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70-723: The modern era of the Port of Yangon began in 1880 with the Commissioners for the Port of Rangoon. In 1954, the Board of Management for the Port of Rangoon was established, and then in 1972 the Burma Ports Corporation. The MPA was founded in 1989 to replace the Burma Ports Corporation. In March 2011, MPA began collaborating with a Chinese firm, CCCC TDC Tianjin Dredging, to begin dredging the Port of Yangon, to increase

140-512: A cliff face compress air in cracks on the cliff face. This exerts pressure on the surrounding rock, and can progressively splinter and remove pieces. Over time, the cracks can grow, sometimes forming a cave . The splinters fall to the sea bed where they are subjected to further wave action. Attrition occurs when waves cause loose pieces of rock debris ( scree ) to collide with each other, grinding and chipping each other, progressively becoming smaller, smoother and rounder. Scree also collides with

210-402: A dragline . This technique is often used in excavation of bay mud . Most of these dredges are crane barges with spuds , steel piles that can be lowered and raised to position the dredge. A backhoe/dipper dredger has a backhoe like on some excavators . A crude but usable backhoe dredger can be made by mounting a land-type backhoe excavator on a pontoon . The six largest backhoe dredgers in

280-487: A sandblasting effect. This effect helps to erode, smooth and polish rocks. The definition of erosion is grinding and wearing away of rock surfaces through the mechanical action of other rock or sand particles. According to the IPCC, sea level rise caused by climate change will increase coastal erosion worldwide, significantly changing the coasts and low-lying coastal areas. Hydraulic action occurs when waves striking

350-490: A capacity of 6,000 cubic metres per hour (59,000 cu ft/ks). An even larger dredger, retired in 1980, was the U.S. Army Corps of Engineers Essayons , which was 525.17 feet (160.07 m) long. The Mallard II , a clamshell dredger that maintains levees in San Francisco Bay , has operated continuously since being built in 1936. Dredgers are often equipped with dredge monitoring software to help

420-439: A few different types of dredge hoses that differ in terms of working pressure, float-ability, armored or not etc. Suction hoses, discharge armored hoses and self-floating hoses are some of the popular types engineered for transporting and discharging dredge materials. Some even had the pipes or hoses customised to exact dredging needs etc. Other times, it is pumped into barges (also called scows ), which deposit it elsewhere while

490-567: A few situations the excavation is undertaken by a specialist floating plant , known as a dredger. Usually the main objectives of dredging is to recover material of value, or to create a greater depth of water. Dredging systems can either be shore-based, brought to a location based on barges , or built into purpose-built vessels. Dredging can have environmental impacts: it can disturb marine sediments , leading to both short- and long-term water pollution , damage or destroy seabed ecosystems , and can release legacy human-sourced toxins captured in

560-471: A final solution to erosion. Seawalls can also deprive public access to the beach and drastically alter the natural state of the beach. Groynes also drastically alter the natural state of the beach. Some claim that groynes could reduce the interval between beach nourishment projects though they are not seen as a solution to beach nourishment. Other criticisms of seawalls are that they can be expensive, difficult to maintain, and can sometimes cause further damage to

630-475: A horrific case of coastal erosion is due to an increase in the frequency and the intensity of storms it experienced. These natural events had destroyed the Hampton Pier, Hernecliffe Gardens, a set of villas, several roads, and many other structures that once lay on Hampton-On-Sea. After this destruction, in 1899 they started building a sea wall to protect the rest of the remaining land and buildings. However,

700-482: A long tube like some vacuum cleaners but on a larger scale. A plain suction dredger has no tool at the end of the suction pipe to disturb the material. A trailing suction hopper dredger (TSHD) trails its suction pipe when working. The pipe, which is fitted with a dredge drag head , loads the dredge spoil into one or more hoppers in the vessel. When the hoppers are full, the TSHD sails to a disposal area and either dumps

770-421: A more permanent solution than soft-erosion control methods. Seawalls and groynes serve as semi-permanent infrastructure. These structures are not immune from normal wear-and-tear and will have to be refurbished or rebuilt. It is estimated the average life span of a seawall is 50–100 years and the average for a groyne is 30–40 years. Because of their relative permanence, it is assumed that these structures can be

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840-496: A process known as dewatering. Current dewatering techniques employ either centrifuges, geotube containers, large textile based filters or polymer flocculant /congealant based apparatus. In many projects, slurry dewatering is performed in large inland settling pits, although this is becoming less and less common as mechanical dewatering techniques continue to improve. Similarly, many groups (most notable in east Asia) are performing research towards utilizing dewatered sediments for

