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COASTAL PROCESSES
By
Krishnaprasad V N
M090160CE
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Introduction
* Coastal processes are the set of mechanisms that
operate along a coastline, bringing about various
combinations of erosion and deposition.
» The land water interface along the coastline is always in
a highly dynamic state and nature works towards
maintaining an equilibrium condition.
* The energy due to tide, waves, wind and currents is
constantly working in the coastal zone. Dissipation of
energy (due to tide, wind, waves and current) is often
provided by the beaches, mudflats, marshes and
3001015.
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Human use of the coasts also requires space and
herein lays the conflict, which results in unstable
coastal systems. This induces coastal erosion,
sediment transport and accretion.
Coastal processes are highly unpredictable and is a
challenge to coastal scientists.
Some of the coastal processes are
Sediment transport
currents
Denudation
Deposition
Erosion
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» Flooding
> Diffraction
» Refraction
» Sediment transport is the one of the important
coastal process.
* The analysis and prediction of sediment
transport have great commercial, aesthetic,
social, and scientific importance owing to the
sustainable development and coastal zone
management.
_
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Sediment Transport and Coastal Processes
Definitions
» accretion - natural or artificial deposition of sediment
in a particular location
» beach nourishment - the restoration of a beach by
the mechanical placement of sand on the beach for
recreational and/ or shore protection purposes.
+ cross shore transport - the displacement of sediment
perpendicular to the shore
» erosion - the removal of sediment from a
particular location by the action of wind or water.
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» _groin - shore perpendicular structure, installed
singly or as a field of groins, designed to trap
sand from the littoral drift system .
» longshore _ transport littoral drift) - the
displacement of sediment down the shore
(parallel to the shore).
» offshore transport - movement of sediment or
water away from the shore
_
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» longshore bar - offshore ridge or mound of sand,
gravel or other loose material running parallel to
the shore which is submerged (at least at high tide)
and located a short distance from the shore.
» tombolo - the combination of an offshore rock or
island which is connected to the beach by a sand
spit.
» sand spit - low tongue of land or a relatively long,
narrow shoal extending from the land.
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» Gabions: Boulders and rocks are wired into mesh
cages and usually placed in front of areas
vulnerable to heavy to moderate erosion.
» onshore transport - movement of sediment or
water toward the shore.
_
صفحه 9:
1.Coastal profile
» The offshore
» Nearshore
» Beach
- Coast
د _
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Figure 1 ‘SHORE ۰ ۲ Coastal
NE ۴ interne
Shoretace
or itoral Zone
Zone of nearshore cuments تا tant
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Processes responsible for initiating
sediment Transportation
» Changes in water level
> Tides
» Waves
» Currents
» Stream outflow
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Coastal response
» The areas, most directly affected by the forces of
the sea are the beaches, the gulfs, and the
nearshore zone regions that experience the full
impact of the sea’s energy.
» There are two general types of dynamic beach
response to wave motion:
Response to normal condition
Response to storm condition
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< Normal condition prevail most of the time, and the
wave energy is easily dissipated by the beach’s
natural defence mechanisms.
When storm’ conditions generates waves
containing increased amounts of energy, the coast
must respond with extraordinary measures, such
as sacrificing large section of beach and dune.
Alternate erosion and accretion may be seasonal
on some beaches; the winter storm waves erode
the beach, and the summer waves rebuilds it.
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» Another nearshore dynamical system 15
littoral transport - the movement of sediments in
the nearshore zone by waves and currents.
» Littoral transport is divided into two general
classes:
» Longshore transport: Transport parallel to
the shore.
» Onshore-offshore transport:
Transport perpendicular to the shore.
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Modes of Sediment transport
Sediments are transported as,
» Bed load transport
The bed load is the part of the total load that is
more or less in contact with the bed during the
transport. It primarily includes grains that roll,
slide or bounce along the bed.
» experiment and theory suggest that the rate of
bedload transport (qb) is proportional to the cube
of the shear Velocity,
سس سس 3 با و وان رز ۶
تس
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» Suspended load transport.
