Coastal erosion

subhash k km tech-iczm

Introduction• Shorelines are dynamic landforms. With every

passing wave, and each changing tide, the profile of the shoreline changes.

• Large scale development of coastal areas will bring about problems if development is not properly planned and managed.

• Proper coastal management and protection are needed to preserve quality of coastal environment.

Physical parameters- identify coastal erosion

• Coastal geomorphology• Wind• Waves• Tides• Vegetation

Erosion Processes• Abrasion• Scouring• Hydraulic Action• Solution• Attrition

IMPACTS OF COASTAL EROSION

• danger to life in the case of sudden onset landslide events

• destruction of buildings • destruction of infrastructure -water, sewage and

gas pipes,roads• loss of land, resulting in coastal cliffs or shorelines

retreating closer to other buildings• land instability at neighbouring slopes and

properties• loss of beach amenity due to cliff collapse or sea

wall construction

EROSION MITIGATIONTypes of Approach• Hard engineering• Soft Engineering

Types of Approach

• Hard engineering

- Refers to the construction of physical structures to defend against the erosive power of waves.

- Also known as the structural approach.

Types of Approach• Soft Engineering- Focuses on planning and management so

that both coastal areas and properties will not be damaged by erosion.

- Also aims to change individual behaviour or attitude towards coastal protection by encouraging minimal human interference and allowing nature to take its course.

- Also known as the non-structural approach.

Hard Engineering Measures• Seawalls (+)- Seawalls absorb the energy of waves

before they can erode away loose materials.

- most common form of shore protection structure

- Seawalls can be made of concrete, rocks or wood.

- They are effective in protecting cliffs from erosion.

Hard Engineering Measures• Seawalls (-)- They can only absorb the energy of

oncoming waves. They do not prevent the powerful backwash of refracted waves from washing away the beach materials beneath the walls.

- Base of seawalls will be undermined (weakened) and leads to their collapse

Hard Engineering Measures- On a high-energy site-stone riprap

should be placed in front of the wall to protect the toe of the wall from erosion.

- Seawalls are costly to build and maintain as constant repairs have to be made to prevent their collapse

Hard Engineering Measures• Breakwaters (+)

• Breakwaters

- Protect coast and harbour by reducing force of high energy waves before they reach the shore.

- They can either be built with one end attached to the coast or built away from coast.

Hard Engineering Measures• Breakwaters (-)

- Materials deposited in the zones behind the breakwater are protected while those in the zones located away from the breakwater are not.

- Zones will not receive any new supply and thus be eroded away.

DEPOSITION

EROSION

Hard Engineering Measures• Groynes

- Built at right angles to the shore to prevent longshore drift.

- These structures absorb or reduce the energy of the waves and cause materials to be deposited on the side of the groyne facing the longshore drift.

Hard Engineering Measures• Groynes (-)

- Erosion can still take place on the part of the coast that is not protected by groynes.

Hard Engineering Measures• Gabions

- Gabions are wire cages usually filled with crushed rocks, and then piled up along the shore to prevent or reduce coastal erosion by weakening wave energy.

Hard Engineering Measures• Gabions (-)

- This method offers only short-term protection (about five to ten years).

- Wire cages need regular maintenance as they are easily corroded by sea water.

- Affected by excessive trampling and vandalism.

Hard Engineering Measures• Gabions (-)

- Can be unsightly and become a danger along the beach.

Hard Engineering Measures

• Revetment

- The most effective type of shore protection

- Beach material builds up behind them- Need replacing more frequently than

other structures

Hard Engineering Measures

• Bulkheads- Used when deep water must be

maintained up to the shoreline for boating or shipping

- Constructed-either steel sheet piles or treated timbers

- These types of structures have very little resistance to wave action

Soft Engineering Measures• Beach nourishment

- Refers to constant replenishment of large quantities of sand to the beach system.

- Beach is therefore extended seawards, leading to improvement of both beach quality and storm protection.

Soft Engineering Measures• Beach nourishment (-)

- Very expensive and impractical to constantly transport large quantities of sand to fill up beach.

- Imported sand that can be continuously eroded and transported away could have serious consequences on wildlife living within coastal environment.

Soft Engineering Measures• Beach nourishment (-)

- E.g. Coral reefs at Waikiki Beach of Hawaii are destroyed as imported sand gets washed out to sea and covers the corals.

- Corals are deprived of sunlight they need to survive

Soft Engineering Measures• Beach nourishment (-)

- E.g. Singapore – Large scale land reclamation along coast has led to muddy and polluted water and has destroyed coral reefs.

Soft Engineering Measures• Relocation of Property

- Coastal planners protect man-made structures (e.g. buildings) by relocating them and letting nature reclaim the beach in its own time.

- No building of new properties of structures would be allowed in coastal areas that are vulnerable to coastal erosion.

Soft Engineering Measures• Relocation of Property

- E.g. England – “Green Line” policy which discourages any building beyond a given line. Any properties built beyond the given line will not be defended or protected should they be threatened by coastal erosion.

Soft Engineering Measures• Relocation of Property (-)

- Opposition by people with considerable investments in coastal areas.

- This approach will be important in future coastal management due to rising sea level as a result of global warming.

Soft Engineering Measures• Planting of Mangroves

- Mangroves have long and curved roots that prop up from soil. It traps sediments and reduce coastal erosion.

- Mangoves can secure and trap enough sediments to form small islands – extend coastal land seawards.

Soft Engineering Measures• Planting of Mangroves (-)

- Not all coastal regions can support mangroves, especially in coastal regions with destructive waves.

- As sediments build up along coast after planting mangroves, depth of coast may become shallower and thus affect port activities – (E.g. Pose a problem for countries like Singapore that depends heavily on maritime trade.)

Soft Engineering Measures• Growth of coral reefs

- Coral reefs weaken wave energy.

- Artificial reefs can be created along coast by placing environmentally friendly and durable materials such as steel or concrete on sea floor.

Another example of man made structure as coral reef

REFERENCES• by Jim DeStefano & John Roberge, 2004 August STRUCTURE magazine.• Concrete Shore Protection, Portland Cement Association, 33 West Grand Avenue, Chicago, Illinois,1955.•Designing for Bank Erosion Control with Vegetation, Knuston, P. L., Reprint 78-2, U. S. Army Coastal Engineering Research Center, February 1978, (NTIS #AO51 571).•Designing Retaining Walls, Bulkheads and Seawalls of Treated Timber, American Wood Preservers Institute

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