COASTAL DEFENCE

BY,

SUMAN KUMAR

M-TECH (OENA)

IIT KHARAGPUR

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE,

INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

What is Coastal defence ?

• This refers to sea defence or coastal protection.

• Coastal defence is defense against flooding and erosion and techniques that allow erosion to claim land.

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE,

INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

Why is Coastal defence is Important?

• The economy growth of countries.

• Erosion threatening beaches or coastal settlements.

• Development of tourism.

• flooding if sea levels rise.

• Problem with sewage and/or pollution.

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE,

INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

Types of Approach for coastal defence:

• Hard Engineering Approach.

• Soft Engineering Approach.

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE,

INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

Hard Engineering:

• It involves the use of man made structures to defend against the erosive power of waves.

• Also known as the structural approach.

• Some structures used are gabions, groynes, sea walls, revetments and rip raps.

11/14/2014 DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE, INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

Soft Engineering:

• These are methods that try to work with nature to protect the coast.

• Beaches or naturally formed materials are used to control or re-direct erosion processes.

• Some coastal features used are sand dunes and mangroves. Beach replenishment is also a form of soft engineering.

• Also known as the non-structural approach.

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE,

INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

Hard Engineering

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE, INDIAN

INSTITUTE OF TECHNOLOGY (KHARAGPUR)

Breakwaters

• They help to protect the coast and harbour by reducing the force of high energy waves before they reach the waves

• They create a zone of calm water behind them

•However, materials deposited in the zones behind them are protected but in the zones located away from the breakwater they are not, and will be eroded away

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE,

INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

11/14/2014

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.

• 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.

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE,

INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

11/14/2014

Groynes

•Often described as fences built at right angles to the coastline

•The aim is to stop the movement of material along the beach due to longshore drift

•They are effective at reducing erosion in the area they are constructed in by causing significant build up of beach material.

•However, they deprive areas further down the coastline of material as they stop longshore drift

11/14/2014 DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE, INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

11/14/2014

Seawalls• Seawalls absorb the energy of waves

before they can erode away loose materials.

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

• They are effective in protecting cliffs from erosion.

• 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.

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE,

INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

Seawalls

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

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

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE,

INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

Rip-Raps/Rock Armour

•Huge boulders (10 tonnes min.) stacked on beaches for use as a sea wall

•Energy of the wave is dissipated effectively as water is let through the spaces between the boulders

•These boulders must be big enough to withstand being eroded themselves

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE,

INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

Revetment

•Wooden structures that break the force of waves

•Beach material builds up behind them

•Need replacing more frequently than other structures

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE,

INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

Soft Engineering

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE,

INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

Beach Replenishment

• This process involves the transportation of sand from elsewhere to be distributed along the beach.

• Environmentally, this is the preferred method as it keeps the beauty of the landscape of the beach.

• It is very costly and time consuming as regular replenishments are required.

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE, INDIAN

INSTITUTE OF TECHNOLOGY (KHARAGPUR)

Relocation of Property

• Coastal planners are trying to protect man-made structures by relocating them and let nature reclaim the beach slowly.

• No building of new structures is allowed in coastal areas vulnerable to coastal erosion.

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

• This approach is likely to be opposed by people who want to invest in the coastal areas.

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE,

INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

Planting of Mangroves

• Mangroves can help to trap sediments and reduce coastal erosion.

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

• E.g. Aceh – More than 5,000 mangrove seedlings were planted to rehabilitate coast devastated by 2004 tsunami.

• However some areas with violent destructive waves may not support mangroves.

• The depth of the coast may also become shallower, affecting coastal transportation and port activities.

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE,

INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

Stabilising Dunes

• Coastal dunes can be found along the shore where lots of sand and strong winds exist. The winds carry and deposit the land on the coast further inland, forming coastal dunes gradually which help defend the coast from the sea.

• However they are very fragile and thus access points to the beach are controlled and designated.

• Shrubs and trees are planted to stabilise the dunes to anchor the sand.

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE,

INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

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.

• These man-made reefs enhance fishing opportunities and serve as undersea barriers to reduce impact of wave energy.

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE,

INDIAN INSTITUTE OF TECHNOLOGY (KHARAGPUR)

11/14/2014DEPARTMENT OF OCEAN ENGINEERING AND NAVAL ARCHITECTURE, INDIAN INSTITUTE OF TECHNOLOGY

(KHARAGPUR)

11/14/2014