Page 61 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

Tidal Energy Chapter | 3 53

FIG. 3.7 Standing wave shown at two time intervals (t = 0and t = T/2).

FIG. 3.8 Standing wave plotted along a channel at the time of HW at the coast. In this case,
maximum elevations occur, but there are zero currents along the channel. Nodes are plotted as filled
circles at 3L/4, L/4, etc.

zero (slack water). By contrast, 1/4 of a wave period later (Fig. 3.9), combination
of the incoming and reflected waves leads to zero elevations along the channel,
but maximum amplified currents, which occur at the nodes.
Because the currents at the nodes of a standing wave are double those of
a progressive wave (Eq. 3.6), the maximum current velocity at the nodes of a
standing wave is

g
V = R (3.6)
h
3.5 RESONANCE

Resonance occurs when a water body is forced at its natural period. If a sea has
a natural period of 12 h, it will resonate with this forcing, resulting in amplified
tides. Consider a channel which is exactly L/4 long; for example, this could be