Presentation on theme: "Coastal Management Lesson Objective: Understand why methods of engineering to protect the coast have advantages and disadvantages."— Presentation transcript:
Coastal Management Lesson Objective: Understand why methods of engineering to protect the coast have advantages and disadvantages
MANAGING THE COASTLINE “Hundreds of thousands of tonnes of chalk fell 500ft to the beach on Sunday morning, in what is thought to have been the biggest sudden loss of coastline in Britain in living memory.” “Beach hit by 200 tonne cliff collapse” “Cliff collapse in Egypt leaves 24 dead” “Beach closes after cliff collapse” “Holbeck Hall Hotel crashes into the sea” “My house is only worth £1!”
Coasts : Coastal Protection / Hard and Soft Engineering Strategies Possible Questions : Why might certain coastal protection methods be selected instead of others? What is the difference between Hard and Soft coastal protection techniques? There are many different ways to reduce the rate of coastal erosion. Some are more expensive than others, some last longer, some are less of an eye- sore..etc. There are 2 main categories of coastal protection: HARD Engineering Strategies : Building or creating something which will interfere with coastal processes – usually to reduce the power of breaking waves against cliffs. SOFT Engineering Strategies : Working with the natural processes of sea and sand in a more environmentally sustainable way. Using the natural processes to bring about an intended effect. 17
How do groynes work? Groynes trap the sand which is carried along the coast by longshore drift. The sand acts as a natural protection against the force of the waves. The waves break onto the beach and not the cliffs.
Groynes These are built at right angles to the coastline. They aim to slow down longshore drift and trap sediment. They can be made of timber, concrete or rock. Wood groynes are around £5,000 each. Rock groynes cost around £200,000 each. Advantages: More beach material to dissipate wave energy which slows down cliff erosion. Cheap in comparison to other hard engineering methods Disadvantages: Beaches further along the coast are starved of beach material due to their affect on long shore drift.
What are the disadvantages of groynes? In 1991 a rock revetment and two rock groynes were built. Sand accumulated and halted erosion. South of Mappleton, the rate of erosion has increased significantly. Material that usually moves south via longshore drift is becoming trapped within the groynes. Now there is no beach to protect the cliffs - the sea reaches the base of the soft cliffs and erosion occurs. Mappleton is located on the Holderness Coastline.
Sea Walls Sea walls aim to prevent erosion by providing a barrier. They reflect the seas energy. The walls are placed parallel to the shore. They are usually expensive and can cost up to £5000 per square metre. Advantages: Provide excellent defence where wave energy is high They reassure the public They have a long life span. Disadvantages: Expensive Ugly- can put off tourists They can make it harder to access the beach Recurved sea walls cause greater erosion at the base of the wall
What problems do sea walls create? Why might a sea wall cause more coastal erosion?
Rip rap/ Rock Armour These are often large boulders placed along the base of a cliff to absorb energy from waves. They cost about £3000 per metre Advantages: Cheap and efficient Disadvantages: Ugly Dangerous access to beach Costs increase when rock is imported
Gabions These are steel wire baskets filled with rock and built into an eroding cliff to improve stability They are fairly cheap, costing about £350 per metre. Advantages: Cheap Fairly effective Disadvantages: Unattractive They cannot withstand strong waves
Revetments Traditionally these have been wooden slatted barriers constructed towards the rear of beaches to protect the base of cliffs. Energy from waves is dissipated by them breaking against the revetments. Cost: £2000 per metre Advantages: Less beach material is eroded compared to a sea wall Cheaper and less intrusive than a sea wall Disadvantages: Short life span Unsuitable where wave energy is high
Offshore reef Enormous concrete and natural blocks are sunk offshore to alter the wave direction Cost £1,950 per metre Advantages The waves break further offshore so reduce erosive power The allow the build up of sand Disadvantages May be reduced by heavy storms Difficult to install
Beach Replenishment Beach replenishment is also known as beach nourishment. Beach nourishment is the process of dumping or pumping sand from elsewhere onto an eroding shoreline to create a new beach or to widen the existing beach. Beach nourishment does not stop erosion, it simply gives the erosional forces (usually waves) something else to "chew on" for awhile. The waves erode the nourished sand instead of destroying houses, roads or parking lots. Because nourishment doesn't stop erosion, nourishment must be repeated to maintain the beach.
