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4/15/2013
1
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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2
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
4/15/2013
3
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
4/15/2013
4
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
4/15/2013
5
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
Revetment Training walls/ Jetty
Seawalls Groins
COASTAL PROTECTION WORKS
Types
Bulkheads Offshore breakwaters
Artificial beach nourishmentBio-shields
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
4/15/2013
6
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
Revetment Training walls/ Jetty
Seawalls Groins
COASTAL PROTECTION WORKS
Types
Bulkheads Offshore breakwaters
Artificial beach nourishmentBio-shields
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
4/15/2013
7
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
Revetment Training walls/ Jetty
Seawalls Groins
COASTAL PROTECTION WORKS
Types
Bulkheads Offshore breakwaters
Artificial beach nourishmentBio-shields
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
4/15/2013
8
DETACHEDBREAKWATER
BreakwaterWave direction
Initial shoreline
Updrift erosion Downdrift erosion
Salient
Wave direction
DEFINITION SKETCHOF SEGMENTEDDETACHEDBREAKWATER
Breakwater
SalientTombolo
Erosion DepositionInitial shoreline
Tombolo
Breakwater
Revetment Training walls/ Jetty
Seawalls Groins
COASTAL PROTECTION WORKS
Types
Bulkheads Offshore breakwaters
Artificial beach nourishmentBio-shields
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
4/15/2013
9
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
4/15/2013
10
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
4/15/2013
11
• 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
4/15/2013
12
• 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…..