910-548: A rate of over 100 feet per year, earning the area the nickname "Washaway Beach". Much of the original town has collapsed into the ocean. The area is said to be the fastest-eroding shore of the United States' West Coast. Measures were finally taken to slow the erosion, with substantial slowing of the process noted in 2018. Fort Ricasoli , a historic 17th century fortress in Malta is being threatened by coastal erosion, as it

980-411: A shipping channel through coral reefs . A bucket dredger is equipped with a bucket dredge, which is a device that picks up sediment by mechanical means, often with many circulating buckets attached to a wheel or chain . A grab dredger picks up seabed material with a clam shell bucket , which hangs from an onboard crane or a crane barge , or is carried by a hydraulic arm, or is mounted like on

1050-576: A solution that keeps in mind environmental justice . Typically, there has been low public support for "retreating". However, if a community does decide to relocate their buildings along the coast it is common that they will then turn the land into public open space or transfer it into land trusts in order to protect it. These relocation practices are very cost-efficient, can buffer storm surges, safeguard coastal homes and businesses, lower carbon and other pollutants, create nursery habitats for important fish species, restore open space and wildlife, and bring back

1120-697: A whole new range of solutions to coastal erosion, not just structural solutions. Solutions that have potential include native vegetation, wetland protection and restoration, and relocation or removal of structures and debris. The solutions to coastal erosion that include vegetation are called "living shorelines". Living shorelines use plants and other natural elements. Living shorelines are found to be more resilient against storms, improve water quality, increase biodiversity, and provide fishery habitats. Marshes and oyster reefs are examples of vegetation that can be used for living shorelines; they act as natural barriers to waves. Fifteen feet of marsh can absorb fifty percent of

1190-421: A year, 1917, directly due to earlier dredging of shingle in the bay in front of it. The California coast, which has soft cliffs of sedimentary rock and is heavily populated, regularly has incidents of house damage as cliffs erodes. Devil's Slide , Santa Barbara , the coast just north of Ensenada , and Malibu are regularly affected. The Holderness coastline on the east coast of England, just north of

1260-526: Is a bar or blade which is pulled over the seabed behind any suitable ship or boat. It has an effect similar to that of a bulldozer on land. The chain-operated steam dredger Bertha , built in 1844 to a design by Brunel and as of 2009 was the oldest operational steam vessel in Britain, was of this type. This is an early type of dredger which was formerly used in shallow water in the Netherlands. It

1330-467: Is currently operated by a Hong Kong-based firm, while another in Ahlone Township is run by Asia World . On 17 March 2012, Japanese firm MOL, began a twice-weekly container service between the Port of Yangon and Singapore. 8 new Yangon area ports complete jun1st 2019. The Myanma Port Authority headquarters are located on Strand Road, Yangon , housed in a colonial-era building, designated in

1400-511: Is due to waves causing erosion of the primarily sedimentary material on which the buildings foundations sit. Dunwich , the capital of the English medieval wool trade, disappeared over the period of a few centuries due to redistribution of sediment by waves. Human interference can also increase coastal erosion: Hallsands in Devon , England, was a coastal village washed away over the course of

1470-729: Is forbidden unless authorized by a permit issued by the Army Corps of Engineers . Due to potential environmental impacts, dredging is often restricted to licensed areas, with vessel activity monitored closely using automatic GPS systems. According to a Rabobank outlook report in 2013, the largest dredging companies in the world are in order of size, based on dredging sales in 2012 Notable dredging companies in North America Notable dredging companies in South Asia Coastal erosion Coastal erosion

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1540-424: Is mainly used in harbours and other shallow water. Excavator dredge attachments The excavator dredge attachment uses the characteristics of cutter-suction dredgers, consisting of cutter heads and a suction pump for transferring material. These hydraulic attachments mount onto the boom arm of an excavator allowing an operator to maneuver the attachment along the shoreline and in shallow water for dredging. This

1610-439: Is removed from the foreshore depends on the power of the waves crossing the beach . This energy must reach a critical level to remove material from the debris lobe. Debris lobes can be very persistent and can take many years to completely disappear. Beaches dissipate wave energy on the foreshore and provide a measure of protection to the adjoining land. The stability of the foreshore, or its resistance to lowering. Once stable,