It is the part of the total load that is moving in
suspension without continuous contact with the bed.
* The determination of the rate of suspended load
transport is straightforward by comparison with
measurement of the rate of bedload transport.
> Wash load transport
< ۱] consists of very fine particles transported in
water.
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Figure 2
Dissolved Load
oa ۳ Load
Bed Load
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Longshore drift
> It is the movement of sediments most often sand,
along a coast parallel to its shoreline - a zig-zag
movement of sediment along the beach .
» The largest beach sediment is found where the
process begins, updrift, and the smallest, most
easily moved, downdrift.
» Where waves are strong, the coast will
be eroded and sediment carried away and where
they are weak sediment will be deposited.
» Longshore drift can have undesirable effects for
humans, such as beach erosion. To prevent this,
sea defences such as groynes or gabions may be
۰ aC
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» Where beaches have become severely eroded by
longshore drift and little material is replaced by
natural processes, then the material may be
artificially replaced by beach nourishment.
» Net rate:
» Gross rate:
_
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Model theories for the calculation of
longshore drift are
» Energy flux model
» Energetic model
» Suspended transport model
_
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Energy flux model
> It is the simplest model used for the calculation of
total amount of material moved along the
shoreline.
» It is based on the amount of energy available in
the waves arriving at the shoreline.
» The flux energy in the wave direction is
determined to be as F dl,
> where
< ۴ -the energy flux of the waves per unit crest
width(F=ECg),
» E- the energy per unit surface area= &gH?/8
e water density
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» g-acceleration due to gravity
» Cg-Group velocity
» To determine the amount of this energy flux
per unit length of coastline consider the
figure,
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< 02,
» where @is the angle the wave ray makes
with the onshore(y)direction.
» Now it is supposed that the energy flux in
the alongshore direction is responsible for
the longshore sediment _ transport;
therefore we multiply the energy flux per
unit legth(dx=unity) of beach by sin® to
obtain
< ۲6۵565۱۴ ۴۱۶۲95۱۳96050 (p
gCgsin20)/16
_
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» For many years this alongshore energy flux per unit
length of beach,P! was correlated empirically with
the volume sand moved by the waves.
» Amount material moved,
Where C is a dimensional constant of proportionality.
The power n has been found to be close to unity.
equation A as being dimensionally incorrect. Then
corrected the equation as
> Q=KPi/[(p .- 0(9)1-0([
_
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» Where -p is the porosity of the sediment, which is
typically about .3 to .4. the porosity is introduced
to convert sand weight to sad volume, taking into
account the voids that occur with in the sand.
< K-is the dimensionless parameter, its value
ranging from 0.2 to 2.
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Longshore Sediment transport rate at different
locations in India
& Longshore sediment transport rate (LSTR)
reported for different locations along the Indian
coast shows local reversals in the transport
direction in a number of locations along the west
coast.
> It was found that the annual gross sediment
transport rate was high (»1 * 108m3) along the
coast of south Orissa and south Kerala.
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Sediment transport rate at different locations in
India
» Location Net transport Gross
transport
(m3/yr) (m3/yr)
» West coast of India
» Kalbadevi 118,580 South 147,621
» Ambolgarh 189,594 South 299,997
» Vengurla 53,040 South 120,141
» Calangute 90,000 South 120,000
» Colva 160,000 North 160,000
» Arge 69,350 North 200,773
» Gangavali 142,018 South 177,239
» Kasarkod 40,186 North 77,502
» Maravanthe 25,372 North 29,836
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Kasargod 736,772 South 958,478
Kannur 19,434 South 561,576
Kozhikode 114,665 South 256,697
Nattika 192,818 North 660,276
Andhakaranazhi 202,096 South 599,484
Alleppey 9 North 62,519
Kollam 383,784 South 805,296
Thiruvananthapuram 99,159 North 1231,153
Kolachel 0 West 946,500
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East coast of India
Ovari 1,500 South 251,300
Tiruchendur 64,100 North 87,500
Kannirajapuram 117,447 North 145,979
Naripayur 36,600 South 122,500
Muthupettai 5,200 South 8,900
Poompuhar 146,000 North
478,800
Pondichery 134,400 North 237,000
Periyakalapet 486,900 North 657,600
Tikkavanipalem 177,000 North 405,000
Gopalpur 830,046 North 949,520
Prayagi 887,528 North 997,594
Puri 735,436 North 926,637
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Onshore-offshore transport
» Cross shore transport is a result of the water
motions due to the waves and the undertow.
