Coastal Protection Aspects of Intake-outfall Structures-print

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Coastal protection aspects of intake‐outfall structures

Prof. S.A. Sannasiraj

Prof. V. SundarDepartment of Ocean Engineering, IIT Madras

Options for Outfall systems

• Shoreline discharge system

• Offshore discharge system (Tunnel/ Pipeline)

Coastal Processes

• Typical beach profile and coastal zone- Beaches dissipate wave energy and are constantly adjusting to the

wave environment (shoaling, wave breaking, sand bar & surf zone)

• Littoral Transport (sediment transport)Littoral Transport (sediment transport)- Long shore transport (parallel to the shoreline, long shore current)- Offshore-onshore transport (perpendicular to the shoreline)

Basic controls of a coastal system

Energy

Coastal h lMaterials

WavesTidesWind

morphologyMaterials

HumanManagement

SedimentsVegetation

Shore protection structures

DredgingIntake/ Outfall

systems

TERMS DESCRIBING COASTAL ZONE

Deep watersIntermediate waters Shallow waterSurf zone

2A

2B

A>>B A>B A=B

Breaking of

Wave Length Decreases

MECHANICS OF SEDIMENT TRANSPORT

Breaking of waves

Waves feel the sea bed.Particle orbit do not close particularly for steeper wave.Mass transport velocity dominates and transports the sediments.

(Sedimentsthrown into suspension)

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SURF ZONE

PATH OF SAND GRAINS

CREST OF BERM

Line of Breakers

CURRENTSLONGSHORE

MECHANICS OF SEDIMENT TRANSPORT (PLAN)

WAVE ATTACK

SAND GRAINS OUTSIDE

OBLIQUE

SURF ZONEPATH OF

xy

tQtxq

Longshore net volume change

txxQtQ / Cross-shore net volume change

Water Level Datum

toq

tQ

Q

Bd

cd

tq Total volume change CB DDyxV

= txqtxxQ /

0t

01

qx

Q

DDt

y

CB

ONE-D Shoreline Evolution Model

KRAUS and HARIKAI (1983) proposed a numerical scheme to solve the one line model using Crank Nicholson implicit finite difference method. The non-dimensional equation of shoreline is,

CQQBy

22

,,,1,

1,11,1,

tntntntnn

ntntntn

yqxQQBCandx

tBwhere

CQQBy

0

45

90270

315

0

45

90270

315

Wave rose diagram - December

10

90

135

180

225

270

0% 5% 10% 15% 20%

90

135

180

225

270

0 1 2 3 4

Hs (m)

0

45

90270

315

N

0 20 40 60 80 100

11

135

180

225

Maximum Wave height exceedance during an annual year

Month No. of days exceeding

Hmax < 1.5m 1.5m < Hmax < 2.0m 2.0m > Hmax

January 12.75 9.75 8.5

February 18.5 7.25 2.25

March 27.25 3.75 0

April 19.25 9.5 1.25

May 12.5 14.25 4.25

June 18.5 10.25 1.25

12

Ju e 8 5 0 5 5

July 9 10.5 11.5

August 16.5 10 4.5

September 17.25 9.75 3

October 10.5 8.5 12

November 3.5 6 20.5

December 8.75 10.75 11

Annualyear

174.25 110.25 80.0

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Nearshore wave propagation

Either

Spectral Models

WAM, SWAN, WaveWatchIII

Phase resolving Models (for diffraction problems)

Mild-slope equation (MSE)

ARTIFICIAL LATERAL BOUNDARY

INCIDENT WAVE - BOUNDARY CONDITION :

incos(1iKx

(0,0)

Y-AXIS

X AXIS

Mild Slope Equation

BOUNDARY CONDITION : 0iKn

SHORELINE – BOUNDARY CONDITION : 0iKn

SHORLEINE

= is the reflection coefficient0 for full reflection

1 for full absorption

X-AXIS

RECTANGULAR DOMAIN OF DISCRIETIZATION

Sediment transport models

• Suspension & bedload movement

• Longshore sediment transport• Longshore sediment transport

• Cross‐shore sediment transport

ANY INTRUSION IN THE COASTAL ZONE AFFECT THE BEHAVIOUR IN LITTORAL ZONE

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A Coast is said to be eroding when the loss of material due to various reasons exceed the material supplied to it.

