Page 237 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

226 Fundamentals of Ocean Renewable Energy

FIG. 8.22 Simulated annual mean (2014) wave power distribution (in kW/m) for Galway Bay.
Arrows show mean wave direction at selected grid points.

by the principal semidiurnal lunar (M 2 ) and solar (S 2 ) constituents, take around
two-and-a-half hours to propagate around Orkney from the western to the
eastern approaches of the Pentland Firth (Fig. 8.24), leading to a considerable
phase lag across Orkney. This phase lag results in a strong pressure gradient
across Orkney, driving very strong tidal flows through the Pentland Firth and
along the Firths of Orkney. For example, the M 2 phase lag between the Atlantic
approach of Westray Firth and the North Sea approach of the connecting
6
Stronsay Firth (Fig. 8.23) is around 65 degrees, that is 2.25 h, and the resulting
flow is channelled through constrictions, which narrow to around 5 km, with
water depths in the range 25–50 m.

6. Recall from Section 3.3 that a 30-degrees phase lag in M 2 represents around 1 h.