Tides are caused by the gravitational pull of the moon and sun, and the rotation of the Earth. As the moon orbits the Earth, the tide moves around the Earth as a bulge in the ocean.

High tides occur as the water advances to its furthest extent on the shoreline. Low tide is when it recedes to its furthest extent. The difference between high and low tide is called the tidal range. The incoming tide is called the ‘flood tide’ and the outgoing tide is called the ‘ebb tide’.

Tidal range varies over time as the earth, moon and sun move relative to each other. The greatest tidal range occurs when the moon and sun are in alignment with the Earth and their gravitational pull is combined. These are called ‘spring tides’ and happen twice a month when there is a new or a full moon.

When the moon faces the Earth at a right angle to the sun, the gravitational forces are weakest. This creates the smallest tidal range, called a ‘neap tide’. Neap tides occur twice a month when the first and last quarter moons appear.

Most shorelines experience two high and two low tides within a twenty-four-hour period, though some areas have just one of each.

A coastline’s physical features, such as a wide sandy beach or a rocky cove, along with the depth of the water just offshore, affect the height of the tides.

The funnel shape of the Severn Estuary is unique in the UK and creates some of the highest tides on earth; the highest tide in the Severn Estuary is 14.7m (48ft), but storm-surges, caused by strong winds and low atmospheric pressure, can add 1-2m. The Great Flood of 1607 was caused when a high spring tide coincided with a low pressure storm.