The Seychelles Government have been taking seriously the threat of coastal erosion for well over a decade now and only two weeks ago published a 5 year plan to continue to combat it.
“18/07/2019 · A major share of infrastructure, population and economic activities in Seychelles are located in the country’s coastal zone. In order to protect its coastline from ongoing erosion, the government of Seychelles has embarked on an ambitious and wide-reaching coastal management” Ministry of environment, Energy and Climate change 2019
This EarthCache looks at one of the existing actions that the Seychelles has undertaken to reduce the effects of coastal erosion and why the materials used have been chosen.
Port Glaud Island
There are many ways to combat the threat of coastal erosion. Which is the action of the sea in causing damage to the coast. i.e. cliff collapse or the washing away of beaches.
Hard Engineering
Hard Engineering involves building structures to protect the coastline.
Advantages and disadvantages of hard engineering schemes
- Sea Wall: Placed at the base of a cliff to reflect the waves energy. They are very expensive at approximately £10,000 per km, but extremely effective at protecting areas from flooding. Environmentally ugly.
- Gabions: Cages of wire filled with rocks to absorb the waves energy, they are effective and cheap but environmentally ugly.
- Groynes: Can be made of wood or rock and are long vertical structures placed at right angles to the beach to trap sediment. This builds up the beach and protects the cliffs from erosion. They are effective at building up the beach therefore protecting cliffs from wave attack. Can result in areas further down the coast being starved of beach material resulting in more erosion! Only last 25-30 years.
- Rip Rap (Rock armour): Large rocks placed at the bottom of the cliff or artificial spit to absorb the wave energy, they are effective at dispersing the waves energy and cheap. Environmentally they may appear ugly and may put off tourists.
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Soft Engineering
Soft Engineering is a less environmentally noticeable way of managing the coastline.
- Beach Nourishment: Large amounts of sand are added to beaches to build them up and help absorb wave energy. This protects tourism as well as the coast and is easy to carry out and fairly cheap. But it does not last very long as sand will continue to be transported along the coast by longshore drift.
- Managed Retreat: This allows the natural erosional processses of the sea to occur, areas of low value land are allowed to flood hopefully protecting more important areas further down the coast.
- Cliff Stabilisation: Cliffs are covered in matting and vegetation planted to help make them more stable and resistant to erosion.
- Do nothing: Obviously the cheapest and most environmentally friendly option! However, you have to weigh up the COSTS (to people, tourism and buildings) against the BENEFITS (advantages of letting the area return to its natural processes). If the costs greatly outweigh the benefits e.g. by having to rehome many people or losing valuable tourist facilities then other options will have to be looked at.
For the purposes of this EarthCache you will need to visit the published coordinates at low tide and answer some questions about the rock armour set up at the base of the cliff.
Types of Rocks
Sedimentary
Sedimentary rocks are formed on the surface of the Earth, either in water or on land. They are called secondary, because they often result from the accumulation of small pieces broken off from pre-existing rocks. Most sedimentary rocks become cemented together by minerals and chemicals present when they are formed, and others are held together by electrical attraction. Some, however, remain loose, crumbly and unconsolidated. Gravel is an overall name for large sediment grain size, which includes boulder, cobble, and pebble. Sand includes sediment grains ranging in size from 2mm to 0.625 mm. Silt is the name of a sediment grain that range in size from 0.625 mm to 0.0039 mm. Mud is the smallest grain size and is also known as clay. It is important to distinguish the grain size called clay from the mineral called clay. Clay sized grains are too small to see individually without the aid of a microscope. Layers are often seen as the sediments are dropped in cycles and disturbances between the cycles often causes a break in the rock formation and a new layer begins when it resumes. Fossils are another easy way to identify sedimentary rocks. Sandstone and limestone are examples of sedimentary rocks.
Igneous
Igneous rocks get their name from the Latin word ignis, meaning "fire." Under the surface of the Earth the magma is kept liquid by high temperature and high pressure. As the volcano erupts hot magma reaches the surface. Afterward the lava rapidly cools down and solidifies. The crystals formed by cooling magma are usually small. Magma doesn't always reach the surface. Sometimes it is trapped underground in pockets inside of other rocks. In this case the magma cools down more slowly forming larger crystals and coarse-grained rocks. How the rocks form will depend not only on the different cooling temperatures of the magma but also its chemical composition. Igneous rocks have individual crystals that are visible to the naked eye. Crystals can range from less than a millimeter to centimeters in diameter. Igneous rocks with a porphyritic texture show two or more distinct crystal sizes—coarse-grained crystals, visible to the naked eye, surrounded by fine-grained crystals. There would be no evidence of layers or foliation in an igneous rock. Visible crystals or phenocrysts are an easy way to identify an igneous rock. An example of an igneous rock would be granite.
Metamorphic
Sedimentary and igneous and metamorphic rocks which were subjected to intense pressure and/or heat and as a result underwent a complete change. Metamorphic rocks form deep within the Earth's crust. The process of metamorphism does not melt the rocks, but transforms them into other rocks which are denser and more compact. New minerals are created either by the rearrangement of a mineral's components or by reactions with fluids that enter the rocks. There are two distinctive metamorphic textures. The first is Foliation. This represents a distinct plane of weakness in the rock. Foliation is caused by the re-alignment of minerals when they are subjected to high pressure and temperature. Individual minerals align themselves perpendicular to the stress field such that their long axes are in the direction of these planes (which may look like the cleavage planes of minerals). Usually, a series of foliation planes can be seen parallel to each other in the rock. Well-developed foliation is characteristic of most metamorphic rocks. Metamorphic rocks often break easily along foliation planes. Slate or gneiss are good examples of this type of metamorphic rock. The second type is Granular. This describes a metamorphic rock consisting of interlocking equant crystals (granules), almost entirely of one mineral. A granular texture is developed if a rock's chemical composition is close to that of a particular mineral. This mineral will crystallise if the rock is subjected to high pressure and temperature. A granular texture is characteristic of some metamorphic rocks. Marble and schist are examples of this type of metamorphic rock. As the grade of metamorphism increases (more temperature and pressure), both crystal size and the coarseness of foliation increase. Therefore, gneiss represents more intense metamorphism (or a higher grade) than schist.
So to the questions: Please either email or message me the answers to the questions below. One can immediately log the cache as a find after the answers have been sent.
1) Where you are standing now is man made leading to what was once an island. Is this evidence of hard or soft engineering? And what type?
2) Two types of rock have been used here. Please name the types used also describing their texture, appearance and size. Are these rocks local?
3) Using your answer to Question 2? How do those characteristics make these rocks suitable to be rock armour?
4) Do you notice any fossils? If so, are they animal or vegetable?
5) Do you notice any other sea defences whilst standing at GZ ? If so, what is the compass bearing?
6) What is the evidence that this man-made sea defence is working?
Optional but desirable:
7) Please take a photo of yourself with the island behind you.
References:
www.gfdrr.org/en/publication/seychelles-coastal-management-plan,
www.geologypage.com/2017/03/types-of-rocks.html
https://en.wikipedia.org/wiki/Rip_Raps,
http://www.geography-site.co.uk/pages/physical/coastal/defences.html