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.

Lunga and the Grey Dog
View to Lunga from Scarba, across Scotland's "Grey Dog" tidal race.

Notable tidal races

See also

  • Gut (coastal geography) – A narrow coastal body of water, a channel or strait, usually one that is subject to strong tidal currents, or a small creek
  • Rip current – Narrow current of water which moves directly away from the shore, cutting through the lines of breaking waves
  • Tidal bore – A water wave traveling upstream a river or narrow bay due to incoming tide


  1. ^ "Nature"
  2. ^ The-Bitches.co.uk
  3. ^ UK Riversguidebook
  4. ^ CanoeWales Archived 2014-09-11 at the Wayback Machine
  5. ^ "Bitches and Tides"
Current (stream)

A current, in a river or stream, is the flow of water influenced by gravity as the water moves downhill to reduce its potential energy. The current varies spatially as well as temporally within the stream, dependent upon the flow volume of water, stream gradient, and channel geometry. In tidal zones, the current in rivers and streams may reverse on the flood tide before resuming on the ebb tide.

Dalton's law

In chemistry and physics, Dalton's law (also called Dalton's law of partial pressures) states that in a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases. This empirical law was observed by John Dalton in 1801 and published in 1802. and is related to the ideal gas laws.

Dursey Island

Dursey Island (Irish: Baoi Bhéarra or Oileán Baoi) lies at the southwestern tip of the Beara Peninsula in the west of County Cork in Ireland. Dursey Island is 6.5 kilometres long and 1.5 kilometres wide. The island is separated from the mainland by a narrow stretch of water, Dursey Sound, which has a very strong tidal race, with the submerged Flag Rock close to the centre of the channel. The island has just six or so permanent residents, and is connected to the mainland by Ireland's only cable car. Dursey has no shops, pubs or restaurants. At one point there was a post office on the island; this has now closed

Falls of Lora

The Falls of Lora is a tidal race which forms at the mouth of Loch Etive when a particularly high tide runs out from the loch. They form white water rapids for two to five days either side of the spring tides.The falls of Lora are generated when the water level in the Firth of Lorn (i.e. the open sea) drops below the level of the water in Loch Etive as the tide goes out. As the seawater in Loch Etive pours out through the narrow mouth of the loch, it passes over a rocky shelf which causes the rapids to form. As the tide rises again there is a period of slack water when the levels are the same on either side. However, due to the narrow entrance to the Loch, the tide rises more quickly than the water can flow into the Loch. Thus there is still considerable turbulence at high tide caused by flow into the Loch. Thus, unlike most situations where slack water is at high and low tides, in the case of the Falls of Lora slack water occurs when the levels on either side are the same, not when the tidal change is at its least. As a result, the tidal range is much greater on the coast than it is inside the loch. A 3 metres (9.8 ft) range at Oban may produce only a 1.3 metres (4.3 ft) at Bonawe on the loch shore.The loch mouth is spanned by Connel Bridge.

The race is popular with white water kayakers and divers as well as tourists and photographers.

Gut (coastal geography)

A gut is a narrow coastal body of water, a channel or strait, usually one that is subject to strong tidal currents flowing back and forth. A gut may also be a small creek.

Holm of Skaw

The Holm of Skaw is a small islet off the northeast coast of the island of Unst.

It is just northeast of the settlement of Skaw.

The island is 57 feet (17 m) in height.

There is a lighthouse on the island.

Tidal currents are slack between Holm of Skaw and Herma Ness at high water, and the passage may be made by small boats.

The Skaw Röst, a dangerous tidal race, forms off the shore of the Holm of Skaw and Lamba Ness.

Inhaca Island

Inhaca Island (Ilha da Inhaca in Portuguese) is a subtropical island of Mozambique off the East African coast.

The 52 km2 (20 sq mi) island separates Maputo Bay (Baía de Maputo) to the west from the Indian Ocean off its eastern shores. The island's irregular coastline approaches mainland Machangulo Peninsula at Ponta Torres where a 500-metre-wide (1,600 ft) tidal race separates the two headlands. In administrative terms Inhaca is a municipal district of the municipality of Maputo, while the Machangulo peninsula is included under the Lubombo Transfrontier Conservation Area and is part of the district of Matutuíne, Maputo Province.

Jack Sound

Jack Sound is a treacherous body of water about 800 metres (2,600 ft) wide between the island of Skomer and the Pembrokeshire mainland that contains numerous reefs and a tidal race of up to 6 knots.

