Tidal currents

The interesting behavior of tidal currents in open offshore waters is due to the lack of physical boundaries. A tidal current here tends to move about a point in a rotary-type current. Therefore, tliis type of current will tend to move any wastes discharged offshore in an elliptictil pattern on which may be superimposed a net current drift. 

Bi-yden, I. G. Naik, S. and Fraenkel, P. (1998). Matching Tidal Current Plants to Local Flow Conditions. Energy 23 699-709. 

Data after Fontana and Greene source International Nickel Co. t Immersed in tidal current. t Immersed in sea-water flume. 

A dock had to be built where the water temperature is rarely above 40°F (5°C) the tides are 30 ft (9m) high there is a tidal current of 6 or 7 knots (1 1-13 km/h) and the water is so turbid that the divers must be guided mainly by touch. The docks also had to be protected by ice-breakers. These are built into the harbor to prevent large icebergs from hitting the dock before they are reduced into smaller pieces. 

Tidal energy There are two different technologies tidal barriers and tidal currents. 

Tidal-current (or stream) power is derived from water turbines submerged in the wide expanse of a tidal flow or current there is no constructed barrier. Such a turbine is, therefore, the water-equivalent of a wind turbine. As yet, there are no commercial tidal-current power plants. 

In the rivers, high concentrations of NP in sediment were observed [25] over relatively long stretches of about 10 km downstream from a sewage effluent discharge point (compared to elevated concentrations over a stretch of only 1 km length in a similar study conducted in the Detroit river [27]). Owing to the tidal current the horizontal mixing of surface sediments is possibly more extensive than in nontidal areas [25]. 

As shown in Illustrative Examples 19.3 and 19.4, often it is not immediately known whether an exchange process is controlled by transport across a boundary layer or by transport in the bulk phase. In Illustrative Example 19.3 we look at the case of resuspension of particles from the polluted sediments of Boston Harbor. We are interested in the question of what fraction of the pollutants sorbed to the particles (such as polychlorinated biphenyls) can diffuse into the open water column while the particles are resuspended due to turbulence produced by tidal currents in the bay. To answer this question we need to assess the possible role of the boundary layer around the particles. 

The time, t, required for the passage of water across the Campbellrand platform is uncertain. The modern Bahamas are probably a reasonable analogue for the Campbellrand platform. On the Bahama Banks tidal currents of 25 cm s are common, and velocities of 1 m s have been recorded in channels (SeUwood, 1986). At a rate of 25 cm s it would have taken seawater —1 month (4 X 10" min) to traverse the —800 km diameter of the Campbellrand platform. This period is much longer than the time required to precipitate Fe oxyhydroxide after the oxidation of Fe to Fe (Grundl and Delwiche, 1993). The best estimate of the residence time of seawater on the Grand BahamaBankis —1 yr(Morseefa/., 1984 Millero, personal communication). The pH of the solutions from which the iron formations were deposited was probably less than that of seawater today, but probably not lower than 7.0. 

In considering the effect of river inflows on corals in the bay, we focused on soil grains rather than salinity because the effect of low salinity due to inflows from the rivers is smaller and shorter than that of soil grains. In considering the effect of oceanic mechanical force on corals in the bay, we focused on wave height rather than tidal current because tidal full and residual currents have a smaller effect on corals than does wave height. 

Water masses on the earth always have a tendency to align their surfaces perpendicular to the direction of gravity. However, because of the enormous mass of the oceans, gravity disturbances cause oscillations forcing the water particles to move on trajectories that are nearly horizontal to their normal position. These horizontal movements, the tidal currents, have the same periodicities as the rising and falhng of the water level, which is caused by the attraction and withdrawal of water masses. 

The temporal evolution of tides in a particular area does not necessarily follow local gravity disturbances. However, as gravity disturbances everywhere depend exclusively on the apparent position of the tide-generating celestial bodies relative to the earth, it is possible to compute the tides and tidal currents at any location rather precisely from the apparent motions of the moon and sun. 

Wind and density induced currents near the bottom temporarily are very weak. If there were no tidal currents, which are weak but are always ( ) present oxygen depletion in the near-bottom layers of the Baltic Sea probably would occur more frequently. Land uplift and subsidence play a major role in current studies of sea level rise. However, when evaluating gauge data, also the tides have to be taken into account, especially in historical datasets (Liebsch, 1997),... 

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