The hydraulic jump is the most commonly used choice of design engineers for energy dissipation below spillways and outlets. A properly designed hydraulic jump can provide for 60-70% energy dissipation of the energy in the basin itself, limiting the damage to structures and the streambed. Even with such efficient energy dissipation, stilling basins must be carefully designed to avoid serious damage due to uplift, vibration, cavitation, and abrasion. An extensive literature has been developed for this type of engineering.

Recreational

While travelling down river, kayaking and canoeing paddlers will often stop and playboat in standing waves and hydraulic jumps. The standing waves and shock fronts of hydraulic jumps make for popular locations for such recreation.

Similarly, kayakers and surfers have been known to ride tidal bores up rivers.

==References and notes

reflist

Adapted from the Wikipedia article Hydraulic jump, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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According to the Einstein field equation, the gravitational field is locally coupled not only to the distribution of non-gravitational mass-energy, but also to the distribution of momentum and stress (e.g. pressure, viscous stresses in a perfect fluid).

In the case of test particles in a vacuum solution or electrovacuum solution, this turns out to imply that in addition to the tidal acceleration experienced by small clouds of test particles (spinning or not), ”spinning” test particles may experience additional accelerations due to spin-spin forces.

Adapted from the Wikipedia article Test particle, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Although technological advances

in instrumentation and modeling have produced greater knowledge of internal tide and near-inertial wave generation, Garrett and Kunze (2007) observed 33 years later that “The fate of the radiated [large-scale internal tides] is still uncertain. They may scatter into [smaller scale waves] on further encounter with islands

or the rough seafloor

, or transfer their energy to smaller-scale internal waves in the ocean interior

” or “break on distant continental slopes

”.

It is now known that most of the internal tide energy generated at tall, steep midocean topography radiates away as large-scale internal waves. This radiated internal tide energy is one of the main sources of energy into the deep ocean, roughly half of the wind energy input

. Broader interest in internal tides is spurred by their impact on the magnitude and spatial inhomogeneity of mixing, which in turn has first order effect on the meridional overturning circulation

.

The internal tidal energy in one tidal period going through an area perpendicular to the direction of propagation is called the energy flux and is measured in Watts/m^2. The energy flux at one point can be summed over depth- this is the depth-integrated energy flux and is measured in Watts/m. The Hawaiian Ridge produces depth-integrated energy fluxes as large as 10& kW/m. The longest wavelength waves are the fastest and thus carry most of the energy flux. Near Hawaii, the typical wavelength of the longest internal tide is about 150& km while the next longest is about 75& km. These waves are called mode 1 and mode 2, respectively. Although Fig. 1 shows there is no sea surface expression of the internal tide, there actually is a displacement of a few centimeters. These sea surface expressions of the internal tide at different wavelengths can be detected with the Topex/Poseidon or Jason-1 satellites (Fig. 2).

Near 15 N, 175 W on the Line Islands Ridge, the mode-1 internal tides scatter off the topography, possibly creating turbulence and mixing, and producing smaller wavelength mode 2 internal tides.

The inescapable conclusion is that energy is lost from the surface tide to the internal tide at midocean topography and continental shelves, but the energy in the internal tide is not necessarily lost in the same place. Internal tides may propagate thousands of kilometers or more before breaking and mixing the abyssal ocean.

Adapted from the Wikipedia article Internal tide, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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The tidal bulges on Earth are carried ahead of the Earth–Moon axis by a small amount as a result of the Earth’s rotation. This is a direct consequence of friction and the dissipation of energy as water moves over the ocean bottom and into or out of bays and estuaries. Each bulge exerts a small amount of gravitational attraction on the Moon, with the bulge closest to the Moon pulling in a direction slightly forward along the Moon’s orbit, because the Earth’s rotation has carried the bulge forward. The opposing bulge has the opposite effect, but the closer bulge dominates due to its comparative closer distance to the Moon. As a result, some of the Earth’s rotational momentum is gradually being transferred to the Moon’s orbital momentum, and this causes the Moon to slowly recede from Earth at the rate of approximately 38& millimetres per year. In keeping with the conservation of angular momentum, the Earth’s rotation is gradually slowing, and the Earth’s day thus lengthens by about 17 microseconds every year. (This would make each Earth day one second longer every 60,000 years or so, and one minute longer every four million years). Looking back, the day was 15 minutes shorter when the dinosaurs roamed the Earth 65 million years ago.) See tidal acceleration for a more detailed description and references.

The Moon is gradually receding from the Earth into a higher orbit, and calculations suggest that this would continue for about fifty billion years. By that time, the Earth and Moon would become caught up in what is called a “spin–orbit resonance” in which the Moon will circle the Earth in about 47 days (currently 29 days), and both Moon and Earth would rotate around their axes in the same time, always facing each other with the same side. However, the slowdown of the Earth’s rotation is not occurring fast enough for the rotation to lengthen to a month before other effects change the situation: about 2.1 billion years from now, the increase of the Sun’s radiation will have caused the Earth’s oceans to vaporize, removing the bulk of the tidal friction and acceleration.

Adapted from the Wikipedia article Orbit of the Moon, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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On December 18, 2007 the Pacific Gas & Electric Company, the largest national utility company, announced a commercial agreement to purchase power generated by wave energy. This decision was made in part to be competitive in the public electrical energy market in the state of California under stringent renewable energy restrictions. Currently, California state law requires that publicly owned utilities are required to generate 20% of their electricity from renewable energy sources such as wind, solar and wave power by 2010. After the General Election on November 4, 2008 this law may be subject to change to an even more stringent law, which states that publicly owned utilities would be required to increase their proportion of electricity from renewable resources to 20% by 2010, 40% by 2020 and 50% by 2025.

