ii
DECLARATION
I hereby declare that the work incorporated in this dissertation, which is in
partial fulfillment of M. Sc. Degree course, at Goa University; is original and
carried out at the National Institute of Oceanography, Dona Paula, Goa, and it
has not been submitted in part or as a whole for any degree or diploma at any
other university.
Date: 14.04.2012 (Mandar V. Joglekar)
Place: Dona Paula

iii
ACKNOWLEDGEMENT
I am greatly indebted to Dr. S. R. Shetye, Director, National Institute of
Oceanography, Dona Paula, Goa, for granting permission and providing the
facilities to carry out dissertation work at the National Institute of
Oceanography, Goa.
I am extremely greatful to my supervisor Dr. A. K. Chaubey, Chief Scientist,
National Institute of Oceanography, Dona Paula, Goa, for providing great
opportunity to work under his valuable guidance, supporting and
encouraging me. I profusely thank him for caring, patience, and providing me
with an excellent atmosphere for doing research work at this stage. It is to him
that I owe success of this dissertation.
I am whole heartedly thankful to Dr. G. N. Nayak, Dean, Life Sciences and
Environment, Goa University, for his excellent and constant guidance as well
as directives during the course of my academic career and this dissertation
work.
I would like to thank Dr. H. B. Menon, Head - Department of Marine
Sciences, Goa University, for providing and availing me an opportunity to
work on topic of my own interest for this dissertation work. I also owe my
work success to my teachers Dr. S. Updhyay, Dr. V. M. Matta, Dr. C. U.
Rivonkar, Dr. Aftab Can for their patient guidance during the course of my
academic career.
I would also like to thank Dr. M. V. Ramana, Chief Scientist, National
Institute of Oceanography, Dona Paula, Goa, for his suggestions regarding
basic concepts of the features in this study.
I would like to thank Dr. K. Srinivas, Senior Technical Officer (3), National
Institute of Oceanography, Dona Paula, Goa, for his timely critical remarks,
valuable suggestions in the discussion during this dissertation work.

iv
I would like to convey my thanks to Mr. Mithun Gawas, for his extended
support in installation of the required softwares and technical support from
time to time.
I would like to greatly thank Mr. Datta Harmalkar, in helping me from time
to time for generation of maps used in this dissertation work and also its
scanning and printing..
I would like to convey my thanks to Mr. Avinash Pandey, Mr. Shahadur
Prasad, Mr. Javed Mujawar, for their help and support during the
dissertation work.
I express my sincere thanks to NIO Library staff for their extended help and
kind gesture during library survey of dissertation work.
The valuable help rendered by Mr. Ritej Banaulikar and Ms. Myra George
during scanning, printing is highly acknowledged.
Most importantly I would like to thank my loving parents for their most
supportive gesture, unconditional love, care, continuous encouragement. It is
because of their unending support I am able to reach here so far. It is to them I
owe my all career successes and my future.
I would like to specially thank Ms. Neha Kudalkar, for her patient, pertinent,
kind and moral support which was most needed every time to carry out this
dissertation work and academic career throughout, I definitely owe this to her
too.
I would also like to thank all my friends for their help in the dissertation
work.

v
CONTENTS
CHAPTER 1 - INTRODUCTION
1.1 General Background 2
1.2 Study Area 3
1.3 Objectives of the study 3
1.4 The Seismic reflection method 5
CHAPTER 2 - GENERAL SETTING
2.1 Palk Bay 8
2.2 Gulf of Mannar 9
2.3 Adams Bridge 10
2.4 Coastal Features
2.4.1 Rameshwaram Island 12
2.4.2 Mannar Island 13
2.4.3 Coral reefs 14
2.5 Sediment dynamics in the Palk Bay
2.5.1 Sedimentation 15
2.5.2 Sediment exchange between Palk Bay and Gulf of
Mannar 17
2.6 Tectonic Setting 19
2.7 Sea level changes 22
CHAPTER 3 - DATA AND METHODOLOGY
3.1 Data 26
3.2 Methodology
3.2.1 Identification and mapping of paleochannels 28
3.2.2 Channel dimensions 28
3.2.2 Quantitative measurements of geomorphologic
parameter for buried channel systems 30