1680-587: Is suffering from this problem as well. Hampton-on-Sea is located in Kent, England. It was at one time very popular for its oyster fishing and was very reliant on the sea. Hampton-on-Sea has undergone the effects of coastal erosion since before the 1800s. Hampton-on-Sea's coastal erosion worsened with the increase in global warming and climate change. Global warming is causing a rise in sea level, more intense and frequent storms, and an increase in ocean temperature and precipitation levels. Another reason Hampton-on-Sea had such

1750-601: Is the loss or displacement of land, or the long-term removal of sediment and rocks along the coastline due to the action of waves , currents , tides , wind-driven water, waterborne ice, or other impacts of storms. The landward retreat of the shoreline can be measured and described over a temporal scale of tides, seasons, and other short-term cyclic processes. Coastal erosion may be caused by hydraulic action, abrasion , impact and corrosion by wind and water, and other forces, natural or unnatural. On non-rocky coasts, coastal erosion results in rock formations in areas where

1820-410: Is usually sucked up by a wear-resistant centrifugal pump and discharged either through a pipe line or to a barge. Cutter-suction dredgers are most often used in geological areas consisting of hard surface materials (for example gravel deposits or surface bedrock) where a standard suction dredger would be ineffective. They can, if sufficiently powerful, be used instead of underwater blasting. As of 2024,

1890-486: The Leiv Eriksson are: 46,000 cubic metre hopper and a design dredging depth of 155 m. Next largest is HAM 318 ( Van Oord ) with its 37,293 cubic metre hopper and a maximum dredging depth of 101 m. A cutter-suction dredger's (CSD) suction tube has a cutting mechanism at the suction inlet. The cutting mechanism loosens the bed material and transports it to the suction mouth. The dredged material

1960-557: The Humber Estuary , is one of the fastest eroding coastlines in Europe due to its soft clay cliffs and powerful waves. Groynes and other artificial measures to keep it under control has only accelerated the process further down the coast, because longshore drift starves the beaches of sand, leaving them more exposed. The white cliffs of Dover have also been affected. The coastline of North Cove, Washington has been eroding at

2030-506: The Kra Isthmus ): Dredging Dredging is the excavation of material from a water environment. Possible reasons for dredging include improving existing water features ; reshaping land and water features to alter drainage , navigability , and commercial use; constructing dams , dikes , and other controls for streams and shorelines; and recovering valuable mineral deposits or marine life having commercial value. In all but

2100-481: The Nile were channelled and wharfs built at the time of the pyramids (4000 BC), there was extensive harbour building in the eastern Mediterranean from 1000 BC and the disturbed sediment layers gives evidence of dredging. At Marseille , dredging phases are recorded from the third century BC onwards, the most extensive during the first century AD. The remains of three dredging boats have been unearthed; they were abandoned at

2170-782: The Yangon City Heritage List . The coast of Myanmar fronts on the Indian Ocean 's Bay of Bengal and Andaman Sea . Major port facilities administered by the MPA include: (only facility to handle vessels with 200 metre in length, 9-metre draft and 20,000-tonnage) Other significant ports in Myanmar include: ( 90% of trade handle in Yangon ports not incl Thilawa as of 2019) In Rakhine State : In Ayeyarwady Division : In Mon State : In Tanintharyi Division (of

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2240-566: The Eastern seaboard of the United States. Locations such as Florida have noticed increased coastal erosion. In reaction to these increases Florida and its individual counties have increased budgets to replenish the eroded sands that attract visitors to Florida and help support its multibillion-dollar tourism industries. There are three common forms of coastal erosion control methods. These three include: soft-erosion controls, hard-erosion controls, and relocation. Hard-erosion control methods provide

2310-414: The base of the cliff face, chipping small pieces of rock from the cliff or have a corrasion (abrasion) effect, similar to sandpapering. Solution is the process in which acishutds contained in sea water will dissolve some types of rock such as chalk or limestone. Abrasion , also known as corrasion , occurs when waves break on cliff faces and slowly erode it. As the sea pounds cliff faces it also uses

2380-655: The beach if built improperly. As we learn more about hard erosion controls it can be said for certain that these structural solutions cause more problems than they solve. They interfere with the natural water currents and prevent sand from shifting along coasts, along with the high costs to install and maintain them, their tendency to cause erosion in adjacent beaches and dunes, and the unintended diversion of stormwater and into other properties. Natural forms of hard-erosion control include planting or maintaining native vegetation, such as mangrove forests and coral reefs. Soft erosion strategies refer to temporary options of slowing

