1
CHAPTER 1
INTRODUCTION
1.1
GENERAL
Although nature begins with the cause and ends with the experience,
we must follow the opposite course, namely, begin with the experience and by
the means of it investigate the cause.
Leonardo da Vinci, 1452-1519
Notebooks
Diverse and complex natural processes continually change coasts
physically, chemically and biologically, at scales that range from microscopic
(grains of sand) to global (changes in sea level). Regional and local
characteristics of coasts control the differing interactions and relative
importance of these natural processes. Human activity adds yet another
dimension to coastal change by modifying and disturbing, both directly and
indirectly, the coastal environments and the natural processes of change. An
increasing number of people are awakening to the fact that the well-being of
the environment and survival are intricately woven into each other.
The coastal zone occurs at the interface between the three major
natural systems - atmosphere, ocean and land. Processes operating in all three
of these systems are responsible for shaping the coastal zone, and the
interaction between the three different sets of processes makes the coastal
zone an extremely dynamic one. The coastal zone is also a zone of transfer of
material from the land to the ocean system, with sediments eroded by rivers,
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glaciers, etc., being moved to the beach and nearshore, and ultimately some to
the ocean floor. The coastal zone is a large and diverse area comprising a rich
array of social, economic and environmental resources. These zones are
important because majority of the world's population inhabit such zones.
Coastal zones are continually changing because of the dynamic interaction
between the oceans and the land.
1.2
SEDIMENT CHARACTERISTICS
The sediment cycle begins with weathering of rocks and transport of
that material. The eroded material is deposited, reflecting the redistribution of
material within and the geomorphological evolution. The sediments are
transported by water, wind, ice and gravity (turbidity currents). Most beaches
are composed of sediments derived from the disintegration of the land – sand
and gravel eroded from terrestrial rocks. The composition of the beach
sediments reflects the nature of the source rocks and often can be used to
assess relative contributions and transport from the sources to the beaches.
Waves and nearshore currents continuously rework the accumulated beach
sediment, shaping the particles and sorting them by size, shape and density.
The beach takes on a form that reflects the totality of water and sediment
movements.
1.2.1
Granulometry
Grain size distribution is one of the most important characteristics of
sediment. This is true because grain size is a powerful tool for describing a
site’s geomorphic setting, interpreting the geomorphic significance of fluid
dynamics in the natural environment, and distinguishing local versus regional
sediment transport mechanisms. The nomenclature used to classify sediment
particles according to their grain diameter is illustrated in Table 1.1.
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Table 1.1 Particle Size Classification
Grain Size
mm
Descriptive Terminology
Udden
(1914) and
Wentworth
(1922)
Friedman and
Sanders (1978)
-11
2048
Very large
boulders
-10
1024
Large boulders
9
512
-8
Blott and Pye (2001)
Very large
Medium boulders
Large
256
Small boulders
Medium
7
128
Large cobbles
Small
6
64
Small cobbles
Very small
-5
32
Very coarse
pebbles
Very coarse
-4
16
Coarse pebbles
Coarse
-3
8
-2
4
-1
2
0
Cobbles
Pebbles
Boulders
Gravel
Medium pebbles
Medium
Fine pebbles
Fine
Granules
Very fine pebbles
Very fine
1
Very coarse
sand
Very coarse sand
1
500
Coarse sand
Coarse sand
2
250
3
125
Fine sand
Fine sand
4
63
Very fine
sand
Very fine sand
Medium sand Medium sand
Very coarse
Coarse
Medium
Fine
Very fine
Sand
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Table 1.1 (Continued)
Grain Size
mm
Descriptive Terminology
Udden (1914)
and Wentworth
(1922)
Friedman and
Sanders (1978)
Blott and Pye (2001)
5
31
Very coarse silt
Very coarse
6
16
Coarse silt
Coarse
7
8
Medium silt
Medium
8
4
Fine silt
Fine
Very fine silt
Very fine
Clay
Clay
9
2
D = 1/2 mm
Silt
Clay
Silt
Three dominant factors that control the mean grain size of beach
sediments are sediment source, wave energy level and general offshore slope
upon which the beach is constructed. Mean is a function of the size range of
available materials, amount of energy imparted to the sediment which
depends on current velocity or turbulence of the transporting medium. A
complex relationship exists between the energy level of the nearshore waves
and currents, the offshore slope, and the resulting grain sizes of the beach.
