Seabed

The seabed (also known as the seafloor, sea floor, or ocean floor) is the bottom of the ocean.

Elevation
Map showing the underwater topography (bathymetry) of the ocean floor. Like land terrain, the ocean floor has ridges, valleys, plains and volcanoes.

Structure

Oceanic divisions
The major oceanic divisions
Fischeripollis pollen
Example of pollen found at different depths below seafloor in sample cores: Fischeripollis found at 148.36 mbsf (1) and at 142.50 mbsf; Periporopollenites "spinosus" ms. found at 180.40 mbsf (9 and 10).

Most of the oceans have a common structure, created by common physical phenomena, mainly from tectonic movement, and sediment from various sources. The structure of the oceans, starting with the continents, begins usually with a continental shelf, continues to the continental slope – which is a steep descent into the ocean, until reaching the abyssal plain – a topographic plain, the beginning of the seabed, and its main area. The border between the continental slope and the abyssal plain usually has a more gradual descent, and is called the continental rise, which is caused by sediment cascading down the continental slope.

The mid-ocean ridge, as its name implies, is a mountainous rise through the middle of all the oceans, between the continents. Typically a rift runs along the edge of this ridge. Along tectonic plate edges there are typically oceanic trenches – deep valleys, created by the mantle circulation movement from the mid-ocean mountain ridge to the oceanic trench.

Hotspot volcanic island ridges are created by volcanic activity, erupting periodically, as the tectonic plates pass over a hotspot. In areas with volcanic activity and in the oceanic trenches there are hydrothermal vents – releasing high pressure and extremely hot water and chemicals into the typically freezing water around it.

Deep ocean water is divided into layers or zones, each with typical features of salinity, pressure, temperature and marine life, according to their depth. Lying along the top of the abyssal plain is the abyssal zone, whose lower boundary lies at about 6,000 m (20,000 ft). The hadal zone – which includes the oceanic trenches, lies between 6,000–11,000 metres (20,000–36,000 ft) and is the deepest oceanic zone.

Depth below seafloor

Depth below seafloor is a vertical coordinate used in geology, paleontology, oceanography, and petrology (see ocean drilling). The acronym "mbsf" (meaning "meters below the seafloor") is a common convention used for depths below the seafloor.[1][2]

Sediments

Sediments in the seabed vary diversely in their origin, from eroded land materials carried into the ocean by rivers or wind flow, waste and decompositions of sea animals, and precipitation of chemicals within the sea water itself, including some from outer space.[3] There are four basic types of sediment of the sea floor: 1.) "Terrigenous" describes the sediment derived from the materials eroded by rain, rivers, glaciers and that which is blown into the ocean by the wind, such as volcanic ash. 2.) Biogenous material is the sediment made up of the hard parts of sea animals that accumulate on the bottom of the ocean. 3.) Hydrogenous sediment is the dissolved material that precipitates in the ocean when oceanic conditions change, and 4.) cosmogenous sediment comes from extraterrestrial sources. These are the components that make up the seafloor under their genetic classifications.

Terrigenous and biogenous

Terrigenous sediment is the most abundant sediment found on the seafloor, followed by biogenous sediment. The sediment in areas of the ocean floor which is at least 30% biogenous materials is labeled as an ooze. There are two types of oozes: Calcareous oozes and Siliceous oozes. Plankton is the contributor of oozes. Calcareous oozes are predominantly composed of calcium shells found in phytoplankton such as coccolithophores and zooplankton like the foraminiferans. These calcareous oozes are never found deeper than about 4,000 to 5,000 meters because at further depths the calcium dissolves.[4] Similarly, Siliceous oozes are dominated by the siliceous shells of phytoplankton like diatoms and zooplankton such as radiolarians. Depending on the productivity of these planktonic organisms, the shell material that collects when these organisms die may build up at a rate anywhere from 1mm to 1 cm every 1000 years.[4]

Hydrogenous and cosmogenous

Hydrogenous sediments are uncommon. They only occur with changes in oceanic conditions such as temperature and pressure. Rarer still are cosmogenous sediments. Hydrogenous sediments are formed from dissolved chemicals that precipitate from the ocean water, or along the mid-ocean ridges, they can form by metallic elements binding onto rocks that have water of more than 300 °C circulating around them. When these elements mix with the cold sea water they precipitate from the cooling water.[4] Known as manganese nodules, they are composed of layers of different metals like manganese, iron, nickel, cobalt, and copper, and they are always found on the surface of the ocean floor.[4] Cosmogenous sediments are the remains of space debris such as comets and asteroids, made up of silicates and various metals that have impacted the Earth.[5]

