Page 66 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

58 Fundamentals of Ocean Renewable Energy

FIG. 3.11 Propagation of Kevin waves in an idealized rectangular basin in the northern
hemisphere [1].

to the east). This deflection of flow continues until the flow reaches the right-
hand boundary (i.e. coastline), where the build-up of water leads to a pressure
gradient. A Kelvin wave propagates as a result of the balance between these two
forces: Coriolis and pressure gradient. Standing oscillating waves are a special
case of interest. When a Kelvin wave enters a basin, it will be reflected at the
head of the basin; therefore, the tidal wave in a basin can be represented by
two Kelvin waves travelling in opposite directions (Fig. 3.11). These waves
rotate around a node, which is called an amphidrome. The cotidal contour lines
radiate outwards from the amphidrome, and tidal amplitude is zero at this node.
Considering an idealized rectangular basin and using the earlier concept, it
is easy to demonstrate why, in the northern hemisphere, the direction of the
rotation around an amphidrome is anticlockwise.
As an example, consider the propagation of tidal waves in the northwest
European shelf seas (e.g. [2]). The Atlantic semidiurnal Kelvin wave travels
towards the north and transfers the tidal energy into the Celtic Sea between
Brittany and Southern Ireland (Fig. 3.6). Part of this energy propagates into the
English Channel and the Irish Sea. The Atlantic Kelvin wave further progresses
northwards and deflects towards the east (due to Coriolis), travels to the North
of Shetland, and enters the North Sea. Tides in the North Sea are primarily
semidiurnal, with two amphidromes in the southern North Sea and a third
around the southern tip of Norway. The south travelling wave, which moves
along the east coast of the United Kingdom, generates the largest amplitudes in
the North Sea.

3.8 TIDAL ANALYSIS AND PREDICTION
Tidal analysis and prediction are based on the assumption that the tidal signal
can be represented by a finite number N of harmonic terms of the form
H n cos (ω n t − g n ) (3.17)