There is a general tendency for the highest energy beaches with the largest
waves to have the coarsest grains. The more exposed beaches of an island are
generally made up of the coarsest grains (Dobkins and Folk 1970).
Sorting depends on four major factors: (i) the size range of material
supplied to the environment, (ii) type of deposition – with currents working
over grains continuously i.e. swash and backwash giving rise to better sorting
than rapid deposition of sediments, (iii) current characteristics – currents of
relatively constant strength whether low or high will give better sorting than
currents which fluctuate and (iv) time - beach sediments where waves are
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attacking continually caving cliffs or are battling great loads of detritus
brought to the shore by vigorous rivers, will be generally poorly sorted than
beaches on a flat, stable coast receiving little sediment influx. The coarseness
of sediment reflects the bottom topography and the local intensity of
turbulence and wave-energy dissipation. The largest sediment particles
generally are located in the zone of most intense wave breaking, with decrease
in grain sizes both toward deeper water and shoreward across surf and swash
zones (Folk 1980).
Skewness and kurtosis describe how closely the grain size
distribution approaches the normal Gaussian probability curve, and the more
extreme the values, the more non-normal the curve. It has been found that
single-source sediments (e.g. most beach sands, aeolian sands, etc.) tend to
have fairly normal curves, while sediments from multiple sources (such as
mixtures of beach sands with lagoonal clays, or river sands with locallyderived pebbles) show pronounced skewness and kurtosis. Bimodal sediments
exhibit extreme skewness and kurtosis values; although the pure end members
of such mixtures have nearly normal curves, sediments consisting dominantly
of one end member with only a small amount of the other end member are
extremely leptokurtic and skewed, the sign of the skewness depends on which
end member dominates; sediments consisting of sub equal amounts of the two
end members are extremely platykurtic (Folk 1980).
1.2.2
Weathering
Weathering is the breakdown and alteration of rocks and minerals at
or near the Earth's surface into products that are more in equilibrium with the
conditions found in this environment. Weathering is the first step for a
number of other geomorphic and biogeochemical processes. The products of
weathering are the major source for sediment transport. There are three broad
categories of mechanisms for weathering: physical, chemical and biological.
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Physical weathering is the breakdown of mineral or rock
material by entirely mechanical methods brought about by a
variety of causes. Some of the forces originate within the
rock or mineral, while others are applied externally. Both of
these stresses lead to strain and the rupture of the rock. The
processes that may cause mechanical rupture are abrasion,
crystallization, thermal expansion, wetting and drying, and
pressure release.
Chemical weathering involves the alteration of the chemical
and mineralogical composition of the weathered material. A
number of different processes can result in chemical
weathering. The most common chemical weathering
processes are hydrolysis, oxidation, reduction, hydration,
carbonation, and solution.
Biological weathering involves the disintegration of rock
and mineral due to the chemical and/or physical agents of an
organism. The types of organisms that can cause weathering
range from bacteria to plants to animals.
The residue of weathering consists of chemically altered and
unaltered materials. The most common unaltered residue is quartz. Many of
the chemically altered products of weathering become very simple small
compounds or nutrient ions. These residues can then be dissolved or
transported by water, released to the atmosphere as a gas, or taken up by
plants for nutrition. Some of the products of weathering, less resistant
alumino-silicate minerals, become clay particles. Other altered materials are
reconstituted by sedimentary or metamorphic processes to become new rocks
and minerals.
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1.2.3
Heavy Metals
The term heavy metal refers to any metallic chemical element that
has a relatively high density and toxicity, highly toxic or poisonous at low
concentrations. Heavy metals are a group of elements between copper and
lead on the periodic table of the elements; having atomic weights between
63.546 and 200.590 and specific gravities greater than 4.0. Heavy metals are
also defined as a group of elements between Copper and Bismuth on the
Periodic Table, consisting of 55 elements.
Metals can exist in seawater in atleast four different forms viz., in
true solution, as colloidal particles, adsorbed on other colloidal particles and
as a part of living organisms (Forstner and Wittmann 1979). The partitioning
behavior of heavy metals is such that they tend to accumulate in sediments at
levels that are several orders of magnitude higher than the surrounding water.
