Sea level and Chichester Harbour

Our coastline has changed a lot over the past 2 million years. Sea levels have risen and fallen as the Earth's climate has changed.

Each time there is an Ice Age, lots of the Earth's water is frozen, and is trapped as ice (on land or floating on the sea) causing sea levels to fall. When the Earth's climate warms up, ice melts and ends up back in the sea, causing sea levels to rise.

The Chichester Harbour area has changed from open coastline, to dry grassy valleys with freshwater springs 40 km from the sea, to the channel and mudflat harbour that we see today.

Go to the Landscape Timeline for more information on the changes to the coastline in the past.

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Power of the tide, washing away and depositing material

Even without sea level change, the power of the tide coming in twice each day is strong enough to change the coastline. Sand, mud, rocks and pebbles are moved by the sea from one part of the harbour to another. Longshore drift happens along the coast, moving sand and stones from East to West in our area.

Go to Coastal Processes page to find out more

Channels get silted up and need to be dredged to allow boats to continue to get through them.
Comparing old maritime charts with today's charts, we can see some changes to the harbour shoreline, especially at East Head.

Go to Maritime charts page to explore the harbour with a chart that a sailor would have used 200 years ago!

East Head – the spit of land at East Head has moved 90 degrees, from West to North.

This map shows how East Head has changed in shape and direction since 1786 until the present.

Go to the East Head section in “Places” for further information or try our Virtual Field Trip

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Erosion – evidence from groynes

Land at the edge of the harbour is lost by the washing action of the water each time there is a very high tide. This is called “erosion”. This is usually a very slow process within the harbour, as the shoreline is protected from the full force of the sea. But over the years the shoreline has moved inland. We can see this erosion by comparing the position of the groynes near Copperas Point to the present shoreline 20 metres away.

People don't want their land to be washed away, so they have tried to protect it with sea defences.

Sea defences – There are many different types of sea defences. 

Sometimes the land is heavily protected from the sea; sometimes erosion is allowed to happen naturally.
In the past, areas of saltmarsh and sea have been reclaimed for use as farmland, such as at Thorney, Bosham, Nutbourne and Cobnor. Much of this land was claimed back by the sea, sometimes leaving posts and other defences behind.

The North part of Thorney has remained as land, linking it to the mainland.

You can walk on embankments that protect farmland from the sea in many parts of the harbour, including Chidham and Fishbourne.

To see the location of defences in the Harbour go to the interactive map.

For more information on types of sea defences look at the Coastal Processes page

Coastal Squeeze and Managed Realignment

Global warming and the gradual sinking of the South of England is leading to increased sea levels and loss of valuable habitats. In areas with fixed “hard” sea defences such as sea walls, saltmarsh plants cannot move inland to escape rising sea levels as they would if there were no hard sea defences. They are underwater for too long and die. This means that the area of saltmarsh gets smaller, this is called “Coastal Squeeze”. As well as loss of an important habitat, loss of saltmarsh along the edge of the water increases the erosion of sea defences by waves and tidal current. Waves bounce off hard defences much more than saltmarsh and this increases erosion.

Managed Realignment is an alternative to “hard” sea defences. This involves building new sea walls further inland and allowing the sea to flow through gaps in the (old) sea wall, allowing new areas of saltmarsh to develop. This relaxes the problem of “Coastal Squeeze”, but does rely on landowners being willing to allow some of their land to be reclaimed by the sea.

You can see examples of Managed Realignment at Thornham Point and West Chidham here

or take a ride on the Solar Boat virtual field trip

View An Example of a Model of Coastal Managed Realignment

Talking points

In the future we may face rising sea levels and flooding.

Visit the tide simulation section on main website, which predicts tide levels in 2050.

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If you had to plan how to look after Chichester Harbour in the next 50 years, what would you do?

Would you use sea defences or let natural processes happen?

What about the cost of sea defences and the cost of allowing flooding?

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