» Here the sediment transport is perpendicular to
the shore.
Important factors in determining the cross-shore
transport of sand are
» The ratio of wave height to wavelength
» Physical parameters of the beach;
i.e., grain-size distribution,
cohesiveness, beach
slope also play an important role.
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» Model theories for the calculation of
Onshore-offshore transport drift are
» Simple cross shore transport model
> Fall time model
» Traction model for cross shore transport
» Energetics model
» Ripple model
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Simple cross shore transport model
This model was first proposed by Moore and later
modified by Kriebel and Dean.
The basic concept is that, for a uniform sand size
across the profile and an equilibrium beach, there is a
constant energy dissipation rate per unit volume.
It is assumed that the amount of sediment moved will
be dependent on the difference between the actual
energy dissipation rate and that for an equilibrium
profile D*.
qs= K(D- D*.)
where qs is the volumetric cross shore sediment
transport rate per unit width in the offshore direction
and K is a new dimensional constant.
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>If D is greater than equilibrium value D*
there is a greater turbulence level in the surf
zone than that of for the equilibrium profile. If
qs is positive then there will be a sediment
transport in the offshore direction. On the
other hand , for values of p less than
equilibrium value, onshore transport will
occur. The value D can be obtained as,
» D = (5pgk2(gh)°dh/dy)/16
» which is dependent on the water depth and
bottom slope, which has stronger effect.
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Landforms of Littoral Drift
* Onshore offshore drift and longshore drift deposit
and shape debris to produce a variety of landforms.
A spit is an elongated deposit that extends from the
land edge out to sea. It may be shaped to produce
a distinctly curved end, as in Cape Cod. Another
landform that attaches an island to the mainland is
known as a tombolo, while deposits may also
accumulate to create barriers such as a bay barrier.
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» Excellent recent examples of seasonal cross-
shore transport and the resultant change in
beach profile, -Imperial Beach, La Jolla, and
Carlsbad.
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Sediment transport measurements
Coastal engineers, physical oceanographers and
geomorphologists generally carry out measurements
of sediment transport using the following methods :
» Wave refraction studies Determines places of wave
convergence or divergence and provide quantitative
measurement.
» Using geomorphic indicators
* Based on geomorphic indicators sediment transport
direction is decided.
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» Sedimentological methods.
» Beach profile study, sediment budget measurement.
» Sediment trap study Suspended sediments are
collected in sediment trap and are measured
- Artificial tracer tracking Sediment transport
direction detected from the movement of artificial
tracers.
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Conclusion
» Coastal processes are highly unpredictable and is
a challenge to coastal scientists.
Coastal processes are important in the case of
» Prediction of environmental quality and impact
< Habitat stability,
» Public health risks,
< Marine hazards such as ship grounding
» Access to ports, seabed scouring
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< Siltation of harbours, infill of reservoirs and
artificial lakes
» And in coastline protection.
» It is very crucial to understand the nearshore
physical system, the consequent impact on
sediment dynamics, and the coastline’s response
to it.
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Refernces
» Robert G.Dean &Robert A.Dalrymple,Coastal
processes with engineering applications,2002,04.
» Walton Jr., T. L. and Bruno, R. O., Longshore
transport at a detached break water, Phase Il. J.
Coast. Res., 1989, 5, 679-691.
» U.S corps engineers, Coastal engineering manual.
> www.googlebooks.com
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۲۳۷۸۷۲۵
د ع2
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