Size classification

Another way that sediments are described is through their descriptive classification. These sediments vary in size, anywhere from 1/4096 of a mm to greater than 256 mm. The different types are: boulder, cobble, pebble, granule, sand, silt, and clay, each type becoming finer in grain. The grain size indicates the type of sediment and the environment in which it was created. Larger grains sink faster and can only be pushed by rapid flowing water (high energy environment) whereas small grains sink very slowly and can be suspended by slight water movement, accumulating in conditions where water is not moving so quickly.[6] This means that larger grains of sediment may come together in higher energy conditions and smaller grains in lower energy conditions.

Benthos

Benthos is the community of organisms which live on, in, or near the seabed, the area known as the benthic zone.[7] This community lives in or near marine sedimentary environments, from tidal pools along the foreshore, out to the continental shelf, and then down to the abyssal depths. The benthic zone is the ecological region on, in and immediately above the seabed, including the sediment surface and some sub-surface layers. Benthos generally live in close relationship with the substrate bottom, and many such organisms are permanently attached to the bottom. The superficial layer of the soil lining the given body of water, the benthic boundary layer, is an integral part of the benthic zone, and greatly influences the biological activity which takes place there. Examples of contact soil layers include sand bottoms, rocky outcrops, coral, and bay mud.

Features

Pelagiczone
Layers of the pelagic zone

Each area of the seabed has typical features such as common soil composition, typical topography, salinity of water layers above it, marine life, magnetic direction of rocks, and sedimentation.

Seabed topography is flat where sedimentation is heavy and covers the tectonic features. Sediments comes from various sources:

  • Land erosion sediments, brought mainly by rivers
  • Underwater volcanic ash spreading, especially from hydrothermal vents
  • Microorganism activity
  • Sea currents eroding the seabed itself
  • Marine life: corals, fish, algae, crabs, marine plants and other biologically created sediment

Where sedimentation is very light, such as in the Atlantic ocean, especially in the northern and eastern Atlantic, the original tectonic activity can be clearly seen as straight line "cracks" or "vents" thousands of kilometers long.

Marine life is abundant in the deep sea, especially around hydrothermal vents. Large deep sea communities of marine life have been discovered around black and white smokers—vents emitting chemicals toxic to humans and most vertebrates. This marine life receives its energy both from the extreme temperature difference (typically a drop of 150 degrees) and from chemosynthesis by bacteria.

Brine pools are another seabed feature,[8] usually connected to cold seeps.

History of exploration

The seabed has been explored by submersibles such as Alvin and, to some extent, scuba divers with special equipment. The process that continually adds new material to the ocean floor is seafloor spreading and the continental slope. In recent years satellite images show a very clear mapping of the seabed, and are used extensively in the study and exploration of the ocean floor.

Resources

Seabed contains "several hundred years' worth of cobalt and nickel". In 2001-2013, International Seabed Authority issued 13 various licences to seabed mining.[9]

In art and culture

Some children's play songs include elements such as "There's a hole at the bottom of the sea", or "A sailor went to sea... but all that he could see was the bottom of the deep blue sea".

On and under the seabed are archaeological sites of historic interest, such as shipwrecks and sunken towns. This underwater cultural heritage is protected by the UNESCO Convention on the Protection of the Underwater Cultural Heritage. The convention aims at preventing looting and the destruction or loss of historic and cultural information by providing an international legal framework.[10]

