Coastal Systems - How Tides are Created

Tides are caused by the gravitational pull of the moon and the sun.

Tides are long-period waves that appear to move through the oceans due to the gravitational forces exerted by the moon and sun. Their apparent movement towards the coast creates a rise of the sea surface, though due to the earth’s rotation it is the coast rolling into a deeper bulge of ocean that creates the effect.

Where the sea surface rises to its highest point, this is known as high tide and at the lowest point it is known as low tide. The difference between the high and low tide is called the tidal range.

Coastal areas experience two high and two low tides every lunar day (approx. 24 hours & 50 minutes)

Where a section of the earth points towards the moon, then a high tide will occur as gravity pulls the ocean towards the moon. As well as creating the high tide there, the area on the opposite side of the earth, facing away from the moon will also have a high tide at the same time. This occurs as a result of inertia and centrifugal force, as the gravitational pull is weaker here and the ocean bulges out as a result. The areas at 90-degree angle to the moon at this time will experience low tides. As the Earth spins, different areas of the planet face the moon, and this rotation causes the tides to cycle around the planet.

As well as high and low tides, when the moon, earth and sun are in alignment this causes the gravitational pull to increase more than usual and creates a high spring tide. However, neap tides are lower than normal tides and they occur when the sun and moon are at right angles to the earth so the gravitational pull from both has a reduced impact. There are two spring tides each lunar month – at full moon and new moon when the sun, earth and moon are in a line, and two neap tides at quarter and three-quarter moons.

Tides and coastal erosion

Tides increase the rate of coastal erosion. Where tidal range is low, for example, in the Mediterranean, wave energy is less and many cliff faces are unaffected by marine processes.

However, in other places, such as the UK, where the tidal range is greater, this leads to an increase in erosion and can create landforms such as wave cut notches and platforms.