2450-532: The beds of streams. During the renaissance Leonardo da Vinci drew a design for a drag dredger. Dredging machines have been used during the construction of the Suez Canal from the late 1800s to present day expansions and maintenance. The completion of the Panama Canal in 1914, the most expensive U.S. engineering project at the time, relied extensively on dredging. These operate by sucking through

2520-620: The bottom of the harbour during the first and second centuries AD. The Banu Musa brothers during the Muslim Golden Age in while working at the Bayt-Al-Hikmah (house of wisdom) in Baghdad, designed an original invention in their book named ‘ Book of Ingenious Devices ’, a grab machine that does not appear in any earlier Greek works. The grab they described was used to extract objects from underwater, and recover objects from

2590-531: The coastline contains rock layers or fracture zones with varying resistance to erosion. Softer areas become eroded much faster than harder ones, which typically result in landforms such as tunnels , bridges , columns , and pillars . Over time the coast generally evens out. The softer areas fill up with sediment eroded from hard areas, and rock formations are eroded away. Also erosion commonly happens in areas where there are strong winds, loose sand , and soft rocks. The blowing of millions of sharp sand grains creates

2660-605: The construction industry. Dredging is a four-part process: loosening the material, bringing the material to the surface (together extraction), transportation and disposal. The extract can be disposed of locally or transported by barge or in a liquid suspension in pipelines. Disposal can be to infill sites, or the material can be used constructively to replenish eroded sand that has been lost to coastal erosion , or constructively create sea-walls, building land or whole new landforms such as viable islands in coral atolls . Ancient authors refer to harbour dredging. The seven arms of

2730-455: The culture of these coastal communities. Storms can cause erosion hundreds of times faster than normal weather. Before-and-after comparisons can be made using data gathered by manual surveying, laser altimeter , or a GPS unit mounted on an ATV. Remote sensing data such as Landsat scenes can be used for large scale and multi year assessments of coastal erosion. Moreover, geostatistical models can be applied to quantify erosion effects and

2800-495: The dredge continues its work. A number of vessels, notably in the UK and NW Europe de-water the hopper to dry the cargo to enable it to be discharged onto a quayside 'dry'. This is achieved principally using self discharge bucket wheel, drag scraper or excavator via conveyor systems. When contaminated (toxic) sediments are to be removed, or large volume inland disposal sites are unavailable, dredge slurries are reduced to dry solids via

2870-482: The dredge operator position the dredger and monitor the current dredge level. The monitoring software often uses Real Time Kinematic satellite navigation to accurately record where the machine has been operating and to what depth the machine has dredged to. In a "hopper dredger", the dredged materials end up in a large onboard hold called a "hopper." A suction hopper dredger is usually used for maintenance dredging. A hopper dredge usually has doors in its bottom to empty

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2940-425: The dredged materials, but some dredges empty their hoppers by splitting the two-halves of their hulls on large hydraulic hinges. Either way, as the vessel dredges, excess water in the dredged materials is spilled off as the heavier solids settle to the bottom of the hopper. This excess water is returned to the sea to reduce weight and increase the amount of solid material (or slurry) that can be carried in one load. When

3010-443: The effects of erosion. These options, including Sandbag and beach nourishment , are not intended to be long-term solutions or permanent solutions. Another method, beach scraping or beach bulldozing allows for the creation of an artificial dune in front of a building or as means of preserving a building foundation. However, there is a U.S. federal moratorium on beach bulldozing during turtle nesting season, 1 May – 15 November. One of

3080-548: The energy of incoming waves. Relocation of infrastructure any housing farther away from the coast is also an option. The natural processes of both absolute and relative sea level rise and erosion are considered in rebuilding. Depending on factors such as the severity of the erosion, as well as the natural landscape of the property, relocation could simply mean moving inland by a short distance or relocation can be to completely remove improvements from an area. A coproduction approach combined with managed retreat has been proposed as

3150-494: The environment, including the following: The nature of dredging operations and possible environmental impacts requires that the activity often be closely regulated and requires comprehensive regional environmental impact assessments alongside continuous monitoring. For example, in the U.S., the Clean Water Act requires that any discharge of dredged or fill materials into "waters of the United States," including wetlands,

3220-399: The foreshore should widen and become more effective at dissipating the wave energy, so that fewer and less powerful waves reach beyond it. The provision of updrift material coming onto the foreshore beneath the cliff helps to ensure a stable beach. The adjacent bathymetry , or configuration of the seafloor, controls the wave energy arriving at the coast, and can have an important influence on