vi
CHAPTER 4 – PALEOCHANNEL MORPHOLOGY,
DIMENSIONS AND PALEODISCHARGE
ESTIMATES
Introduction
4.1 Paleochannel morphology 35
4.2 Channel dimensions and paleo-flow estimate 52
4.2.1 Channel Depth 52
4.2.2 Channel width 52
4.2.3 w/d ratio 52
4.2.4 Cross-sectional area and Paleodischarge estimates 55
4.2.5 Channel Velocity 55
4.2.6 Channel slope % 56
CHAPTER 5 - DISCUSSION AND CONCLUSIONS
5.1 Discussion 59
5.1.1 Development of observed channel system
5.1.2 Depositional environment in relation to
quantitative geomorphological parameters 59
5.1.3 Timing of formation and ages of buried channel
systems 60
5.2 Conclusion 60
5.3 Future Work 62
63
LIST OF FIGURES vii
LIST OF TABLES ix
LIST OF EQUATIONS x
REFERENCES xi

vii
LIST OF FIGURES
Fig 1.2.1 Study area (rectangular box) in the Palk Bay, East coast
of India. Solid lines in rectangular box represent high
resolution shallow seismic survey tracklines. 4
Fig 1.3.1 Schematic representation of seismic reflection technique 6
Fig 2.3.1 Bathymetry of Palk Bay (after Subba Rao et al, 2008) 11
Fig 2.6.1 Location of fault parallel to Dhanushakodi (Vaz et al.,
2006) 20
Fig 2.7.1 Age-Depth relationship and sea-level values (after Rana
et al, 2007) 23
Fig 2.7.2 Holocene sea level curve for western Indian continental
margin relative to present sea level and generalized
envelope curve (shown by dotted lines) (after Hashimi,
1995) 24
Fig 3.1.1 Map depicting High resolution shallow seismic tracks.
L01, L04, L07 represent track numbers. Trackline
spacing is 150 m. 27
Fig 3.2.1 Geometry of a paleo-channel in seismic section and
nomenclature for channel parameters. 30
Fig 4.1.1 Map showing extent of buried channel system in the
Palk Bay, East coast of India. 36
Fig 4.1.2 Seismic image of Channel-1 along trackline L01 39
Fig 4.1.5 Seismic image of Channel-2 & Channel-3 along trackline
L01 40
L04 41
L07 41

viii

ix
LIST OF TABLES
Table 2.4.1 Beach profile of Rameshwaram island 12
Table 2.4.2 Longshore sediment transport rate 18
Table 4.2.1 Paleochannel depths, width and w/dmean ratios 53
Table 4.2.2 Average width/dmean ratios for paleochannels in the
study area 54
Table 4.2.3 Cross-sectional areas for paleochannels in the study
area 55
Table 4.2.4 Computed approximate paleodischarge values using
equations of fluvial environment and tidal
environment for paleochannels in the study area. 56
Table 4.2.5 Computed approximate channel velocity values using
equations of fluvial environment and tidal
environment for paleochannels in the study area. 57
Table 4.2.6 Computed values of various parameters for using
fluvial environment and tidal environment equations
for paleochannels in the study area 57

x
LIST OF EQUATIONS
Eqn 3.1.1 Equation to calculate depth using TWTT
28
Eqn 3.2.1 Equation to calculate Mean channel depth
29
Eqn 3.2.2 Paleodischarge (tidal equation) 31
Eqn 3.2.3 Paleodischarge (fluvial equation) 32
Eqn 3.2.4 Equation to calculate velocity of the paleochannel by
continuity equation 32
Eqn 3.2.5 Equation to calculate slope % between two channel base
depths 32

