Tidal Current Energy

Wave
crest
Wave
trough
Water
depth, h
Figure 8.1 . Wave definitions.
Wave
height, H
Wave Energy
Zero upcrossing
period, T z
Wavelength, l
Wave velocity
(Celerity), C
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kinetic energy of the subsurface water particle movements. It should be noted
that the wave motion is a moving energy packet and that the water particles do
not move with the wave. They are simply agitated when the waves arrive and
oscillate around some fixed position. Only the energy is transmitted through
the water. An important point to note, though, is that waves begin to lose their
energy as they come into shallower water near the shore.
It is possible to calculate this transfer of energy by the waves, which then represents
the power. A straightforward equation results which gives the power in
kW�m � 1 as follows:
Power per meter kW m
� HT ⋅
2 �1
Mean
water
level
where H is the height (meters) of the wave and T its period. The meter width is
measured along the wave crest shown in Figure 8.1 perpendicular to the wave
propagation direction. It must be noted that this is the time-averaged power
over a wave cycle. There is also a dimensional constant equal to 1 that is not
shown. The usual assumption for these monochromatic waves is that they are
of small height compared with the wavelength and are referred to as linear
waves. When the heights become large the theories must be modified to include
‘ non-linear’ terms in the description of the waves.
In the real ocean the situation described by monochromatic waves is not
usually true, as successive wave heights and wave periods vary. A typical water
surface is shown in Figure 8.2 .
In the case of real sea waves, each wave has a different wave height and
period, and so it is necessary to utilize some characteristic value to describe the
sea state at any particular time. These characteristic values are the significant