Federally, under the Marine Renewable Energy Research and Development Act of 2007 the United States has committed $200 million in federal funds toward wave energy technology to be allocated from 2008 through 2012. The United States Department of Energy (DOE) is currently responsible for the allocation of $50 million per fiscal year for research, development, demonstration and commercial application of ocean energy. In 2008, the first year of federal allocation toward wave energy, there are a total of fourteen recipients. The most notable recipients of this year include Oregon State University and the University of Hawaii. Oregon State University in partnership with the University of Washington, will implement the development of the Northwest National Marine Renewable Energy Center for wave and tidal energy. The second recipient, University of Hawaii will develop and implement the National Renewable Marine Energy Center in Hawaii.

The Grays Harbor Ocean Energy Company of Seattle has applied to the Federal Energy Regulatory Commission for permits to harness energy from waves off the coastline of California, Hawaii, Massachusetts, New Jersey, New York and Rhode Island. The $28 billion project would be the largest renewable energy project in the nation.

Adapted from the Wikipedia article Wave farm, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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The Barrage was not supported in the 2003 Energy Review due to “strong environmental concerns” (The same paper also described nuclear power as “an unattractive option”).

The RSPB opposes any Severn Barrage because of the effect it will have the feeding grounds 85,000 birds depend on, stating “The impact a barrage would have is huge. This is one of the most important sites in the UK for wild birds and the chances of them surviving if it went ahead are fairly slim. There wouldn’t be enough room left for all the birds and there wouldn’t be enough food for those that remained. The estuary is one of the UK’s most important sites for water birds and its wildlife value must be taken fully into account.”

The present strong tidal currents in the estuary serve to lift up silt sediment and so keep the water thick with fine particles. This blocks light-penetration and means that the Severn Estuary marine environment is actually a relative desert, in terms of both plant and fish life.

The barrage will not create a “lagoon” – as both opponents and supporters have sometimes claimed. Tidal power stations by definition require that the tide flows through the barrage, but the tidal range in the Severn would be halved. There are claims that the migration of fish would be hampered, but these are contested. The Severn bore would also be weakened or eliminated. Any barrage would be likely to stimulate coastal erosion in some areas, and create a negative visual impact upon the landscape (subjective, similar to wind turbines). There would also be negative consequences of the huge amount of concrete (and other materials) needed, with the quarrying of stone likely to impact on other areas.

DEFRA claims that the environmental effects of the barrage still need more analysis before final conclusions can be drawn. The Sustainable Development Commission is investigating UK tidal resources, including tidal power in the Severn Estuary and its environmental impact, and should report mid-2007.

Tidal lagoon alternative

Friends of the Earth support the idea of tidal power, but oppose barrages because of the environmental impact. They have proposed their own plans based on the concept of ”tidal lagoons”,

whereby man-made lagoons in the estuary would fill and drain through turbines. Their proposals would include lagoons covering up to 60% of the area covered by the barrage, which in some smaller configurations would not impound water in the ecologically sensitive inter-tidal areas of the estuary. The lagoons could be sub-divided so power would be generated at more states of the tide than a barrage, with lower peak output, giving economic advantages to set against the higher construction cost of longer barriers. This idea is based on a prototype now being designed at Swansea bay. However leading figures in the construction industry are sceptical that the lagoons can be economic.

A set of Tidal lagoons known as the “Russell Lagoon concept” were studied and dismissed by the 1981 Bondi Committee report, rejected on the grounds of both economics and environmental damage. Studies suggested that tidal currents around and between the lagoons would become extremely fierce and damaging.

Effects of different site locations

One of the complicating factors in assessing the impacts of a barrage is the large number of possible locations and sizes for the barrage. Generally, the larger the barrage the bigger its environmental impact, and the bigger the amount of energy it could create – and therefore the bigger carbon offset it could have by way of its renewable power generation.

The largest barrages (sited beyond Hinkley Point and towards Minehead on the English side and Aberthaw on the Welsh side) would significantly affect the entire Severn Estuary and much of the Bristol Channel, but could generate 15& GW peak power and protect the whole of the Somerset levels against flooding and sea-level rise caused by Global Warming. The smallest barrages (sited at Aust/Chepstow) would affect only the river and estuary in Gloucestershire, but would also only generate perhaps 0.75& GW peak power.
Adapted from the Wikipedia article Severn Barrage, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Eday Partnership, the local development trust is active in promoting the island’s economy and has instigated numerous projects, including Eday Heritage Centre, and the purchase of a new diesel tank for the island. A community-owned wind turbine is planned. Eday’s various community projects contributed £380,000 to the island’s economy from 2005-7.

In July 2008 the island celebrated the opening of the Eday Heritage and Visitor Centre in the restored former Baptist Church. There is a heritage display area, a permanent archive, a café and a tourist information point. The Eday Oral History Project, which records life on the island in the past, is also housed within the centre.

Adapted from the Wikipedia article Eday, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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If you waited until a white dwarf or neutron star was sufficiently cold, and if you had a rocket which could survive the enormous gravitational and tidal forces, you could land on the surface of the star. Typical cooling times for white dwarfs to become black dwarfs however are much larger than the present age of the Universe. Similarly for the hypothetical iron star.

Adapted from the Wikipedia article Compact star, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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In 2004 the British Wind Energy Association expanded its remit to include wave and tidal energy, and to use the Association’s experience to guide these technologies along the same path to commercialisation. In December 2009, to reflect this expansion in the industries it represented, members resolved to adopted the new name of RenewableUK.

Adapted from the Wikipedia article RenewableUK, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Adapted from the Wikipedia article Aquanator, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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