Further, their deposition rates are generally related to their rates of input in the
surrounding water (Forstner 1990). Sediments are preferable monitoring tools
since contaminant concentrations are orders of magnitude higher and they
show less variation in time and space, allowing more consistent assessment of
spatial and temporal contamination (Caccia et al 2003). Industrial and urban
activities have contributed to the increase of metal contamination into marine
environment and have directly influenced the coastal ecosystems (Buccolieri
et al 2006).
1.2.4
Mineralogy
A mineral is a naturally-occurring, homogeneous solid with a
definite, but generally not fixed, chemical composition and an ordered atomic
arrangement. It is usually formed by inorganic processes. Minerals are
integral to every aspect of our life - from aesthetically pleasing gem stones to
more mundane but essential components of concrete. In geology, minerals are
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useful in unraveling certain questions, such as where do large bodies of
sediment come from (source), how do they move around (transport
processes), and where do they settle (distribution). Although over 100
different minerals have been recorded from sediments, they probably form no
more than 0.1 - 0.5% of the terrigenous fraction of sediments. Despite their
small amount, they are of great value in determining provenance, tracing
transport paths, mapping dispersal patterns, depicting the action of hydraulic
regimes and selection processes, locating potential economic deposits and
understanding diagenetic processes (Morton and Hallsworth 1999). Minerals
are classified based on:
Physical characteristics - traits are used to identify and
describe mineral species which include color, streak, luster,
density, hardness, cleavage, fracture, tenacity, and crystal
habit.
Occurrences and Environments - one of the most diagnostic
features of a mineral is the geological environment in which it
is occurs i.e. igneous minerals, metamorphic minerals,
sedimentary minerals and hydrothermal minerals
Density - or Specific Gravity is an important property of
minerals. As a rule, organics, carbonates, and sulfates are
light; oxides, sulfides, and elements are heavy; silicates and
phosphates are in between.
Chemistry – Classification based on the anionic element
(negatively charge atom)
or polyanionic group (strongly
bound group of atoms consisting of a cation plus several
anions typically oxygen that has a net negative charge) of
elements that occur in the mineral viz., sulfides, halides,
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oxides, hydroxides, carbonates, sulfates, phosphates, borates,
silicates and native elements (contains no anion or polyanion).
The heavy mineral content of sediment is a function of five complex
variables:
i.
Lithology of the source area
ii.
Stability of minerals
iii.
Durability of the mineral to long continued abrasion
iv.
Hydraulic factor - minerals of a certain shape or specific
gravity will be carried father away leading to changes in
mineral ratios from specimen to specimen.
v.
Post-depositional survival factor - on intrastratal solution by
migrating connate water or on surficial weathering some of
the less stable minerals (Garnet, Pyroxene, Amphibole, and
Staurolite) may be destroyed or etched
1.3
COASTAL PROCESSES
Coastal geomorphic processes depend primarily on wave breaking,
alongshore and onshore-offshore currents, littoral sediment transport,
accretion/deposition of beach sediments. The various features and processes
influencing the coastal environment are discussed below.
1.3.1
Physical Processes
The main agents responsible for deposition and erosion along
coastlines are waves, tides and currents. The formation of coasts is also
heavily influenced by their lithology. Coastal processes act on timescales that
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range from few seconds of a wave breaking to many millennia of sea-level
change.
Waves - Waves are generated by offshore and nearshore
winds. Waves are usually described by their height (the
distance between crest and trough), length (the distance
between two consecutive crests) or period (the time between
two wave crests passing a fixed point). The height of a wave
produced by wind depends on the wind strength (wind speed),
length of time the wind blows (duration) and the distance over
which the wave travels (the fetch). As waves travel away from
their source, they spread out and their wavelength increases.
Waves with longer wavelengths or periods are known as
'swell'. As they travel into shallower water, waves become
steeper and 'break'. The angle at which waves reach the beach
affects how much sediment can be transported along the shore.
Tides - Tides are oscillations of ocean waters due to the
gravitational forces exerted by the Moon and the Sun upon the
oceans. The rising tide is usually referred to as flood, whereas
falling tide is called as ebb tide. Tidal currents are the
horizontal water movements corresponding to the rise and fall
(flood and ebb) of the tide. High tides are the highest when the
Earth, Moon and Sun are all lined up, about every two weeks.