Beach nourishment/ repenishment Beaches are built up, usually by pumping sand onto the beach The source of the material is usually from offshore dredging. This has to be undertaken on a regular basis (once every five, or less, years). Pumping material can be expensive (at least £1,250 per cubic metre) unless it is a by-product of channel dredging. Advantages: Retains the natural appearance of the beach Disadvantages: Off shore dredging of sand and shingle increases erosion in other areas and affects ecosystems. Large storms will require beach replenishment, increasing costs. Can be expensive, depending on the source of the sand
Managed retreat This is when areas of coast are allowed to erode. This is usually in areas where the land is of low value. Cost: depends on amount of compensation that needs to be paid to people affected by erosion. Advantages: Managed retreat retains the natural balance of the coastal system. Eroded material encourages the development of beaches and salt marshes. Disadvantages: People lose their livelihood e.g. farmers. These people will need to be compensated.
Cliff regrading Making the cliff face longer so that it is less steep Advantages: Cheap Natural- will encourage wildlife back into the area Disadvantages: Not effective on its own- need other defences at the base of the cliff Some homes on the cliff may need to be demolished
What may influence decision Choosing which methods of coastal protection is most appropriate for an area of coastline may take into account the following: COST : concrete sea wall is expensive. Wood revetments are cheaper LIFETIME : rock groynes may last decades. Beach rebuilding will have to be carried out every few months ENVIRONMENTAL IMPACT : offshore reefs will not affect the look of the beach but concrete will look awful and put tourists off using the coastline SUSTAINABILITY : using hardwood from tropical rainforests for timber groynes is not sustainable – they will deteriorate and need replacing faster than new hardwood trees grow. Beach rebuilding is sustainable as you are moving sand from where it has been deposited, to where it has been eroded from.
How can the coastline be protected? Hard engineering techniques aim to stop coastal processes from occurring. Soft engineering methods try to work with nature to protect the coast.
Match the following statements to your pictures of hard and soft engineering worksheet. Write in pencil if you are unsure! If you finish, use page 58/59 of the textbook to add more detail.
How does it work?AdvantagesDisadvantages Wooden structures break the force of the waves and trap beach material behind them. Much cheaper than a sea wall. Effective at breaking the force of the waves. Less durable than a sea wall – may need replacing quicker. Does not give total protection to the base of the cliff. Cost approx £1000 per metre Large boulders on the beach – lesson the force of the waves by absorbing the wave energy within the gaps between the rocks. Relatively cheap. Use natural material/rocks Environmentally ugly. Can be removed by the waves due to the washing away of sand and shingle beneath. Build up the beach by replenishing beach material, particularly at the base of structures to provide a ‘natural’ solution to absorbing wave energy. Provides a natural solution. Natural looking – meaning it is difficult to tell management is taking place. Can be expensive to keep transporting large amount of sand – sediment moved by longshore drfit so will need frequent replenishment unless used with other defences. Concrete wall, curved under side to deflect the power of the waves. They reflect rather than absorb wave energy. Most effective means of preventing erosion. Most expensive (up to £2.5 million/km). Deflected waves often scour the base, undermining it – it may collapse in the future. Cages of boulders built into the cliff face – small rocks help to absorb the wave energy. Effective where severe erosion and cheaper than sea walls. Environmentally ugly (usually used in large numbers) Cost is approx £350 per metre. Wooden or in some cases steel structures that stop longshore drift and build up/ anchor the beach, protecting the base of a cliff. Stops longshore drift encouraging the build up of the beach and effectively reducing erosion. Can increase erosion further down the coast by stopping longshore drift and starving areas further down the coast of sediment. Cost approx £5-6000 each.
Links to Holderness case study http://www.youtube.com/watch?v=AG9eO C-ErO0http://www.youtube.com/watch?v=AG9eO C-ErO0
Consider…. 1. Why are settlements defended while agricultural land is left to the actions of the sea? 2. Some people are for coastal protection schemes and others are against. Explain these different opinions.