Need for protection:

COASTAL EROSION

Causes are due to Nature or Man made

Intake/ OutfallA protrusionRemoval of materialHard obstruction (submerged)

OPTIONS FOR COASTAL PROTECTION

• DO NOTHING

• REMOVE THE CAUSES FOR COASTAL EROSION

Best Solution (Always not feasible)

• SUPPLY SEDIMENTS TO THE AFFECTED AREA

• REDUCE LOADS

By constructing breakwaters in front of the coast

• INCREASE THE STRENGTH

By constructing shore defense structures ( hard measure)

At the seaward side of the dune, seaward of the dune base [C]

Replenishment of coast with sand.At the landward side of the dune [A]At the seaward side of the dune, landward of the dune base [B]

SOFT MEASURES

[seawalls]Parallel to the coastline on shore

Normal to the coastline [groins]

HARD MEASURES

Sea

Deposition Groins

Wave direction

Erosion

REMEDIAL MEASURES FOR COASTAL EROSION

Natural !!

Bioshield

Structural response NonStructural response

base [C][ ]

Offshore breakwater

Land

Sea

Seawall

Sea

Land

Parallel to the coastal offshore break waters

Wave direction

C B A

Geomorphology

Material characteristics and sources

Tides, Winds and Storms

Waves and Currents

COASTAL PROTECTION WORKSPhysical factors needed for selection

Shoreline details

Bathymetry

Littoral drift

Revetment Training walls/ Jetty

Seawalls Groins

COASTAL PROTECTION WORKS

Types

Bulkheads Offshore breakwaters

Artificial beach nourishmentBio-shields

REVETMENTS

• To improve stability of slope p y pof shores

• Riprap revetment

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Topsoil and seed0.5m min

Elev. 2.7m

Gravel Blanket 0.3m thickOver Regraded bank

Poured Concrete

2

1Existing Beach

Quarrystone(Armour)

QUARRYSTONE REVETMENT

Elev. ‐0.3 m

Over Regraded bankExisting Beach

Elev. 0.0m MSL


Seawalls Groins


Types



Constructed parallel to shoreline

Separates land from water area

PRIMARY PURPOSE

Seawalls : To protect areas in the rear of the beach

SEAWALLS AND BULKHEADS

pagainst the effect of heavy storm action

Bulkheads : To prevent sliding of the land with secondary purpose of affording protection to the backshore against damage by wave action

BELGIUMHOLLANDGREAT BRITAINDENMARKGERMANY

HOLLANDDENMARKGERMANY

SPAIN ITALY

EUROPE

TEXASGREAT BRITAIN

SEA LEVELSEA LEVEL

SEA LEVEL

SEA LEVEL

PALM BEACH JUPITER ISLANDMIAMI BEACH

SLOPING WALLSRUBBLE MOUNDS

ABSORBLNGVERTICAL WALLS

NON ENERGY - ABSORBING

TYPES OF SEAWALLS

GREAT BRITAINHOLLANDGERMANY

GREAT BRITAINHOLLANDGERMANY

SCANDINAVIA

GREATBRITAINSCANDINAVIAPORTUGAL

GREAT BRITAINFRANCE

GREAT BRITAINSPAIN

SEA LEVEL

SEA LEVEL

SEA LEVEL

SEA LEVEL

SEA LEVELSEA LEVEL

SEA LEVEL

SEA LEVEL

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SEA WALLS

Generally Massive and expensive

Should be considered onlywhere the adjoining shore is highly developed andstorm attack is severe.

Cross-shore sediment movement is dominant

Along-shore sediment movement causes toe erosion


Seawalls Groins


Types



GROINS

Constructed usually perpendicular to theshore

Extends from a point landward of possibleshoreline recession into the water beyondbreaker zone

Helps building up of beach in addition

Classification

Permeable and impermeable

High or low

Fixed or adjustable

p g pas shore protection measure

SHORELINEGROIN ADJUSTED

BEACH GROIN ADJUSTED SHORELINE

ORIGINAL SHORELINE

SHORELINE EVOLUTION DUE TO A GROIN

DIRECTION OF NET LONGSHORE TRANSPORTOCEAN

Wave direction

Deposition

Erosion

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Shoreline advancement

Groin

SHORELINE CHANGES DUE TO GROINS

Littoral Drift

Shoreline recession

BEACH

GROINS EROSION DEPOSIT1ON

INITIAL SHORELINE

SHORELINE EVOLUTION DUE TO A GROIN FIELDSHORELINE EVOLUTION DUE TO A GROIN FIELD

DIRECTION OF NET LONGSHORE TRANSPORTWave direction

Timber, Steel, Stone, Concrete

Material usedDeposition or shoreline advance towards the sea(on its updrift side)

Erosion or shoreline recession towards the land (on its downdrift side)