Such tides are known as spring tides. When the Moon is
perpendicular to the Earth - Sun line (also about every two
weeks), tides are the lowest, called neap tides. The tidal range
is the vertical distance between high tide and low tide, and this
coincides with the swash zone of the beach. The slope of the
shoreline and the tidal range determine the amount of shore
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exposed to wave action. A low tidal range tends to produce a
narrower beach, which is prone to higher erosion.
Currents - Currents are generated by the action of tides (ebb
and flood currents), waves breaking at an oblique angle with
the shore (longshore currents), and the backwash of waves on
the foreshore (rip currents). The primary driving force behind
ocean currents is constant winds. When the surface current
reaches a carrier, such as the coast, water tends to pile up
against the land. Of all currents, those that flow near coasts
have substantial effect on coastal landforms. The most
important type of current in the coastal zone is alongshore
current. Longshore current is a current that flows in shallow
water, parallel to the shoreline which transports sediments
along coasts, sometimes they are powerful enough to erode
coast.
Storms - Storms result in raised water levels (known as storm
surge) and highly energetic waves induced by extreme winds
(Cyclones). Combined with high tides, storms may result in
catastrophic damages along the coast. Beside damages to
coastal infrastructure, storms cause beaches and dunes to
retreat tenths of meter in a few hours, or may considerably
undermine cliff stability. Although storms are sporadic, they
are the primary cause of beach erosion along many coasts.
Storms carry sand seaward, forming offshore bars; much of
this sand migrates landward during calm weather. Some areas
are more storm prone than others. Storms often are
concentrated in specific seasons. These seasonal trends result
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in a general difference between the "eroding" beach and the
"building" beach.
Human activity can interfere with the processes within a sediment
cell by disrupting the supply of sediment and therefore the sediment budget of
the cell. Human intervention is caused due to coastal engineering and
management practices viz., Groynes and Seawalls, dredging, construction of
dams and cliff protection measures, non-management such as blocking
structures, jetties or harbour walls. On many occasions these structures block
the movement of sediment, which can lead to beach deposition in the updrift
with complementing erosion in downdrift.
1.3.2
Coastal Erosion and Deposition
The landward displacement of the shoreline caused by the forces of
waves and currents is termed as coastal erosion. The effects of waves,
currents, tides and winds are primary natural factors that influence the coast.
The other aspects eroding the coastline include: the sand sources and sinks,
changes in relative sea level, geomorphological characteristics of the shore
and sand, etc. Other anthropological effects that trigger beach erosion are:
construction of artificial structures, mining of beach sand, offshore dredging.
Erosion moves sand from the shore and deposits it elsewhere. The sand can be
moved to another beach, to the deeper ocean bottom, into an ocean trench or
onto the landside of a dune. The removal of sand from the sand-sharing
system results in permanent changes in beach shape and structure. It generally
takes months or years to note the impact of erosion; therefore, this is generally
classified as a "long term coastal hazard".
Deposition is the geological process by which material is added to a
landform or land mass. Previously eroded sediment, at the loss of enough
kinetic energy is deposited, building up layers of sediment. Deposition occurs
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when the forces responsible for sediment transportation are no longer
sufficient to overcome the forces of particle weight and friction, creating a
resistance to motion. Coastal erosion and deposition are very dynamic in
nature. When the energy of waves changes, the balance between erosion and
deposition also shifts. Normally, beaches grow during quiet weather and
retreat (they are eroded) during storms. This occurs in areas where the amount
of deposited material exceeds that of the depleted material.
1.3.3
Sediment Transport
Sediments on most beaches range from fine sands to cobbles. The
size, character of sediments and the slope of the beach are related to the forces
that the beach is exposed and the type of material available on the coast. The
longshore current plays an important role in transporting the sediments in the
surf zone. When particles reach the shore as sand, they are moved alongshore
by waves and currents. This longshore transport is a constant process, and
great volumes may be transported. Beach material is also derived from
erosion of the coastal formation caused by waves and currents, and in some
cases, brought onshore by the movement of sediment from deeper water. In
some regions, a sizable fraction of the beach material is composed of marine
shell fragments, coral reef fragments, and volcanic materials. Clay and silt do
not usually settle on beaches because the waves create such turbulence in the
water along the shore that these fine particles are kept in suspension. The
particles settle and deposit on the bottom only after moving away from the
beaches into the quieter water of lagoons and estuaries or the deeper water
offshore.