See also

References

  1. ^ Flood, Roger D.; Piper, D.J.W. (1997). "Preface: Depth Below Seafloor Conventions". In Flood; Piper; Klaus, A.; Peterson, L.C. (eds.). Proceedings of the Ocean Drilling Program, Scientific Results. 155. p. 3. doi:10.2973/odp.proc.sr.155.200.1997. we follow Ocean Drilling Program (ODP) meters below seafloor (mbsf) convention
  2. ^ Parkes, R. John; Henrik Sass (2007). Sulphate-reducing bacteria environmental and engineered systems. Edited by Larry L. Barton University of New Mexico. Sulphate-reducing bacteria environmental and engineered systems. Cambridge University Press. pp. 329–358. doi:10.1017/CBO9780511541490.012. Retrieved 11 June 2010. metres below the seafloor (mbsf)
  3. ^ Murray, Richard W. "Ocean-Floor Sediments," Water Encyclopedia
  4. ^ a b c d "The Bottom of the Ocean," Marine Science
  5. ^ "Types of Marine Sediments", Article Myriad
  6. ^ Tripati, Aradhna, Lab 6-Marine Sediments, Marine Sediments Reading, E&SSCI15-1, UCLA, 2012
  7. ^ Benthos from the Census of Antarctic Marine Life website
  8. ^ Wefer, Gerold; Billet, David; Hebbeln, Dierk; Jorgensen, Bo Barker; Schlüter, Michael; Weering, Tjeerd C. E. Van (2013-11-11). Ocean Margin Systems. Springer Science & Business Media. ISBN 978-3-662-05127-6.
  9. ^ Shukman, David (14 March 2013). "UK firm joins ocean mineral rush". Retrieved 26 March 2018 – via www.bbc.com.
  10. ^ Safeguarding the Underwater Cultural Heritage UNESCO. Retrieved 12 September 2012.

Further reading

  • Roger Hekinian: Sea Floor Exploration: Scientific Adventures Diving into the Abyss. Springer, 2014. ISBN 978-3-319-03202-3 (print); ISBN 978-3-319-03203-0 (eBook)
  • Stéphane Sainson: Electromagnetic Seabed Logging. A new tool for geoscientists. Springer, 2016. ISBN 978-3-319-45353-8 (print); ISBN 978-3-319-45355-2 (eBook)

External links

Acoustic seabed classification

Acoustic seabed classification is the partitioning of a seabed acoustic image into discrete physical entities or classes. This is a particularly active area of development in the field of seabed mapping, marine geophysics, underwater acoustics and benthic habitat mapping. Seabed classification is one route to characterizing the seabed and its habitats. Seabed characterization makes the link between the classified regions and the seabed physical, geological, chemical or biological properties. Acoustic seabed classification is possible using a wide range of acoustic imaging systems including multibeam echosounders, sidescan sonar, single-beam echosounders, interferometric systems and sub-bottom profilers. Seabed classification based on acoustic properties can be divided into two main categories; surficial seabed classification and sub-surface seabed classification. Sub-surface imaging technologies use lower frequency sound to provide higher penetration, whereas surficial imaging technologies provide higher resolution imagery by utilizing higher frequencies (especially in shallow water).

Anchor

An anchor is a device, normally made of metal, used to connect a vessel to the bed of a body of water to prevent the craft from drifting due to wind or current. The word derives from Latin ancora, which itself comes from the Greek ἄγκυρα (ankura).Anchors can either be temporary or permanent. Permanent anchors are used in the creation of a mooring, and are rarely moved; a specialist service is normally needed to move or maintain them. Vessels carry one or more temporary anchors, which may be of different designs and weights.

A sea anchor is a drogue, not in contact with the seabed. It is used to control a drifting vessel, or to limit the speed of a sailing yacht running "under bare poles" in a storm.

Benthic lander

Benthic landers are observational platforms that sit on the seabed or benthic zone to record physical, chemical or biological activity. The landers are autonomous and have deployment durations from a few days (for biological studies) to several years (for physical oceanography studies).

Benthic landers come in a variety of shapes and sizes depending upon the instrumentation they carry, and are typically capable of working at any ocean depth.

Cable protection system

A cable protection system (CPS) protects subsea power cables against various factors that negatively impact on the cable lifetime, normally used when entering an offshore structure. When a subsea power cable is laid, there is an area where the cable can be subjected to increased dynamic forces, which the cable is not necessarily designed to survive over the lifetime of the installation.

Cable protection systems are used to allow the specification, and thus cost, of a subsea power cable to be reduced, by removing the need to include additional armoring of the cable. The resulting cables can be produced more cheaply, whilst still providing the 20 years + lifetime required.

Offshore windfarm developers in particular have adopted the use of Cable protection systems due to the dynamic area where the cable comes from the seabed and enters the monopile/J-tube. This is in part due to the potential for localised scouring to occur near the structure.

A CPS generally consists of three sections, a Centraliser or Monopile interface, a protection system for the dynamic area, and a protection system for the static area.

The installation of J-Tubes for offshore renewable monopiles was viewed as a costly approach, and a 'latching' type of cable protection system which penetrates the outer wall of the monopile, via a specifically designed angled aperture enables the simplification of monopile design, and removes the need for additional works post pile driving which usually involved the use of divers. This approach is becoming the industry standard in monopile design, assisting developers to reduce their costs for construction.