2
1.1 General Background
The oceans, our last frontier constitutes 71% of the Earth‟s crust. The
oceans floor is as much varied and irregular as land. From the ancient
times we, the humans have constantly striving for the knowledge of
the earthen features above and below its surface. It is the last century
that comprehensive idea of the sea-floor has been gathered. The major
features include ridges, sea-mounts & guyots, sedimentary basins
deep-sea trenches etc.
The study of interaction between sea and land in the coastal area is
important to investigate paleo-environment changes. Paleochannels
evolution in the coastal area since the last glaciation is under the
control of global regression, transgression and the fluctuation of the
sea-level. It is a result of the sea-land interaction. The sediment
characters and geometries of paleochannel fills enable us to analyze the
channel morphology and hydrologic conditions, and then retrodict
paleo-geographic environments.
Buried channel systems contain a mass of information for sedimentary
and evolutionary process. Further, geomorphology of buried channel
system as well as paleo-drainages in near shore region provides a
broader understanding of the variables affecting deposition, erosion
and preservation of sedimentary strata. Therefore, identifying &
delineating paleochannels, characterising its morphometric
parameters, establishing hydrodynamic conditions and explaining
their filling process are of great significant for understanding
sedimentation and paleo-geographic environment changes.
Investigations of buried channel system, especially those of Quaternary
age, have advanced through the application of modern geophysical
research methods, like ground penetrating radar, and high resolution/
very high resolution seismic surveys. In this study, high-resolution

3
shallow reflection seismic method is used to map shallow subsurface
features in the Palk Bay. A brief description of the method is presented
at the end of this chapter.
1.2 Study Area
The study area (Fig. 1.2.1) covers 300 m wide corridor in the Palk Bay
and lies east of the maritime boundary between India and Sri Lanka
which is potential region for the fishing activity. The geographical
extents of the area of study lie between latitudes 9°11‟30”N & 9° 14'
30”N, and longitudes 79° 31' 30”E & 79° 35' 30”E. The area is a part of
shallow basin northeast of the Adams Bridge with water depths
ranging from 10 to 16 m.
1.3 Objectives of the study
The present study primarily aims on identification & delineation of
buried channel systems in the Palk Bay based on high resolution
shallow seismic reflection data. The study also quantitatively describes
morphometric parameters and hydrodynamic conditions of the paleo
channel. In this study high resolution shallow seismic reflection data
along three track lines (L01, L04, and L07) spaced at 150 meters, is used
to investigate the abovesaid features. The main objectives of this
dissertation work are to:
 Identify buried channel systems using high resolution shallow
seismic data
 Delineate buried channel system
 Estimate channel dimensions and paleo-discharges
 Hypothesize time of formation of the channel observed in the
seismic sections

5
 Understand the process of formation of features under
investigation.
1.4 The seismic reflection method
Seismic reflection method is the most widely used geophysical
technique. Seismic reflection profiling involves the measurement of the
two-way travel time of seismic waves transmitted from surface and
reflected back to the surface at the interfaces between contrasting
geological layers (Fig. 1.4.1). Reflection of the transmitted energy will
only occur when there is a contrast in the acoustic impedance (product
of the seismic velocity and density) between these layers (Telford,
2004). The strength of the contrast in the acoustic impedance of the two
layers determines the amplitude of the reflected signal. The reflected
signal is detected at sea surface using an array of high frequency
hydrophones.
There are two types of seismic reflection techniques: (i) Single channel
reflection and (ii) Multichannel reflection. In single channel reflection
technique only one source and receiver are used with an equal distance
between the source and receiver. This is repeated for several positions
along a line. Multichannel systems use one source and several
receivers, which measure at the same time. There are different ways
with which the source and receivers can be oriented to get the best data
quality and coverage.

