3.1.3.2 Coastal Landscapes in the UK - Keyhaven Salt Marsh, Southampshire Case Study

Question 1

What is a salt marsh?

Answer 1

• Salt marshes are coastal wetlands that are flooded and drained bysaltwater brought in by the tides
• Salt marshes are diverse ecosystems that play an important role in protecting shoreline regions against coastal erosion, reducing flooding risk and stabilising shorelines

Question 2

How do salt marshes form?

Answer 2

1. Salt marshes only form in low energy environments where there is shelter from wind and waves
2. Depositional landforms such as spits can help to provide this shelter
3. The low energy environment located in waters behind the spit traps material being transported by the tides. This material accumulates to form a salt marsh
4. Over time, these areas will be colonised by plants and animals especially adapted to salty conditions
5. Salt water tolerant plants also trap material, allowing the salt marsh to expand and form new land

Question 3

Diagram showing formation of a salt marsh:

Answer 3

Question 4

Where is Keyhaven Salt Marsh located?

Answer 4

Keyhaven Salt Marsh is located in Hampshire on the southwest coast of England

Question 5

How did Keyhaven Salt Marsh form?

Answer 5

The salt marsh has formed in a low energy environment, provided by the shelter of Hurst Castle Spit

Question 6

What has Keyhaven Salt Marsh been recognised as and why?

Answer 6

• Keyhaven Salt Marsh has been recognised as a site of Special Scientific Interest (SSSI) due to its role in protecting the southwest coastline against erosion and flooding events
• The salt marsh’s ecosystem is also home to a diverse range of wildlife, including many species of marine animals that thrive off the salt water tolerant vegetation

Question 7

How does Keyhaven Salt Marsh protect the coastline?

Answer 7

• Keyhaven provide a natural defence against the erosion of the coastline
• Research has found that these diverse ecosystems are significantly effective in reducing waves and erosion, and in stabilising and growing shorelines
• However, there has been a conflict of opinion over just how important salt marshes are to the protection of the coastline

Question 8

Role of marsh plants at Keyhaven Salt Marsh in protecting the coastline:

Answer 8

• In Keyhaven, pioneer plants such as Cordgrass reduce the energy of waves as they hit the shoreline. This provides environmental benefits such as a reduction in coastal erosion as the water’s energy is dissipated as it flows around the tall cord grass
• Plants found in marshes can also help to stabilise growing shorelines. The stabilisation of land occurs when the tangled roots of Cordgrass and Sea Lavender trap material in the Keyhaven Salt Marsh and grow roots below the surface. The deposited roots raise the level of the stable surface. Raised areas of land provide protection to nearby houses from coastal erosion and flooding
• As the level of the mudflats rises, it is less frequently covered by sea water. The conditions become less harsh as rainwater begins to wash out some of the salt and decomposing plant matter improves the fertility of the newly forming soil. This environmental benefit creates new land to act as a barrier to coastal erosion.

Question 9

Limitations of Keyhaven Salt marsh in protecting the coastline:

Answer 9

• Due to rising sea-levels erosion waves are hitting Keyhaven Marsh with strong force. As such, the salt marsh has been retreating by up to 6m per year, reducing its effectiveness and importance to the protection of the coastline.
• A sea wall was built in the 1980s to offer further protection from coastal erosion and flooding. The Environment Agency (EA) have stated that the sea wall at Keyhaven is a very important flood defence structure and provides social benefits to the area such as protecting the community and their homes.
• In 1996 £5 million was spent using rock armour and beach nourishment to increase the height and width ofHurst Castlespit.
• The EA argue that these coastal management schemes offer the region a more robust and reliable form of protection against coastal erosion.
• Human use of salt marsh areas varies but can include tourism in areas designated as nature reserves. In recent years the Keyhaven Salt Marsh has been eroded by tourists, reducing the ecosystem’s effectiveness at protecting the coastline.