3290-527: The form of a scoop made of chain mesh, and are towed by a fishing boat . Clam-specific dredges can utilize hydraulic injection to target deeper into the sand. Dredging can be destructive to the seabed and some scallop dredging has been replaced by collecting via scuba diving . As of June 2018, the largest dredger in Asia is MV  Tian Kun Hao , a 140-metre (460 ft) long dredger constructed in China, with

3360-444: The function of a natural storm beach , may be a soft-erosion control alternative in high energy environments such as open coastlines. Over the years beach nourishment has become a very controversial shore protection measure: It has the potential to negatively impact several of the natural resources. Some large issues with these beach nourishment projects are that they must follow a wide range of complex laws and regulations, as well as

3430-400: The high expenses it takes to complete these projects. Just because sand is added to a beach does not mean it will stay there. Some communities will bring in large volumes of sand repeatedly only for it to be washed away with the next big storm. Despite these factors, beach nourishment is still used often in many communities. Lately, the U.S. Army Corps of Engineers emphasized the need to consider

3500-447: The hopper is filled with slurry , the dredger stops dredging and goes to a dump site and empties its hopper. Some hopper dredges are designed so they can also be emptied from above using pumps if dump sites are unavailable or if the dredge material is contaminated. Sometimes the slurry of dredgings and water is pumped straight into pipes which deposit it on nearby land. These pipes are also commonly known as dredge hoses , too. There are

3570-469: The material could well suit the building industry, or could be used for beach nourishment. Dredging can disturb aquatic ecosystems , often with adverse impacts. In addition, dredge spoils may contain toxic chemicals that may have an adverse effect on the disposal area; furthermore, the process of dredging often dislodges chemicals residing in benthic substrates and injects them into the water column . Dredging can have numerous significant impacts on

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3640-467: The material through doors in the hull or pumps the material out of the hoppers. Some dredges also self-offload using drag buckets and conveyors. As of 2008 the largest trailing suction hopper dredgers in the world were Jan De Nul 's Cristobal Colon (launched 4 July 2008 ) and her sister ship Leiv Eriksson (launched 4 September 2009 ). Main design specifications for the Cristobal Colon and

3710-416: The most common methods of soft erosion control is beach nourishment projects. These projects involve dredging sand and moving it to the beaches as a means of reestablishing the sand lost due to erosion. In some situations, beach nourishment is not a suitable measure to take for erosion control, such as in areas with sand sinks or frequent and large storms. Dynamic revetment , which uses loose cobble to mimic

3780-467: The most powerful cutter-suction dredger in the world is DEME 's Spartacus , which entered service in 2021. The auger dredge system functions like a cutter suction dredger, but the cutting tool is a rotating Archimedean screw set at right angles to the suction pipe. Mud Cat invented the auger dredge in the 1970s. These use the Venturi effect of a concentrated high-speed stream of water to pull

3850-475: The natural temporal and spatial evolution of tracked coastal coastal profiles. The results can be used to determine the required temporal and spatial distances between the measured profiles for ecomic tracking. A place where erosion of a cliffed coast has occurred is at Wamberal in the Central Coast region of New South Wales where houses built on top of the cliffs began to collapse into the sea. This

3920-426: The nearby water, together with bed material, into a pipe. An airlift is a type of small suction dredge. It is sometimes used like other dredges. At other times, an airlift is handheld underwater by a diver . It works by blowing air into the pipe, and that air, being lighter than water, rises inside the pipe, dragging water with it. Some bucket dredgers and grab dredgers are powerful enough to rip out coral to make

3990-667: The production of concretes and construction block, although the high organic content (in many cases) of this material is a hindrance toward such ends. The proper management of contaminated sediments is a modern-day issue of significant concern. Because of a variety of maintenance activities, thousands of tonnes of contaminated sediment are dredged worldwide from commercial ports and other aquatic areas at high level of industrialization. Dredged material can be reused after appropriate decontamination. A variety of processes has been proposed and tested at different scales of application ( technologies for environmental remediation ). Once decontaminated,

4060-473: The rate of cliff erosion. Shoals and bars offer protection from wave erosion by causing storm waves to break and dissipate their energy before reaching the shore. Given the dynamic nature of the seafloor, changes in the location of shoals and bars may cause the locus of beach or cliff erosion to change position along the shore. Coastal erosion has been greatly affected by the rising sea levels globally. There has been great measures of increased coastal erosion on