It is used by boats to avoid a three-mile detour around the island, and has its share of sunken shipwrecks.

The most popular wreck for divers is The Lucy, which sank in good condition in 1967 after being abandoned by its crew owing to its cargo of calcium carbide.The sound is part of the Skomer Marine Conservation Zone.

Lunga (Slate Islands)

Lunga is one of the Slate Islands in the Firth of Lorn in Argyll and Bute, Scotland. The "Grey Dog" tidal race, which runs in the sea channel to the south, reaches 8 knots in full flood. The name 'Lunga' is derived from the Old Norse for 'isle of the longships', but almost all other place names are Gaelic in origin. The population was never substantial and today the main activity is an adventure centre on the northern headland of Rubha Fiola. The surrounding seas are fished for prawns and scallops and there is a salmon farm off the south eastern shores. The Special Area of Conservation of which the island is part hosts a growing number of outdoor leisure pursuits.

NCI Portland Bill

NCI Portland Bill is a National Coastwatch Institution (NCI) lookout station on the Isle of Portland, Dorset, England. The station is situated 50 metres above sea level on the cliff edge, half a mile north of the tip of Portland Bill. It is located close to the Old Higher Lighthouse.

The coast of Portland Bill has been notorious for the many shipwrecks over the centuries. The dangerous coastline has proven more hazardous due to the strong tidal race known as the Portland Race. Portland Bill remains important as a way-point for coastal traffic.


Normocapnia or normocarbia is a state of normal arterial carbon dioxide pressure, usually about 40 mmHg.

Pierre le Pelley III

Pierre le Pelley III, 15th Seigneur of Sark (1799–1839) was Seigneur of Sark from 1820 to 1839. He drowned when the boat carrying him to Guernsey was lost in a tidal race just off the coast of Sark.

Portland Bill

Portland Bill is a narrow promontory (or bill) at the southern end of the Isle of Portland, and the southernmost point of Dorset, England. One of Portland's most popular destinations is Portland Bill Lighthouse. Portland's coast has been notorious for the number of shipwrecked vessels over the centuries. The dangerous coastline features shallow reefs and the Shambles sandbank, made more hazardous due to the strong Portland tidal race.The Bill is an important way-point for coastal traffic, and three lighthouses have been built to protect shipping. The original two worked as a pair from 1716, and they were replaced in 1906 by the current one.

Stratification (water)

Water stratification is when water masses with different properties - salinity (halocline), oxygenation (chemocline), density (pycnocline), temperature (thermocline) - form layers that act as barriers to water mixing which could lead to anoxia or euxinia. These layers are normally arranged according to density, with the least dense water masses sitting above the more dense layers.

Water stratification also creates barriers to nutrient mixing between layers. This can affect the primary production in an area by limiting photosynthetic processes. When nutrients from the benthos cannot travel up into the photic zone, phytoplankton may be limited by nutrient availability. Lower primary production also leads to lower net productivity in waters.

The Bitches

The Bitches are a tidal race and set of rocks between Ramsey Island and the west Welsh coastline near St Davids. They are a popular tourist destination and a playspot for extreme sports enthusiasts such as whitewater kayakers and surfers.

The Swinge

The Swinge is the strait between Alderney and Burhou in the Channel Islands. It often sees a furious tidal race (the Alderney Race), and Braye Harbour which faces it, has a mile long breakwater to cope with this.

The etymology of the Swinge is probably Old Norse, related to Old Icelandic swinnr (swift, rapid)Corbet Rock lies in the Swinge. Corbet Rock is said to have been named after the ancient Corbet family of the Channel Islands.

The Little Swinge is between Burhou and Les Nannels. The Race is the strait on the other side of Alderney.

Wave base

The wave base, in physical oceanography, is the maximum depth at which a water wave's passage causes significant water motion. For water depths deeper than the wave base, bottom sediments and the seafloor are no longer stirred by the wave motion above.

Waves and shallow water

When waves travel into areas of shallow water, they begin to be affected by the ocean bottom. The free orbital motion of the water is disrupted, and water particles in orbital motion no longer return to their original position. As the water becomes shallower, the swell becomes higher and steeper, ultimately assuming the familiar sharp-crested wave shape. After the wave breaks, it becomes a wave of translation and erosion of the ocean bottom intensifies.

Ocean zones
Sea level

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