Tides

I love glaring at the coast, watching and listening to the splashes, water rushes in and out. One of my favourite climbing view is at Brean Down where the tidal range is the second largest in the world. What causes tides and why tidal range can differ significantly from place to place?

Formation

(captured from NOAA)

A short answer is the moon’s gravitational pull, which generates the tidal force. There are two tidal bulges, one on the side closest to the moon and the other on the opposite side, i.e. furthest from the moon.

Tidal force comes from differences in gravity over Earth’s surface. On the side closest to the moon, the moon’s gravitational force pulls water towards it, whereas on the side furthest from the moon, the moon’s gravitational pull is the weakest.

At new moon or full moon, when the Earth, moon and Sun line up, the lunar and solar gravitational pull reinforce each other and cause more extreme tides, known as spring tides.

Force due to moon’s gravitational pull (left) and tidal force: gravitational pull at a location-average gravitational pull over the whole Earth (right). (NOAA)

Tidal Range

Coastal tidal range vary widely across the globe, ranges from 0 to 16m. It is influenced by the volume of water near the coast and basin where the water sits in. When water is forced into narrow inlets and bays, there is nowhere for the water to spread out, so it can only move up and down throughout the day. The Bay of Fundy in Southeastern Canada has the world’s highest tides of 16m.

Bristol Channel, the second largest tidal range in world, is shaped like a funnel with the Atlantic Ocean directly to the west of it. The high ground and mountains to the south of Bristol and Wales are in favour of the prevailing westerly winds which further push the mass of water into the channel.

Brean Down during high tides (left) and during low tides. (photos taken Sept 2018)