by

Abhishek Mondal (12NA30002)&

Sagar Patnaik (12NA30018)

COASTAL PROTECTION

Importance of Coast

Tourism

Sports like sailing, surfing, diving etc

Fishing

Natural resources like petroleum, natural gas

Facilitation of industries

Transport

Ecosystem

Coastal Protection

• Defence against flooding and erosion

• Aimed at protection against coastline retreat

Need of Coastal Protection

• Erosion: Many coastlines are being eroded by stronger storms and also rising seal levels

• Tropical storms: Increasing frequency and strength of storms causes flooding, storm surges and wind damage

Breakwater

Gabions

Groynes

Revetments

Sea Wall

Beach Nourishment

Dune Stabilization

Marshland

Mangroves

Coral Reefs

COASTAL PROTECTION TECHNIQUES

Hard Engineering Soft Engineering

HARD ENGINEERING• Protects coastal settlements along the coastline

• Deflects the power of waves

Advantages • Effective protection of coastal reefs

• Used in the areas where space is limited

• Lasts for a longer period

Disadvantages• Building and maintenance cost is high

• Poor aesthetics violates natural beauty of beach

• Protection of a stretch of coastline against longshore drift

• Installed at both onshore (Fixed) & offshore (Floating)

• Usually made of granite

Advantages

• Reduces power of waves to prevent erosion

• Influences longshore transport of sediment

• Acts as multipurpose artificial reefs

Disadvantages

• Relatively difficult to build

• Vulnerable to strong wave action

BREAKWATER

Types of Breakwater

Rubble Mound Breakwater

Caisson Breakwater

Porous Breakwater

Floating Breakwater

Submerged active Breakwater

Flexible Breakwater

Vertical Wall Breakwater

Armour Units Plays a massive role in construction of

breakwater

Different in shape and size e.g Tetrapod, Dolos, Sealock etc.

• Strong metal cage filled with pebbles and stones

• Protects the coastline by stopping the waves hitting the cliffs

Advantages

• Made of natural materials

• Cheap to construct

Disadvantages

• Easily damaged by powerful storm waves

• Cages tend to rust quickly

GABIONS

• Straight barriers perpendicular to the coastline

• Slows down longshore drift

Advantages• Prevents the movement of beach

material along the coast.

• Forms beaches, a natural defence against erosion and an attraction for tourists

• Easy to construct

Disadvantages• Induces local scour at the toes of structures

• Many structures required instead of one

• Costly to build and maintain

GROYNES

• Sloping structures placed along the base of a cliff

• Prevents the cliff from being eroded

Advantages

• Catches sediment from long shore drift to build up a beaches

• Low maintenance cost

Disadvantages

• Expensive to build

• Doesn’t protect the beach against flooding

REVETMENTS

• Sloped and curved concrete walls

• Breaks up the energy of waves and prevent water going over the top of the wall during storms

Advantages

• Lasts for around 30 years

• Protects the foot of the cliff from erosion

• Prevents flooding

Disadvantages• Expensive to build

• Makes the back swash very strong which erodes the beach quickly

SEA WALL

SOFT ENGINEERING

• Uses natural systems for coastal defence

• Maintains fundamental structures and natural topography of the coast

Advantages • Relatively less expensive

• Retention of the original form and fundamental structure of the landforms

• Long term and sustainable with less impact on the environment

Disadvantages • Need for regular maintenance

• Less likely to be effective against extreme storm events

BEACH NOURISHMENT

Replaces beach or cliff material removed by erosion or longshore drift

Natural defence against erosion and coastal flooding

Relatively inexpensive option but requires constant maintenance

Attracts tourists

Central Boca Raton Beach Nourishment Project

DUNE BUILDING Dunes form due to vegetation trapping and sand

stabilising Dunes extend several kilometers inland and to

height more than 100 meters

REVEGETATION Mangrove trees have prop roots that bind the loose

soil and protect it from erosion Marshland controls the area of action of waves to

reduce wave speed and height Coral reef structure buffers shorelines against

waves, storms and floods, helping to prevent loss of life and erosion

Mangrove Trees, Sundarban Dead tree plantation on beach

Marshland Coral Reef

PHYSICAL ASPECTS OF COASTAL PROTECTION

The motive behind coastal protection is to break the energy of waves, thus reducing loss of lives and property damage. Thus basic mathematical description of waves play an important role in coastal protection. Waves are categorized into two main types -

Travelling Wave: A wave in which the positions of maximum and minimum amplitude travel through the medium. Motion of water particles are elliptical.

