Scotland's Pentland Firth might generate 10 gigawatts from tides alone
Water squeezed through a narrow strait moves with enormous force, and unlike wind or waves, tides arrive on a predictable schedule. Estimates suggest turbines in Scotland's Pentland Firth could produce around 10 gigawatts, while New Zealand's Cook Strait might yield 5.6 gigawatts from a flow carrying about 15 in total.
A strait is a channel of water linking two seas or basins, usually squeezed between two landmasses. The water surface sits at roughly the same height at both ends, so flow can run either way or switch, although some straits carry one dominant current. That is what separates them from rivers and most canals, whose flow follows a drop in elevation. Geographically, a strait is the mirror image of an isthmus: one joins two oceans between lands, the other joins two lands between oceans. Names vary, including channel, passage and, in Scotland, firth or kyle, and a few straits are even called canals, such as Pearse Canal.
The constricted current shapes the seabed. Sand tends to pile up at both exits, building underwater fans or deltas. Some straits cannot be navigated at all because they are too narrow or shallow, or blocked by reefs or clusters of islands.
Because they concentrate shipping, straits carry huge economic weight, and control of them has been fought over. International law therefore sets special rules. Where a strait used for international navigation lies within a country's territorial waters, which extend 12 nautical miles from shore, ships enjoy transit passage that cannot be suspended, as through the Strait of Gibraltar or the Strait of Dover. Elsewhere the stricter regime of innocent passage applies, allowing ships through but banning fishing, research, espionage and similar activities, and subjecting them to the coastal state's safety and environmental rules. Innocent passage also lets vessels take shortcuts between an island and the mainland, as in the Strait of Messina.
Straits even lend their names to wildlife. Cook Strait gives its name to a giant weta, whose scientific name means wrinkled terrible grasshopper, and to a kōwhai tree described only in 2001 and named after the scientist Brian Molloy.
Source: Strait
