire for advantage. Nearly 4,000,000 (four million) tons of stone were quarried and transported, using about a dozen ships innovatively designed by the two engineers. A paper to the Royal Society suggests that Whidbey found many fossils as a result of

986-450: The material requirements—and hence costs—increase significantly. Caisson breakwaters 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

1020-623: The most exposed locations in very deep water, armour units are most often formed of concrete cubes, which have been used up to ~ 195 tonnes Archived 2019-05-12 at the Wayback Machine for the tip of the breakwater at Punta Langosteira near La Coruña, Spain. Preliminary design of armour unit size is often undertaken using the Hudson's equation , Van der Meer and more recently Van Gent et al.; these methods are all described in CIRIA 683 "The Rock Manual" and

1054-414: The name suggests, means the breakwater consists of one unbroken barrier, while multiple breakwaters (in numbers anywhere from two to twenty) are positioned with gaps in between (160–980 feet or 50–300 metres). The length of the gap is largely governed by the interacting wavelengths. Breakwaters may be either fixed or floating, and impermeable or permeable to allow sediment transfer shoreward of the structures,

Plymouth Breakwater - Misplaced Pages Continue

1088-453: The oval masonry sea fort started in 1861 and the main structure was completed in 1865. It has its foundations on Shovel Rock and is 35 yards inside the Breakwater. After several changes in plan, the fort was finally armed in 1879 with fourteen 12.5-inch and four 10-inch rifled muzzle-loading guns in armoured casemates . Although the fort had been disarmed before World War I , it served as

1122-426: The quarrying necessary to the breakwater. The foundation stone was laid on Shovel Rock on 8 August 1812. It followed a line over Panther Rock, Shovel and St. Carlos Rocks, and was sufficiently completed by 1814 to shelter ships of the line. Napoleon was reported as commenting that the breakwater was a grand thing, as he passed by it on the way to exile on St. Helena in 1815. Severe storm damage in 1817 and 1824 prompted

1156-515: The western tip of the breakwater. Designed for the Admiralty by Walker & Burgess , construction of the granite tower began on 22 February 1841 and was completed on 9 November 1843; William Stuart was superintendent of the works. The light became operational in June 1844; soon afterwards a horse-drawn omnibus was driven along the breakwater from end to end, with a full complement of passengers accompanied by

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