6
Fig. 1.4.1 Schematic representation of seismic reflection technique.

8
GENERAL SETTING
Along the southern coast of India, the Gulf of Mannar and Palk Bay lie
on the south and north respectively of the narrow peninsular extension
of the mainland that is connected to the Pamban Island. The area is
exposed to the both the southwest and northeast monsoons. The
maximum atmospheric temperature of 35°C was observed in April in
the Palk Bay as against 320C in March. The surface water temperature
was highest in August (31°C) and in April (32.80C) in Palk Bay,
compared to 32.60C and 31.80C respectively in the Gulf of Mannar.
(Sulochanan & Muniyandi, 2005).
2.1 Palk Bay
A bay is a body of water surrounded by land from most of the sides
but one or two. Palk Bay covers an area of 12,285 km2 with Bay of
Bengal to the north and Gulf of Mannar to the south (Subba Rao et al.,
2008). It extends between latitude 9º 17‟ - 10º 15‟ N and longitude 78º
55‟ - 80º 00‟ E (Chandramohan et al., 2001). The Palk Bay is about 110
km long and is surrounded on the northern and western sides by the
coastline of the State of Tamilnadu in the mainland of India
(Kumaraguru et al., 2008). Palk Bay on the SE Coast of India is
considered as one of the major sinks for sediments. The sediments
discharged by the rivers and transported by the surf currents as littoral
drift, settle in permanent, semi-permanent and temporary sinks
(Chandramohan et al., 2001). Palk Bay is very shallow and is largely
occupied by sand banks and shoals (Agarwal, 1988).
The peninsula formation at Mandapam is separated from the
Rameswaram island by the Pamban viaduct, a channel. This charnel
brings in sediments from the Gulf of Mannar to the Palk Bay
particularly during the southwest monsoon (June - September). During

9
the Northeast monsoon season of October to December, the wind and
wave action make the water turbid in Palk Bay (Pillai, 1975).
Loveson et al (1990) classified the coastal zone of Palk Bay into 3
groups; (i) uplands/highlands with scantly vegetation, comprised of
Cuddalore sandstone formations, (ii) along the lower elevations
sedimented Cuddalore sand stones, and (iii) coastal lands mainly of
microdeltas, swamps, and beach ridges based on the geomorphological
features (NEERI, 2004).
Sediments in the Palk Bay are brought by riverine discharge, littoral
transport and currents. Sediments move towards south during the NE
monsoon and north in the SW monsoon. The annual long shore
sediment transport along Nagapattinam Coast, during October to
February (NE monsoon) is around 0.273 X 106 m3 towards south and
0.175 X 106 m3 towards north during the rest of the year. The gross
sediment transport is around 0.448 X 106 m3 in the Bay. The net long
shore sediment transport is between 0.098x106 m3 towards south, in the
Bay (Sanil Kumar et al., 2002).
A strait is a narrow, navigable or non-navigable channel that connects
two larger navigable bodies of water. It can be referred as channel of
water mass that lies between two land masses. Palk strait is a part of
Palk Bay which acts as an inlet of Bay of Bengal. Between Kodikarai
(India) and Kenagesan Thurai (Sri Lanka) known as Palk Strait, the
depth ranges from 2 to 10 m. (Subba Rao, 2005)
2.2 Gulf of Mannar
A Gulf in general can be stated as a part of ocean or sea extending into
land portion. The Gulf of Mannar is a transitional zone between the
Arabian Sea and Indian Ocean proper and is connected with the Bay of
Bengal through a shallow sill, the Palk Strait (Gandhi et al., 2007). The
gulf consists of several coral reef islands and of the 21 coral islands

10
present along the coastline between Tuticorin and Pamban; most of
them are close (2 to 18 km) to the main land. The Gulf of Mannar
receives riverine input through a number of small rivers and streams of
which the Tambraparni River is the major source. The minor rivers are
the Gudar, Vaipar, Karamaniar and Nambiyar (Jonathan et al., 2004).
2.3 Adams Bridge
Adams Bridge is composed of chain of shoals from the Dhanushkodi
tip of Pamban Island (India) and ends at Mannar Island (Sri Lanka).
The submerged island chain acts as a subsurface physical barrier,
between Rameswaram Island and Mannar Island. Indian Ocean waters
enter the Gulf, and the water at the head of the gulf under the influence
of the southwest monsoon is piled up like a cushion (Silas, 1968). The
Bay of Bengal waters entering through Palk Strait have a major
influence on the hydrographic conditions of Palk Bay, while the Gulf
waters influence Palk Bay to a minor extent only (Murty & Varma,
1964). Geological Survey of India (GSI) carried out a special
programme called “Project Rameswaram” that concluded that the
domain between Rameswaram and Talaimannar may have been
exposed around 18,000 years ago.
The bathymetric data shows that across Adams Bridge between
Arippumunai (India) and Thaliamannar (Sri Lanka), the water depths
vary from 1 to 3 m (Subba Rao et al., 2008).