1.4
SEDIMENT TRANSPORT PROCESSES
Waves typically approach the shore at an angle. Swash moves sand
diagonally, while backwash moves it straight down. The net result of this
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zigzag movement is the downwind displacement of sand. Transport along the
beach is known as beach drift. Besides the beach drift, longshore currents also
transport sediments downwind -alongshore drift. Together beach drift and
alongshore drift are called littoral drift. Littoral drift leads to growth of sand
spit across a bay. Onshore-offshore transport has an average net direction
perpendicular to the shore. The four modes of particle transport in water are
sliding, rolling, saltation and suspension. Sliding particles remain in
continuous contact with the bed, merely tilting to and fro as they move.
Rolling grains also remain in continuous contact with the bed, whereas
saltation grains ‘jump’ along the bed in a series of low trajectories. Sediment
particles in these three categories collectively form the bedload. The
suspended load consists of particles in suspension, i.e. particles that follow
long and irregular paths within the water and seldom come in contact with the
bed until they are deposited when the flow slackens. Sliding and rolling are
prevalent in slower flows, saltation and suspension in faster flows.
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 (tilt to and fro
as they move), slide or bounce along the bed. Thus the bed
load movement is governed by the shear velocity at the bed
and effective resistance of the sediment particle.
Suspended load transport - The suspended load is the part of
the total load that is moving in suspension without continuous
contact with the bed as a result of agitation of fluid turbulence.
Many estuary deposits contain large proportion of fine
sediments, which are readily set in motion by tidal currents.
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The primary transport mode of fine sediments is suspended
load and such sediment may amount to 75-95% of the total
load in estuaries.
Wash load transport - The above two modes of transport
together are called total load transport which has effects on the
bed morphology. The third mode of transport, wash load is not
important as it consists of very fine particles transported in
water and not represented in the bed.
The rate of sediment transport or sediment flux is the mass of
sediment that is moved past a given point or through unit area of the water
column in unit time. The total of both bed load and suspended sediment flux
is considered as total sediment transport rate. A sediment budget is a balance
of the sediment volume entering and exiting a particular section of the coast
or an estuary. Sediment budget analysis consists of the evaluation of sediment
fluxes, sources and sinks from different processes that give rise to additions
and subtractions within a control volume (section of coast or an estuary) in
order to gain a better understanding of the coastal or estuarine system. Control
volumes on open coasts pertain to sections of a coast which form sediment
cells and are controlled by cell boundaries either inhibiting or limiting the
amount of transport across the cell boundary. A source increases the quantity
of material within the control volume and a sink reduces it and within the cell
there may be point sources and sinks (tidal inlets) and line sources and sinks
(movements on and off the beach). The inputs are sediments eroded from
backshore cliffs by waves, upcurrent beach by alongshore drift, current and
rivers. The outputs are sediments transported to backshore dunes by offshore
dunes, downcurrent by alongshore drift, current, deep water by tidal currents
and waves.
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1.5
REMOTE SENSING
Remote sensing is the technique of deriving information about
objects on the surface of the earth without physically coming into contact with
them. This process involves making observations using sensors (cameras,
scanners, radiometer, radar etc.) mounted on platforms (aircraft and satellites),
which are at a considerable height from the earth surface and recording the
observations. Remote sensing has enabled mapping, monitoring and
management of various resources viz., agriculture, forestry, geology, water,
ocean etc. Indian Remote Sensing Satellites (IRS) such as IRS 1A, IRS 1B,
IRS 1C, IRS 1D, Cartostat, Oceanstat etc. facilitate a variety of applications
including natural resource monitoring, environmental assessments and
disaster management related activities.
The Indian satellites with their improved spatial resolution, extended
spectral range and increased repetivity have opened up new applications in
coastal zone. Preliminary analysis of IRS 1C, 1D data indicates that coral reef
zonation, identification of tree and shrub mangroves, mudflats, beach, dune
vegetation, saline areas, etc. as well as better understanding of suspended
sediment patterns are possible. The Panchromatic Satellite (PAN) data
combined with the Linear Imaging Self Scanner (LISS) III and LISS IV data
are extremely useful in providing detailed spatial information about
reclamation, construction activity and ecologically sensitive areas, which are
vital for the coastal zone regulatory activities. The information available from
merged PAN and LISS III, IV data about coral reef zonation, especially for
atolls, patch reef and coral pinnacles, is valuable for coral reef conservation.