Groin 3

Groin 4

Groin 5Groin 6

Groin 7

View of existing groins 2 to 7 at Ratchakan street, Kanyakumari

Groin 2

Training Walls• Perpendicular to shoreline• If solid has influence on the shoreline changes

Types

• Solid(Walls on four sides with earth field)• Open (Piled)

Downdrift (erosion area protected with groins)

Updrift (advancement of shoreline)

Jetty

Approach channel


Seawalls Groins


Types



As a shore protection measureParallel to shorelineLocations where longshore drift

is significantIncreasingly adopted compared to other

protection measures in Japan

OFF SHORE BREAKWATERSOFF SHORE BREAKWATERS

Types ExposedSubmerged

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DETACHEDBREAKWATER

BreakwaterWave direction

Initial shoreline

Updrift erosion Downdrift erosion

Salient

Wave direction

DEFINITION SKETCHOF SEGMENTEDDETACHEDBREAKWATER

Breakwater

SalientTombolo

Erosion DepositionInitial shoreline

Tombolo

Breakwater


Seawalls Groins


Types



Merits

ARTIFICIAL BEACH NOURISHMENTUsed without any shore protection measures

Used in combination with other shore protection measures

It satisfies the basic need of the material demand and have all the characteristics of a natural beach

It increases the stability of not only the beach under protection but also the adjacent shores due to the supply of materials through longshore drift

More economical than massive structures as the materials for Nourishment may be taken from offshore

COASTAL ENGG. PROBLEMSIN INDIA

Most major rivers drain into Bay Of Bengal(Imbalance in Sediments from rivers)

Problems along east coast dominant

Length of coastline is about 6500km

Problems along east coast dominantSilting up of entrance channelsClosing of river mouths

Problems concerning aquacultureSilting up of intake structuresErosion along the coast

Average littoral drift along the east coast approximately1 million m3/annum

Seasonal

North East Monsoon (Oct ‐ Dec)

South West Monsoon (May ‐ Sep)South West Monsoon (May Sep)

Non Monsoon (Jan ‐May)

NET DRIFT

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N

Bay Of Bengal

Stretch of the North of Madras Harbour

Groin 6 Beach formed

Beach formed on the south of Groin 6

North of Groin 6

Beach formed on the south of Groin 6

Beach Formation South of Groin 6 (Royapuram) in July‐Aug’04

N

Groin 6

SEAWALLS

• Experience in west coast – viable in many pockets

• Experience in east coast – not viable in many locationsmany locations

Basic controls of a coastal system

Energy

Coastal h lMaterials

WavesTidesWind

morphologyMaterials

HumanManagement

SedimentsVegetation

Shore protection structures

DredgingIntake & outfall

systems

Changes in ONE of three controls(Waves/ Sediments/ Shore protection)

Change in coastal morphology

Coastal system in EQUILIBRIUM or STEADY STATEWhile balance between

I t & O t t f & t i lInputs & Outputs of energy & materials

Any management interference CAUSE

failure in sediment suppy & possibly change in wave pattern

Shoreline Management Plan (SMP)

envisages

Management Interference to plan for adjusting Erosion or Accretion

Reduction in sediment inputs

CoastalErosion

Management Interference(Coastal protection …)

Once entered intoloop, it is difficult

to escape

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Do nothing response

-since expensive engineering projects are often ineffective, should we do anything at all?

-the cost of coastal protection may exceed the value of the land that the structure is protecting

- however, only protecting areas of economically important coastline creates problems elsewhere, so good management that considers the coastline as a p g gdynamic, interdependent system, where manipulation of one area has forces reaction in another is needed

Outfall: Shoreline discharge

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• Problems to be faced – Obstructs the sediment movement if laid on the seabed

– Installation of pipeline in surf zone – trench would fillup

– Stability of pipeline within the surf zone due to scour, dynamic bathymetry

– Pipeline below seabed might expose

– An exposed pipeline accelerates shoreline erosion

– An open jetty solves the problem but expensive

– Shore discharge acts as an obstruction/ energy souce/ sink

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• Solution

– Take the pipeline well below seabed from the shoreline

– The top level of pipeline should be the level of seabed at the end of surf zone

– Then, take it on the seabed

A id t ti f j tt t l t l– Avoid construction of jetty – not only coastal problems but aesthetic, environmental concerns, maintenance etc.

– Shore discharge possible when No littoral movement, No fishing activities/ human habitants, No tourism

Problems near the coast are dynamic and never ends…..

Coastal Protection Aspects of Intake-outfall Structures-print

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