11
Fig. 2.3.1 Bathymetry (m) of the Palk Bay (after Subba Rao et al., 2008)
Adams Bridge consists of several parallel ledges of conglomerate and
sandstone, hard at surface and growing coarser and soft as it descends
till it rests on a bank of sand (Bahuguna et al., 2003). Adams Bridge has
reef crest on Southern side and crescent shaped sand cays along with
intermittent deep channels. There are about 103 small patch reefs lying
in a linear pattern, generally of ovoid shapes found in waters of
moderate depths.
Sand Cays are accumulations of the loose coral sand and beachrock,
generally situated on Lee side of coral platform. Orientations of the
sandy cays of Adams Bridge have changed between 1900 and 2000.

12
Formation and destruction of sand cays has been observed between
these periods suggesting unstable nature of cays (Bahuguna et al.,
2003).
2.4 Coastal Features
There are number of coastal landforms around the study area. Major of
those landforms are described briefly in the following sections:
2.4.1 Rameshwaram Islands
The southeastern tip of peninsular India assumes much importance
from a geological point of view. Geomorphologically, the onshore area
is known for its extensive stretch of longitudinal sand dunes and sandy
beach. (Vaz et al., 2007)
Rameshwaram Island is located on the eastern part of the
Ramanathpuram district of TamilNadu. It is bounded between
latitudes 90 8' 55"N and 90 19'N and longitudes 790 12' 30"E and 790 27'
30"E. Rameshwaram has several topographic expressions, which are
signatures of interaction of marine and aeolian processes (Prabhakaran,
2010).
A beach profile of Rameswaram Island in general exhibit the
characteristics as follows:
Table 2.4.1 Beach profile of Rameshwaram island
Zone Slope Description
Near shore zone
Slope
upto 5°
Situated in front of the beach, submerged
under sea water even during low tide.
Fore shore zone
slope
Slope 5°
to 15°
Situated between high tide and low tide
mark.
Back shore zone Flat
Rarely submerged during storm surge.
Aeolian activity is common.

13
Landforms observed in the Rameswaram island are
 Beaches
 Beach Ridges and Swales
 Dunes and Sandy Plains
 Lagoons and Mud-Flats
 Creeks
 Spits
The coast of Rameshwaram can be classified in two sectors. Northern
coast extends from Pamban through Rameswaram town to
Dhanushkodi and the southern coast extends from Pamban through
Ramkrishnapuram to Dhanushkodi. Beaches occur through the total
length of 72 km without any break. The dominant wave action from
southern coast of Tamilnadu makes these beaches most dynamic
landform of the area. They are composed of fine and medium sand. In
northern portion, the beaches are terminated by coral cliffs. The tail like
portion in the southern area is made up of sandy barrier beaches. The
tail like portion of Rameshwaram Island is a spit that has formed by
the movement of littoral current between Rameswaram and Sri Lanka
(Prabhakaran, 2010).
2.4.2 Mannar Islands
There are 21 uninhabited islands in Gulf of Mannar and each island is
ranging from 0.25 to 129.04 hectares and are located between
Mandapam and Tuticorin of Tamilnadu coast, covering a distance of
about 140 km. The reefs are mostly located around the islands. Gulf of
Mannar (GOM) of southeast coast of India is predominantly coral reef
ecosystem with rich diversity of flora and fauna (Pillai 1975).
The northern and southern shores of Mannar Islands are comprised of
sandy beaches. They are a part of the Mannar Barrier reef, which is
about 140 km long and 25 km wide between Pamban and Tuticorin.