4130-402: The rate of reaction by removing the reacted material. The ability of waves to cause erosion of the cliff face depends on many factors. The hardness (or inversely, the erodibility ) of sea-facing rocks is controlled by the rock strength and the presence of fissures , fractures , and beds of non-cohesive materials such as silt and fine sand . The rate at which cliff fall debris

4200-414: The scree from other wave actions to batter and break off pieces of rock from higher up the cliff face which can be used for this same wave action and attrition. Corrosion or solution/chemical weathering occurs when the sea's pH (anything below pH 7.0) corrodes rocks on a cliff face. Limestone cliff faces, which have a moderately high pH, are particularly affected in this way. Wave action also increases

4270-426: The sea wall did not offer much help: buildings continued to be affected by the erosion. Then a storm came and broke the sea wall, it then flooded the land behind it. These events cause many land investors to back out. Eventually, Hampton-on-Sea had to be abandoned because the erosion overtook so much of the land. By 1916 Hampton-on-Sea had been completely abandoned. By the 1920s only a couple of structures still stood. It

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4340-479: The seabed to bring the sediment in suspension, which then becomes a turbidity current , which flows away down slope, is moved by a second burst of water from the WID or is carried away in natural currents. Water injection results in a lot of sediment in the water which makes measurement with most hydrographic equipment (for instance: singlebeam echosounders) difficult. These dredgers use a chamber with inlets, out of which

4410-469: The seabed with its hull out of the water. Some forms can go on land. Some of these are land-type backhoe excavators whose wheels are on long hinged legs so it can drive into shallow water and keep its cab out of water. Some of these may not have a floatable hull and, if so, cannot work in deep water. Oliver Evans (1755–1819) in 1804 invented the Oruktor Amphibolos, an amphibious dredger which

4480-628: The sediment. These environmental impacts can reduce marine wildlife populations in some cases, contaminate sources of drinking water and interrupt economic activities such as fishing. Dredging is excavation carried out underwater or partially underwater, in shallow waters or ocean waters . It keeps waterways and ports navigable, and assists coastal protection, land reclamation and coastal redevelopment, by gathering up bottom sediments and transporting it elsewhere. Dredging can be done to recover materials of commercial value; these may be high value minerals or sediments such as sand and gravel that are used by

4550-428: The size of vessels that can dock at the port (to 35,000 tons deadweight, up from the current capacity of 15,000). MPA has been involved in contracts to develop Myanmar's Special Economic Zones , including a US$ 8.6 billion deal to develop a deep sea port at Dawei , by Italian-Thai Development . It was reported in 2010 that state-owned ports run by the MPA will be privatised. The country's largest port, Thilawa Port ,

4620-427: The water is pumped with the inlets closed. It is usually suspended from a crane on land or from a small pontoon or barge. Its effectiveness depends on depth pressure. A snagboat is designed to remove big debris such as dead trees and parts of trees from North America waterways. Some of these are any of the above types of dredger, which can operate normally, or by extending legs, also known as spuds, so it stands on

4690-705: The world are currently the Vitruvius, the Mimar Sinan, Postnik Yakovlev (Jan De Nul), the Samson (DEME), the Simson and the Goliath (Van Oord). They featured barge -mounted excavators. Small backhoe dredgers can be track-mounted and work from the bank of ditches. A backhoe dredger is equipped with a half-open shell. The shell is filled moving towards the machine. Usually dredged material is loaded in barges. This machine

4760-446: Was America's first steam-powered road vehicle. These are usually used to recover useful materials from the seabed. Many of them travel on continuous track . A unique variant is intended to walk on legs on the seabed. Fishing dredges are used to collect various species of clams , scallops , oysters or mussels from the seabed. Some dredges are also designed to catch crabs, sea urchins, sea cucumbers, and conch. These dredges have

4830-468: Was a flat-bottomed boat with spikes sticking out of its bottom. As tide current pulled the boat, the spikes scraped seabed material loose, and the tide current washed the material away, hopefully to deeper water. Krabbelaar is the Dutch word for "scratcher". A water injection dredger uses a small jet to inject water under low pressure (to prevent the sediment from exploding into the surrounding waters) into

4900-410: Was built on a fault in the headland which is prone to erosion. A small part of one of the bastion walls has already collapsed since the land under it has eroded, and there are cracks in other walls as well. In El Campello , Spain, the erosion and failure of a Roman fish farm excavated from rock during the first century B.C. was exacerbated by the construction of a close sport harbour. Hampton-on-Sea

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