The distinction between tree and shrub mangroves from LISS III data
provides vital information on biodiversity studies (Ramachandran et al 2000).
With the development of remote sensing technology, monitoring of
water quality from satellites significantly complement the conventional
17
techniques and have found widespread applications. The radiance
measurements in different wavelength ranges are subjected to atmospheric
corrections. In visible data analysis, the water content is generally divided
into three types of constituent: phytoplankton pigments, sediments and
dissolved organic matter (Sathyendranath et al 1989). Each of them has a
significant and specific signature on water color. Spectral qualities of water
bodies are determined by the interaction of the water surface, optical
properties of the water due to turbidity, roughness of the surface, angle of
observation, illumination and in some instances reflection of light from the
bottom.
1.6
NEED FOR THE STUDY
During the last two decades, the coastal environment of southeast
India has experienced intense developments due to industrial revolution.
Kalpakkam, a tiny fishing hamlet dotting the east coast, half way between
Chennai and Pondicherry which has become prominent due to the Madras
Atomic Power Station (MAPS). Indira Gandhi Centre for Atomic Research
(IGCAR) is a premier atomic research centre of India, set up at Kalpakkam.
Kalpakkam is bound by Sadras backwater in the south, Edaiyur backwater in
the north, Buckingham canal in the west and Bay of Bengal in the east. The
MAPS consists of two pressurized heavy water reactors of 235 MWe
capacities each. A 500MWe capacity Prototype Fast Breeder Reactor Project
(PFBR) at about 680m south of MAPS is proposed to be established. For both
MAPS and PFBR units, approach jetties of 468m and 420m length
respectively have been constructed. The MAPS jetty also supports a discharge
pipeline for low level radioactive effluents discharge. From the outfall point,
the discharged warm water from MAPS flowed as a canal and mixed with the
sea at the tip of sand spit. The length of this naturally formed discharge canal
varied (0.5–2.0 km) with season and was mainly controlled by magnitude and
18
direction of longshore currents and littoral sediment transport (Poornima et al
2005). During transition or southerly drift in the monsoon period, erosion of
sand spit opposite to the thermal outfall made the warm water to flow in the
direction of the MAPS intake well.
It is proposed to have a combined thermal discharge from both
PFBR and MAPS and hence an engineering canal of approximately 1.66 km
length with a fixed opening has been constructed and at present warm water
from MAPS (0.9km) flows through the engineering canal. The shore
protection measures at Kalpakkam commenced in 2007 and completed in
2009. During the Asian tsunami in December 2004, the water level at
Kalpakkam coast was reported to reach up to a level of +10.8 m (CWPRS
2006). To protect PFBR and MAPS premises from the possible future attack
of tsunami, a rubblemound bund approximately at 50 m distance landward of
the outfall channel has been constructed. Shore protection work (seawall) has
been constructed taking into account 100 year return storm surge and littoral
processes. It consists of 800-1000 kg stones in armour in double layer on 1:5
slope to match the existing profile of the beach so that the impact of seawall
on the littoral processes is reduced.
Kalpakkam also contains a coastal township associated with MAPS
and many coastal villages. Mahabalipuram, a world heritage site, also famous
as the sixth centre of Pallava art and architecture of south India, is situated 4
km north of the power plant. The shore temple at Mahabalipuram is protected
by a seawall constructed in 1968 for a length of approximately 0.5km which
protrudes slightly into the sea acting as a headland.
With the extent of human intervention on the coastline, the present
study aims at understanding the sediment characteristics and its transport
along the coast through which the impacts of construction of seawall and
engineering canal on sediment dynamics are assessed.
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1.7
OBJECTIVES OF THE PRESENT STUDY
The main objectives of this research are to examine the sediment
dynamics of Kalpakkam coast.
