Page 67 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

Tidal Energy Chapter | 3 59

where H n is the amplitude, g n is the phase lag (e.g. relative to Greenwich), and
ω n is the angular speed of the constituent.
If we are interested in predicting how the surface elevation η varies over
time, then
η n = H n cos (ω n t − g n ) (3.18)
Through observations and/or numerical simulations, we can obtain values
for H and g, and we know the angular speeds of the various constituents (M2,
S2, K1, O1, etc.), and so we have the basis for making a tidal prediction.
At the port of Holyhead (Fig. 3.6), information on the four principal tidal
constituents can be obtained from Admiralty tide tables (Table 3.1). Taking a
timestep of 15 min, we can easily predict the elevation time series at Holyhead
for an arbitrary start time (Fig. 3.12). This time series shows a spring-neap
cycle (due to combination of the M2 and S2 semidiurnal constituents), and

TABLE 3.1 Tidal Data for Holyhead

Speed (degrees
Constituent Name H (m) g (degrees) per solar hour)
M2 Principal lunar 1.81 292 28.984
semidiurnal
S2 Principal solar 0.59 329 30.000
semidiurnal
K1 Lunisolar diurnal 0.11 177 15.041
O1 Lunar diurnal 0.10 029 13.943

FIG. 3.12 Predicted elevation time series at the port of Holyhead, based on a limited number of
tidal constituents (Table 3.1).