Demersal zone

The demersal zone is the part of the sea or ocean (or deep lake) consisting of the part of the water column near to (and significantly affected by) the seabed and the benthos. The demersal zone is just above the benthic zone and forms a layer of the larger profundal zone.

Being just above the ocean floor, the demersal zone is variable in depth and can be part of the photic zone where light can penetrate and photosynthetic organisms grow, or the aphotic zone, which begins between depths of roughly 200 and 1,000 m (700 and 3,300 ft) and extends to the ocean depths, where no light penetrates.

Electric pulse fishing

Electric pulse fishing is a fishing technique sometimes used in trawl fisheries which produces a limited electric field above the seabed to catch fish. The pulse trawl gear consists of a number of electrodes, attached to the gear in the tow direction, that emit short electric pulses. The electrodes replace the tickler chains that are used in traditional beam trawl fishery. The pulse trawl fishery is mainly aimed at flatfish like sole, with by-catch plaice. In addition, the pulse trawl gear is applied in shrimp fisheries on a limited scale. Technically, the use of electricity to catch fish is prohibited in European waters. However, the European Union is able to provide exemptions to this rule since 2007. These exemptions are now mainly used by Dutch and British trawlers in the North Sea.

Electric pulse fishing should not be confused with pulse fishing, which is a fisheries management technique for preventing fish stocks from being overfished by periodically permitting a cycle of fishing followed by a fallow period which allows stocks to reconstitute.

Fundus (seabed)

The fundus is the seabed in a tidal river below low water mark. This can be owned by the foreshore owner (area between high and low water mark) and may require permission and rent, if used for laying a mooring or putting down crab or lobster pots.

Habitat

In ecology, a habitat is the type of natural environment in which a particular species of organism lives. It is characterized by both physical and biological features. A species' habitat is those places where it can find food, shelter, protection and mates for reproduction.

The physical factors are for example soil, moisture, range of temperature, and light intensity as well as biotic factors such as the availability of food and the presence or absence of predators. Every organism has certain habitat needs for the conditions in which it will thrive, but some are tolerant of wide variations while others are very specific in their requirements. A habitat is not necessarily a geographical area, it can be the interior of a stem, a rotten log, a rock or a clump of moss, and for a parasitic organism it is the body of its host, part of the host's body such as the digestive tract, or a single cell within the host's body.

Habitat types include polar, temperate, subtropical and tropical. The terrestrial vegetation type may be forest, steppe, grassland, semi-arid or desert. Fresh water habitats include marshes, streams, rivers, lakes, and ponds, and marine habitats include salt marshes, the coast, the intertidal zone, estuaries, reefs, bays, the open sea, the sea bed, deep water and submarine vents.

Habitats change over time. This may be due to a violent event such as the eruption of a volcano, an earthquake, a tsunami, a wildfire or a change in oceanic currents; or the change may be more gradual over millennia with alterations in the climate, as ice sheets and glaciers advance and retreat, and as different weather patterns bring changes of precipitation and solar radiation. Other changes come as a direct result of human activities; deforestation, the plowing of ancient grasslands, the diversion and damming of rivers, the draining of marshland and the dredging of the seabed. The introduction of alien species can have a devastating effect on native wildlife, through increased predation, through competition for resources or through the introduction of pests and diseases to which the native species have no immunity.

International Seabed Authority

The International Seabed Authority (ISA) (French: Autorité internationale des fonds marins) is an intergovernmental body based in Kingston, Jamaica, that was established to organize, regulate and control all mineral-related activities in the international seabed area beyond the limits of national jurisdiction, an area underlying most of the world's oceans. It is an organization established by the United Nations Convention on the Law of the Sea.

Isopoda

Isopoda is an order of crustaceans that includes woodlice and their relatives. Isopods live in the sea, in fresh water, or on land. All have rigid, segmented exoskeletons, two pairs of antennae, seven pairs of jointed limbs on the thorax, and five pairs of branching appendages on the abdomen that are used in respiration. Females brood their young in a pouch under their thorax. Isopods have various feeding methods: some eat dead or decaying plant and animal matter, others are grazers, or filter feeders, a few are predators, and some are internal or external parasites, mostly of fishes. Aquatic species mostly live on the seabed or bottom of freshwater bodies of water, but some taxa can swim for a short distance. Terrestrial forms move around by crawling and tend to be found in cool, moist places. Some species are able to roll themselves into a ball as a defence mechanism or to conserve moisture.