Wave Equation

Ƞ = A Cos(kx – ωt + φ)Wavelength (λ) = 2π/k

Time Period (T) = 2π/ω

Celerity (c ) = ω/k

Group Velocity (Cg) = dω/dk = (c/2)(1+(2kh/sinh(2kh)))

• Group velocity is velocity of propagation of energy

Standing Wave : A wave in which each point on the axis of the wave has an associated constant amplitude. Minimum amplitude occurs at nodes and maximum amplitude occurs antinodes.

• Standing wave do not travel horizontally.

• Stationary waves have nodes where there is no vertical displacement at any time.

• In between the nodes are positions called antinodes, where the displacement has maximum amplitude.

• Water particles move horizontally beneath node and vertically beneath antinode.

• Linear wave oscillating with fundamental mode of resonating frequency in a closed basin.

• Node at the center

• Water under the node moves only horizontally, while water at both ends of the basin moves vertically.

• Same linear wave oscillating with fundamental mode of resonating frequency in a basin open to the deep ocean.

• Node at connection of the basin to the deep ocean

• Water particle under the node moves only horizontally, while water at the end of the basin moves vertically.

WAVE CONTROL

Reflection : When a wave advances towards a barrier with angle θ to the normal structure surface, it’s reflected back with the same angle. Therefore the direction of wave propagation can be changed by varying the angle of structure’s front face. Wave height can be changed by changing the reflection coefficient (K r = Ar /Ai) of the structure.

Clapotis : When θ = 0, reflected

waves follows the same path of

incident wave and a standing wave

pattern is formed, called “clapotis”. Full

clapotis is observed when Kr = 1 but in

reality Kr < 1 and a partial clapotis is

formed where wave envelope contains some vertical motion at nodes.

Reflection Coefficient

Kr = (ar / ai) = (amax – amin )/(amax + amin )

Where

ai = Amplitude of incident wave

ar = Amplitude of reflected wave

amin = Minimum amplitude of partial clapotis = (a i – ar)

amax = Maximum amplitude of partial clapotis = (a i + ar)

Refraction : A wave obliquely advancing to the current has the nature of changing its propagation direction.

As per Snell’s Law –

Sin(θ1)/C1 = Sin(θ2)/C2

• C1 , C2 are wave celerities at depth h1 & h2

and θ1 , θ2 are incidence angle and refraction

angle respectively.

• Wave height can be controlled by changing

Transmission coefficient (Kt ) of the mediums.

Phase Change : Wave heights can be controlled by changing phase among the incident, reflected and transmitted waves. Valembois (1953) first attempted to decay waves using the resonance phenomena.

In case of Destructive Interference wave height reduces drastically when two or more wave interfere out of phase. In case of Constructive Interference wave height increases as wave interfere in phase.

Wave Breaking : Since wave energy is proportional to the square of the wave height, wave height can be attenuated by converting a major part of wave energy into sound energy and the energy dissipated by rigid vertical wall.

Types of wave breaker very often depends on slope(m)of sea-bottom. SpillingH/λ >= 0.14 tanh (2πh/λ) for wave breaking to take place. Plunging

Surf similarity parameter ξ = m/(Hb/λb)1/2

where Hb = breaking waveheight, λb = breaking wavelength Naturally there’re four types of wave breaking viz. • Spilling ( 0 < ξ < 0.4 ) Collapsing• Plunging ( 0.4 < ξ < 2 )• Collapsing ( ξ = 2 ) • Surging ( ξ > 2 ) Surging

THANK YOU