Sea Defences

The shape of Chichester Harbour is always changing. This is because the wind and the waves wear away the shoreline. This is called erosion.

The material that is worn away is carried off by the tide and current and deposited in other places. This is called deposition. Global warming is expected to cause a rise in sea levels and the South of England also has to face the problem that it is slowly sinking into the sea. This is because during the last ice age the weight of the ice that covered Scotland caused the land to be pressed down while the South of England was raised. Since the ice has melted Scotland has slowly been rising while the south of England has been sinking. Erosion and flooding is a serious issue because people live and work on the land that surrounds the harbour. Click here to view tide simulations and a possible future shape of the harbour.


Erosion

The wind and waves are not the only things that cause erosion around the harbour. Many of the boats that use the harbour could cause erosion by their ‘wash’ if there was not a speed limit of 8 knots in the harbour. Imagine throwing a stone into a pond. The stone causes a ripple to spread wider and wider until it reaches the edges of the pond. A moving boat does a very similar thing and if it is large and travels through the water quickly, its wash can cause quite large waves that eventually crash against the shoreline.

Humans and animals can also cause erosion. Some footpaths are used so often that they eventually crumble into the sea. Rabbits make large holes very quickly that can cause sections of shoreline to collapse.


Deposition

Although some parts of the shore are being eroded away, others are growing! This is due to deposition – the tide carries material up the shore and it is left behind when the tide goes out. The northern tip of East Head is getting bigger, while the Hinge is being washed away. Pilsea Island is also getting bigger. Deposition can affect the deep water channels in the harbour as well as access to marina and boat yards. Dredging (digging out the mud and moving it away) is used to keep channels deep enough, but it is expensive, affects the sea bed and the wildlife that lives there and is only a temporary solution as deposition continues. There is also the problem of where to put the material that is dredged up.


Sea Defences

It is not possible to protect every part of the shoreline from erosion and flooding. We have to make decisions about which are the necessary areas to protect and where nature can take its course.

Over the years the owners of the land around the Harbour shoreline have used many different structures to protect their land from erosion and flooding. Most of these can be described as ‘hard’ sea defences.

A more recent sea defence used in some parts of the harbour is called ‘rip rap’. This is made from large pieces of stone. Where it is necessary to put in sea defences it is probably the best one to use because it absorbs the energy from waves.

Groynes - during the 1840’s a ‘groyne field’ was built from Selsey to West Wittering. The shoreline was being heavily eroded by the large waves from the open sea and people wanted to protect these popular beaches. Groynes are timber barriers that stretch from the top of the beach down into the sea.

Sand and shingle is moved along the beach by ‘longshore drift’. The groynes slow this process down by trapping the material and building up the beaches. In the past groynes were always structures made of wood. Now they are often built from rock, as at Sandy Point, Hayling Island.

Gabions - are not only found protecting the shoreline. They are used inland too. A gabion is a rectangular cage or basket filled with stones and rocks. The gabions are ‘flexible’ because they stones are able to move a little. This allows the gabions to absorb the powerful energy of the waves. They are also ‘permeable’ which means that water will drain through them.

Rock Berm - The Hinge is a very narrow strip of land that connects East Head to the mainland. There has been so much erosion here that a line of rocks known as a ‘berm’ was built along the inside of The Hinge to stop a deep channel forming through this area and creating a new entrance to the harbour.


Natural Sea Defence

Chichester Harbour is prized for its natural landscape so it is not always best to use ‘hard’ sea defences. If sea levels are rising and if the high tides get even higher then habitats such as the mudflats, saltmarsh and shoreline beaches will disappear. This is known as ‘Coastal Squeeze’. This means that the habitats are squeezed between a fixed boundary of sea defences and the rising sea level, unless they can migrate landwards.

Saltmarsh is a very important habitat as well as a natural sea defence. One way of coping with rising sea levels is to create new saltmarsh on the landward side of the shoreline. The sea is allowed to flood pieces of land on the incoming tide and the land eventually becomes saltmarsh. This is called ‘Managed Realignment’. It is a good example of a ‘soft’ sea defence and very suitable for natural landscapes.

Back to top

Gabions being used as a sea defence

Erosion at the top of a groyne

Thornham Point - new saltmarsh was created here by allowing saltwater to flood the land at high tide

A rock berm was put in place at East Head to prevent the sea breaking through the Hing