To divulge the sediment characteristics, weathering trend and
provenance history of Kalpakkam
To assess the long term and short term shoreline changes
along the study area
To understand the variations in sediment transport processes
in view of the construction of seawall and engineering canal at
IGCAR
To integrate field observations, satellite data and numerical
modeling to elucidate the nearshore dynamics of Kalpakkam
1.8
STUDY AREA
1.8.1
Physiography
The study area is situated on the Coromandel Coast at about 70 km
south of Chennai and covers approximately 15 km of coastal tract from
Sadras to Mahabalipuram (Figure 1.1). It hosts one of India’s nuclear power
plant built by Department of Atomic Energy (DAE). Kalpakkam is located in
Kancheepuram District of Tamil Nadu with the geographical coordinates of
North Latitude 12º 33´ and East Longitude 80º 11´. Kalpakkam is a
conglomerate of Puduppattinam, Sadras and a DAE township. Kalpakkam is a
tropical coastal station with a gently sloping plain terrain. Kalpakkam falls
under Universal Transverse Mercator UTM 44. The shoreline of Kalpakkam
is aligned at SSW-NNE and it is open to the Bay of Bengal.
20
21
The area upto 1.6 km from MAPS site is known as exclusion zone
and is under the jurisdiction of MAPS. There is no other major industrial
complex surrounding Kalpakkam. The coast is experiencing different
monsoonal seasons viz., Post monsoon (January to May), Southwest (SW)
monsoon (June to September) and Northeast (NE) monsoon (October to
December). SW monsoon is also known as Pre monsoon season since the
rainfall received is scanty.
The study area has a complex ecosystem, which is partly influenced
by inputs from River Palar, backwaters of Sadras and Edaiyur that transports
effluents from minor urban settlements and industries. In addition, the
Buckingham canal runs parallel to the coast carrying urban sewage, pesticides
and fertilizers derived from agricultural practices and the salt panning
industry. Sadras is connected to the sea during northeast monsoon whereas
Edaiyur is kept open through dredging. This coastal tract is represented by
low angle, siliclastic beaches with a width of about 50 to 100 m which are
backed by coastal sand dunes usually of less than 5 m elevation. The back
barrier morphology consists of a variety of geomorphic units such as stranded
beach ridges, palaeo-lagoons, palaeo-tidal flats and palaeo barriers (beach
ridges) (Anbarasu 1994). These are the features mostly formed due to the
recent and ongoing emergence of the coast.
Wide rocky patches are present in the seabed on this stretch. There
are three types of seabed slopes viz., nearshore waters 0-5m i.e. steep slope
1:20, 6-10 m i.e. moderate slope 1:40 and 10-18 m i.e. flat slope 1:100 occur
in this site. Selvaraj and Ram Mohan (2003) indicated that the inner shelf is
carpeted with a mosaic of sand and silty sand with minor amounts of clay
(maximum 4%). Sediments are poorly sorted and depositional environments
are influenced by mixed river/dune/beach conditions. A palaeo-shoreline was
22
identified based on the higher sand (>90%) and CaCO3 (>15%), and very low
organic matter contents (<0.4%) at a depth around 50–53 m.
1.8.2
Meteorology
The maximum and minimum temperatures would be 46.4°C and
15.7°C for a return period of 100 years. The predominant wind direction in
this area is south with a frequency of 16.2%. The predominant wind speed is
12-19km/h with a frequency of 39.7%. The highest instantaneous wind speed
(over 1 minute period) recorded was 182km/h on 12.11.1985 when a cyclonic
storm crossed the East coast near Kalpakkam. The maximum wind speed at
60m elevation is 100km/h recorded on May 1990, during another cyclonic
weather condition. The highest and lowest annual rainfall of 2112 mm and
567.1 mm were recorded during 1985 and 1968 respectively. The study area
receives about 62% of its annual rainfall during NE monsoon and 33 %
during SW monsoon.
1.8.3
Nearshore Environment
Wave hindcasting studies predicted the wave height of 4.8m for 100
year return period and 4.3m for 50 year return period at the coast of
Kalpakkam. The direction of current is mainly along the coast, either
northward or southward depending on the season and monsoon. The currents
are more influenced by the bay circulations generated by southwest and
northeast monsoon (CWPRS 2002). The waves are predominantly from SE
and SSE directions with 19% and 73% respectively in SW monsoon while
from ENE, E, SE and SSE directions with 42%, 26%, 10% and 17%
respectively in NE monsoon and from ENE, East, SE and SSE directions with
16%, 20%, 28% and 25% respectively during post monsoon period. High
waves upto 3.5m are encountered in most of the occasions. The longshore
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transport takes place upto about 300m from the shore. In a field study
conducted by CWPRS (2002), semi-diurnal tides with spring and neap tides
upto 1.22m and 0.25m respectively were observed. The tidal currents are
quite weak (average around 0.03m/s) and do not exhibit any correlation with
the tide or tidal phase.