14
Different types of reef forms such as shore, platform, patch and
fringing type are also observed in the Gulf of Mannar. The islands have
fringing coral reefs and patch reefs around them. Narrow fringing reefs
are located mostly at a distance of 50 to 100 m from the islands. On the
other hand, patch reefs rise from depths of 2 to 9 m and extend to 1 to 2
km in length with width as much as 50 meters.
The total area occupied by reef and its associated features is 94.3 sq.
km. There are about 96 species of corals belonging to 36 genera in the
Gulf of Mannar (Pillai, 1975).
2.4.3 Coral Reefs:
Small coral reefs of fringing type are found in the Palk Bay and Gulf of
Mannar at the south-eastern coast of India. They are located chiefly
around the various islands lying between Tuticorin and Rameswaram
in the Gulf of Mannar, and in Palk Bay at Mandapam and along the
eastern side of Rameswaram Island (Pillai, 1969).
Coral reefs are the natural barriers and protect the shorelines of islands
and landmasses against natural calamities. Due to their fragile nature,
they can be easily destroyed (Wilson, 2005). Coral reef and seagrass
ecosystems are unique in embracing a plethora of floral and faunal
species with higher biological productivity. The carbonate skeletal
structures of coral reefs are effective barriers which dissipate wave
energy and create low energy environs in the seas (Shridhar, 2008).
The reefs also reduce the action of currents on shorelines, thus
preventing erosion in those areas and help sea grasses to establish and
grow. In India coral reefs are present in the Gulf of Kutch, Gulf of
Mannar, Palk Bay, Lakshadweep, and Andaman & Nicobar Islands.
The coral reefs are formed along the shores of mainland. The natural
parameters like light, temperature, salinity and settlement of silt
(mainly) influences the growth of corals (Wilson, 2005).

15
The Palk Bay region along east coast of India is influenced by both
northwest and southeast monsoon. Apart from monsoon effect causing
siltation, mechanical damage to corals is done by cyclone at times in
this region (Pillai, 1975). Sedimentation affects their mass spawning,
reduces light penetration and hence photosynthesis of the symbionts
resulting in significant differences in rates of extension and calcification
of corals (Lough & Barnes, 2000).
The Palk Bay remains practically calm during most of the months
except at the onset of northeast monsoon when turbulent conditions
prevail. No fresh water streams dilute the sea near Mandapam. The
tidal range is usually within amplitude of a metre (Pillai, 1969). Corals
in Palk Bay are generally found to be healthy and seen with extended
polyps.
Corals are normally quarried illegally from October to May in Gulf of
Mannar. About 100 - 125 tons of coral reefs are removed daily during
peak period of activity. As a result of the continuous and unrestricted
excavation over many years, corals and its associated fauna and flora
get destroyed continuously (Patterson Edward, 2000).
2.5 Sediment Dynamics in the Palk Bay
2.5.1 Sedimentation:
Vaigai, Vaishali and Variyar rivers and the littoral transport by various
sources from the northern part of the Tamil Nadu coast are the major
sediment sources entering the Palk Bay region. The mouth of the Palk
strait is dominated by sand particles, which is due to the accretion
nature of the coastal area (Sundararajan, 2010).
Occurrence of cyclonic storm during north-east monsoon is common in
the Nagapattinam–Poomphur region, which causes erosion along this
region (Jena, 2001). The sediments are transported southerly and

16
deposited in the Palk Bay. Low wave action inside the bay and
protection from the southerly waves encourages the deposition of
sediment (Ramesh, 2004).
Sediments are brought into the Palk Bay by (Subba Rao et al., 2008):
1. Rivers mainly Vaigai, Vaishali, and Variyar on the east coast of
Tamil Nadu.
2. Littoral transport from the northern parts of Tamil Nadu.
3. Littoral transport from the Gulf of Mannar through Pamban
Pass and Adams Bridge.
4. Cyclones that occur, especially in the Nagapattinam - Poomphur
region.
5. Rivers flowing into the Bay from Sri Lanka
6. Currents from the Bay of Bengal.
It has been observed that the shelf sediments of the east coast of India
(Krishna and Godavari basins) are very fine in nature, but show very
low organic matter in the silts and clays. The major sources of
carbonate materials for the sediments in the creek are shell fragments
and calcareous tests of organisms (Subba Rao; 1960).
Geologically, thick section of Quaternary alluvium overlies the
Archean charnockite rocks and these are in turn overlain by the
Holocene tidal flat deposits in Palk Bay. The detailed lithological
observation of cores reveals that the sediments have been depositing in
phases and that there has been pulsating supply of fine sediments onto
the tidal flats and estuaries (NEERI, 2004).
According to studies by Chandramohan et al. (2001), 0.3 × 1010 m3
sediment got deposited over a period of 51 years in the Palk Bay
region. It was assumed that the rate of accumulation is uniform over
these years, it is estimated that between the years 1931 and 1982, the