There are over 10,000 species of isopod worldwide, with around 4,500 species found in marine environments, mostly on the seabed, 500 species in fresh water, and another 5,000 species on land. The order is divided into eleven suborders. The fossil record of isopods dates back to the Carboniferous period (in the Pennsylvanian epoch), at least 300 million years ago, when isopods lived in shallow seas. The name Isopoda is derived from the Greek roots iso- (from ἴσος ísos, meaning "equal") and -pod (from ποδ-, the stem of πούς poús, meaning "foot").

Māori Party

The Māori Party (Māori: Te Pāti Māori) is an indigenous rights-based centre-left political party in New Zealand. Tariana Turia founded the party in 2004 after resigning from the governing centre-left Labour Party, in which she was a minister, over the foreshore and seabed ownership controversy. She and Pita Sharples, a high-profile academic, became the first co-leaders.

The party won four Māori seats in the 2005 election and went into Opposition. After the 2008, 2011 and 2014 elections, where the party won five, three and two Māori seats respectively, it supported a government led by the centre-right National Party, with the co-leaders being ministers outside cabinet. Sharples resigned as male co-leader in 2013 and was replaced by Te Ururoa Flavell, who became a minister after the 2014 election. Turia retired in 2014 and was replaced by Marama Fox as female co-leader. The Māori Party failed to win any seats in the 2017 election and left Parliament.

New Zealand foreshore and seabed controversy

The New Zealand foreshore and seabed controversy is a debate in the politics of New Zealand. It concerns the ownership of the country's foreshore and seabed, with many Māori groups claiming that Māori have a rightful claim to title. These claims are based around historical possession and the Treaty of Waitangi. On 18 November 2004, the New Zealand Parliament passed a law which deems the title to be held by the Crown. This law, the Foreshore and Seabed Act 2004, was enacted on 24 November 2004. Some sections of the Act came into force on 17 January 2005. It was repealed and replaced by the Marine and Coastal Area (Takutai Moana) Act 2011.

Ocean bank

An ocean bank, sometimes referred to as a fishing bank or simply bank, is a part of the seabed which is shallow compared to its surrounding area, such as a shoal or the top of an underwater hill. Somewhat like continental slopes, ocean banks slopes can upwell as tidal and other flows intercept them, resulting sometimes in nutrient rich currents. Because of this, some large banks, such as Dogger Bank and the Grand Banks of Newfoundland, are among the richest fishing grounds in the world.

There are some banks that were reported in the 19th century by navigators, such as Wachusett Reef, whose existence is doubtful.

Offshore drilling

Offshore drilling is a mechanical process where a wellbore is drilled below the seabed. It is typically carried out in order to explore for and subsequently extract petroleum which lies in rock formations beneath the seabed. Most commonly, the term is used to describe drilling activities on the continental shelf, though the term can also be applied to drilling in lakes, inshore waters and inland seas.

Offshore drilling presents environmental challenges, both offshore and onshore from the produced hydrocarbons and the materials used during the drilling operation. Controversies include the ongoing U.S. offshore drilling debate.There are many different types of facilities from which offshore drilling operations take place. These include bottom founded drilling rigs (jackup barges and swamp barges), combined drilling and production facilities either bottom founded or floating platforms, and deepwater mobile offshore drilling units (MODU) including semi-submersibles and drillships. These are capable of operating in water depths up to 3,000 metres (9,800 ft). In shallower waters the mobile units are anchored to the seabed, however in deeper water (more than 1,500 metres (4,900 ft) the semi-submersibles or Drillships are maintained at the required drilling location using dynamic positioning.

Petronius (oil platform)

Petronius is a deepwater compliant tower oil platform operated by Chevron in the Gulf of Mexico, 210 km southeast of New Orleans, United States.

A compliant piled tower design, it is 609.9 metres (2,001 ft) high, and was arguably the tallest free-standing structure in the world, until surpassed by the Burj Khalifa in 2010, although this claim is disputed since only 75 metres of the platform are above water. The multi-deck topsides are 64 metres by 43 metres by 18.3 metres high and hold 21 well slots, and the entire structure weighs around 43,000 tons. Around 8,000 m3 (50,000 barrels) of oil and 2,000,000 m3 (70 million cubic feet) of natural gas are extracted daily by the platform.