1.8.4
Geology
The geological setup of Kalpakkam area consists of Charnockite
suite of rocks and Pyroxene granulites of Archean age as a basement overlain
by recent coastal alluviam with an unconformity. The bedrock occurs at about
10 - 15m depth in this region. MAPS and PFBR are located in the formation
of coastal alluvium. Generally, the bedrock occurs as wave cut platform or
ridge parallel to the coast. The rocks are medium to coarse grained, with
major mineral constituent as quartz, feldspar and pyroxene. The rocks are of
igneous and metamorphic origin and can be identified as charnockite, gneiss
and granite with crystals of garnet distributed in rock specimen. The
Kalpakkam site falls at the depositional environment by the floods and coastal
processes.
1.8.5
Power Plant Operation
The MAPS consists of two pressurized heavy water reactors, of 235
MWe capacity each (presently derated to 170 MWe), Units I and II became
operational on 23rd July, 1983 and 18th September, 1985 respectively. It has a
comprehensive nuclear power production, fuel reprocessing, and waste
treatment facility that includes plutonium fuel fabrication for fast breeder
reactors. It is India’s first fully indegenously constructed nuclear power
station. The station has reactors housed in a reactor building with double shell
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containment ensuring total protection even in the remotest possibility of loss
of coolant accident. The Kalpakkam Atomic Reprocessing Plant (KARP)
facility, with a capacity 100 tonne per annum, with several novel features and
concepts, was successfully commissioned at Kalpakkam in 1998. It
reprocesses spent fuel from the reactors.
MAPS uses the coastal waters of the Bay of Bengal as a heat sink. It
operates based on once through cooling system wherein the water is drawn
from an intake point in the sea, used in the heat exchangers for rejecting waste
heat, and then released back in to the sea at a location away from the intake
point. The seawater cooling system consists of an intake structure (located
420 m away from the shore in the sea), tunnel (468 m long and 3.8 m in
diameter) and fore-bay drawing seawater at the rate of 35m³/s. The tunnel
connects the fore-bay and pump house to the intake structure. From the intake
shaft, water flows by gravity into the fore-bay where 12 pumps draw and
circulate the seawater through the condensers and other heat exchangers. The
coolant flow, when all the 12 pumps are running, is about 3 m/s (MAPS
Design Manual 1975). The intake point is guarded by steel weld mesh screens
to prevent the entry of large objects into the cooling circuit. The intake point
is accessible from land through an approach jetty. The intake well is located at
the end of the jetty. After passing through the steam condensers and other
auxiliary heat exchangers, the seawater is discharged onshore through an
outfall structure (situated on the northern side of the jetty). A schematic
illustration of the power plant operation is shown in Figure 1.2 modified after
Anupkumar et al (2005).
From the outfall point, the discharged seawater flows through the
engineering canal before it mixes with the sea. Figure 1.3 shows MAPS
discharge prior to construction of the engineering canal. The main condenser
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of each unit is designed for a
T (temperature difference between inlet and
outlet) of 10ºC. Figure 1.4 depicts the seawall at Mahabalipuram. The outfall
discharge of the existing MAPS plant is 35m3/s, while the discharge from
PFBR outfall is estimated as 29m3/s. The combined discharge (64m3/s) will
be having a maximum flow velocity of 2 m/s (CWPRS 2006).
Figure 1.2 Schematic of MAPS Operation
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27
Figure 1.4 Aerial and Photographic View of Mahabalipuram
1.9
ORGANIZATION OF THE THESIS
This thesis has seven chapters. Chapter 1 gives an introduction to
the processes involved in sediment transport, reasons for taking up the study,
objectives of the research and an overview of activities IGCAR, Kalpakkam.
Chapter 2 presents a detailed literature review of all the aspects considered in
the study. Chapter 3 discusses the methodology which includes sampling
strategy, data collection, softwares used and data interpretation methods.
Textural characteristics and weathering pattern of the sediments are described
in Chapter 4. In Chapter 5, shoreline dynamics of Kalpakkam coast has been
discussed. Chapter 6 throws light on the variations in sediment transport and
the underlying processes. Finally in Chapter 7, the summary and conclusions
of the research and recommendations for future study are presented.