17
sediment deposition has caused a reduction in water depth of about
0.32 m, i.e. 0.006 m per year.
2.5.2 Sediment exchange between Palk Bay and Gulf of Mannar:
The Pamban Pass, connecting Palk Bay and Gulf of Mannar breaks the
continuity of longshore current between the mainland and
Rameswaram Island, the magnitude of the current on either side of
Pamban Pass is found to be very weak. This reduces the volume of
littoral sediments approaching the Pamban Pass which in turn reduces
the quantity of sediment passing through Pamban Pass from Gulf of
Mannar to Palk Bay (NEERI, 2004). The longshore current direction
prevailed northerly during southwest monsoon and fair weather
period, and southerly during northeast monsoon between Sippikulam
and Uthallai.
The phenomenon of northerly currents along the mainland and
westerly current along Rameswaram create a zone, wherein, most of
the littoral drift will get deposited. Only a fractional proportion is
expected to move from this region by tide induced currents towards
the Adams Bridge. This would reduce the volume of littoral sediment
reaching the Adams Bridge and in turn the quantity of sediment
entering Palk Bay from Gulf of Mannar. These sediments deposited at
shoals is supplied back to the littoral system for the mainland, when
the longshore currents move towards south during the ensuing
northeast monsoon (NEERI, 2004).
The accumulation of littoral drift on either side of Rameswaram Island
occurs during southwest monsoon and removal during northeast
monsoon, making this region as a sediment storage reservoir.
Palk Bay is very shallow and is largely occupied by sandbanks and
submerged shoals. Rameswaram Island, the geological formation of
coral atoll with huge sand cover between India and Sri Lanka plays a

18
vital role on the processes of exchange of littoral drift between east
coast and west coast (NEERI, 2004). It separates the sea in the north by
Palk Bay and south by Gulf of Mannar.
The wave sheltering effect due to Sri Lanka Island, the large siltation in
Palk Bay, the presence of numerous offshore islands in Gulf of Mannar,
the growing sand spit along Dhanushkodi and the shallow reef
(Adams Bridge) between Arimunai (India) and Thalaimannar (Sri
Lanka) largely modify the sediment movement.
Table 2.5.1 Longshore sediment transport rate (NEERI, 2004)
During southwest monsoon, the longshore sediment transport is
considerable (>10 X 103 m3/month) along the spit facing Gulf of
Mannar and negligible on Palk Bay side. Near Arimunai, the longshore
transport direction dominated in easterly direction indicates the
movement from Gulf of Mannar to Palk Bay through Adams Bridge.
Locality Name
Annual net
transport (x 103
m3/year)
Direction of annual
net transport
Sippikulam 1.4 South
Kannirajapuram 25.6 North
Valinokkam 3 North
Vedalai 1 North
Kondugal 10.2 North
Mukkuperiyar West 5.6 North
Mukkuperiyar East 79 North
Dhanushkodi West 22.1 North
Dhanushkodi Mid 32 North
Dhanushkodi East 80 North
Arimunai West 43.7 North
Arimunai East 36.4 North
Mukkuperiyar West (Palk Bay) 2.7 North
Uthalai West (Palk Bay) 4.6 North
Villuvandithirtham 1.6 North
Light House 0.1 South
Ariyaman 23 North