The platform is situated to exploit the Petronius field, discovered in 1995 in Viosca Knoll (block VK 786) and named after Petronius, the Roman writer. The seabed is 535 m (1,754 ft) below the platform. The compliant tower design is more flexible than conventional land structures to cope better with sea forces. It can deflect (sway) in excess of 2% of height. Most buildings are kept to within 0.5% of height in order to have occupants not feel uneasy during periods of movement.

Construction began in 1997 by J Ray McDermott with the seabed mooring system. The contract for the platform was budgeted at $200 million with total costs of around $500 million. The 4000-tonne North Module was installed in November 1998, but the attempt to install the slightly lighter South Module in December of that year ended with the unit on the seabed. A replacement module was built and installed by Saipem 7000 in May 2000.

Riprap

Riprap, also known as rip rap, rip-rap, shot rock, rock armor, or rubble, is man-placed rock or other material used to armor shorelines, streambeds, bridge abutments, pilings and other shoreline structures against scour and water, wave, or ice erosion. Common rock types used include granite and modular concrete blocks. Rubble from building and paving demolition is sometimes used.

Riprap is also used underwater to cap immersed tubes sunken on the seabed to be joined into an undersea tunnel.

Seabed tractor

A seabed tractor belongs to the remotely operated underwater vehicle class, although their operations and the handling of these units are quite different. They can be put into use for submarine cable laying or completion of a bury operation. A typical operation involves the seabed tractor being lowered onto the seabed over the pipeline when the location has been confirmed. The weight of the tractor remains mostly on the gantry, being controlled by heave-compensation gear on the gantry. Only about 40 tonnes of the weight should rest on the seabed. The position reference of the vessel is now transferred to the trim-cube sensors on the seabed tractor support wires, which remain vertical. The position of the vessel is now being controlled by the movements of the tractor, with the trim¬cube feeding back wire angle data to the DP system. The DP, in turn, is correcting the position of the vessel in order to keep the tractor wires vertical. The DP system would be configured with the centre of rotation located on the trencher. Reading can thus be adjusted according to the environment or any other constraints.This type of vehicle consists of a tracked Crawler excavator device, built and configured to carry out the job required. It is controlled tended from the vessel by an umbilical cable. Operating seabed tractors show great similarity with the operation of Remotely operated underwater vehicles.

The control of the seabed tractor will be carried out from its control location, with the operator driving the unit as if they were on board. This can be achieved by using camera’s attached to the unit. These are also operated from the control location.

There are a lot of factors to be taken into account. For example, when laying a cable, the cable itself forms a hazards as it can get caught in the umbilical cable. Seabed tractors are often compared to a remotely operated underwater vehicle, but they usually move somewhat slower, being less agile. They can be used together with a remotely operated underwater vehicle to enhance the overview and to survey the progress and performance.

Star Canopus diving accident

The Star Canopus diving accident was an incident in Scotland in November 1978 that claimed the lives of two British commercial divers. During a routine dive beside the Beryl Alpha platform in the North Sea, the diving bell of the diving support vessel MS Star Canopus was lost when its main lift wire, life support umbilical, and guide wires were severed by an anchor chain of the semi-submersible Haakon Magnus. The bell dropped to the seabed at a depth of over 100 metres (330 ft). Its two occupants, 25-year-old Lothar Michael Ward and 28-year-old Gerard Anthony "Tony" Prangley, were unable to release the bell's drop weight in order to return to the surface because it was secured to the bell frame with secondary locking pins. Since there was not a bell stage to keep the bottom door of the bell off the seabed, the divers could not exit the bell to release the pins. Despite the efforts of three rescue vessels – Intersub 4, Tender Carrier, and Uncle John – the bell was not recovered for over thirteen hours, by which time Ward and Prangley had died of hypothermia and drowning.

Tidal race

Tidal race or tidal rapid is a natural occurrence whereby a fast-moving tide passes through a constriction, resulting in the formation of waves, eddies and hazardous currents. The constriction can be a passage where the sides narrow, for example the Gulf of Corryvreckan and the Saltstraumen maelstrom, or an underwater obstruction (a reef or rising seabed), such as is found at the Portland Race.

In extreme cases, such as Skookumchuck Narrows in British Columbia, through which tides can travel at more than 17 knots, very large whirlpools develop, which can be extremely hazardous to navigation.

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