19
In northeast monsoon, the values of longshore transport rate was
relatively low along the spit facing Gulf of Mannar and negligible in
Palk Bay. The sediment transport direction was consistently towards
west in Gulf of Mannar and east in Palk Bay.
In fair weather period, the longshore sediment transport was low along
the spit facing Gulf of Mannar and also Palk Bay. The transport
direction is westerly near the tip facing Gulf of Mannar. It shows that
in February, April and May the sediment drifts from Palk Bay to Gulf
of Mannar. Consequently, in March, June, July, August and September,
sediments drift from Gulf of Mannar towards Palk Bay. There was no
significant movement of sediment observed during October to January
(NEERI, 2004). No noticeable exchange due to wave induced longshore
transport takes place in northeast monsoon.
Due to low littoral drift taking place during northeast monsoon, the
quantity of sediments entering Gulf of Mannar from Palk Bay will be
much lower than the quantity moving from Gulf of Mannar to Palk
Bay during southwest monsoon. It signifies that the region around
Adams Bridge forms as significant sink for the littoral drift. The
prolonged accumulation may lead to the emergence of new islands.
Once the sediments enter Palk Bay, the environment favours
immediate deposition.
2.6 Tectonic setting
There is evidence (Vaz et al., 2008) of the vertical neo-tectonic
movements due to active fault in the Palk Bay, and around
Dhanushkodi in Rameshwaram island. Presence of active faults on
land or in the nearby sea are discernible from shifting of river course,
disposition of coastline, presence of thermal springs unrelated to
volcanic activity, along E-W line around Manamelkudi, seismic
activity, structural lineament, etc.

20
The coastline along the western Palk Bay is arcuate and smooth except
near Manamelkudi near Vellar river mouth. Here the coastline is
irregular due to the presence of small cuspate forms, micro deltas, etc.
The bathymetry along this stretch also shows a spit perpendicular to
the shore. It extends upto 9 km in offshore to a maximum depth of 1.8
m. The bathymetric contours of 2 m and 5 m off Manamelkudi are
perpendicular to shore. These features suggest upliftment of seabed
with unconsolidated recent sediments by a vertical tectonic movement
along East-West trending fault.
Figure 2.6.1 Location of fault parallel to Dhanushakodi (Vaz et al., 2006)
Similarly minor faults along streamlets sympathetic to the Vellar River
fault also exhibit similar but minor scale geomorphic features in the
offshore. Presence of thermal springs around Manamelkudi is also

21
considered a significant feature in support of fault activation (Vaz et
al.; 2006).
The subsidence and submergence of the southern part of
Dhanushkodi township (Fig 2.6.1) during 1948-1949 along a WNW-
ESE trending fault has been well established. The fault indicates
that the area has undergone neo-tectonic movement. Shallow
bathymetry and side scan sonar surveys, along with seabed sampling
and underwater videography, have suggested that vertical tectonic
movement occured along the fault parallel to the coastline. The fault
has a displacement of about 5 meters which led to the subsidence
of southern part of Dhanushkodi township during 1948-1949. (Vaz
et al., 2007). This is the latest neo-tectonic movement ever recorded
along the area.
The observation of Vaz et al. (2007) revealed that the southern side
of Dhanushkodi township has undergone severe erosion and
subsidence. The bathymetric contours indicate that the scarp face
aligned parallel to the coast from Makundarayarchatram to
Dhanushkodi tip suggesting the actual extent of subsidence. It
further indicates that the fault in the offshore has occurred along
the WNW-ESE direction parallel to the coastline. This detected fault
appears to be sympathetic to the regional lineament through
which the river Vaigai flows in the adjoining mainland. Due to
the submergence of coastal zone the shoreline has migrated
landwards and pre-existing coastal plain deposit were reworked
by nearshore waves and currents.
Vaz et al. (2007) emphasized that this observed fault in the area
may be the surficial manifestation of a deep seated major faulting
at depth, and may pose hazards of reactivation, about which nothing
can be predicted at this time. It is pertinent to note here that
Ramasamy (2006) suggested that the southern part of Indian Peninsula

22
is tectonically active due to presence of north-south trending
extensional and northwest-southeast trending dextral faults.
2.7 Sea-Level changes:
The study area in this work falls under coastal segments and it is
required to relate any process or features to the sea-level changes in the
past and its effect on them. There is still very less data related to sea-
level changes along east coast of India but notable research works are
referred in this study.
Post glacial transgression started earlier than 12,500 years, the sea-level
rise was rapid during the initial stages of transgression. After initiation
of Holocene transgression the sea-level was at stand still at about -80 m
for some time and it again rose to about -60 m depth. The sea-level rise
during Holocene transgession was not continuous and was interrupted
by temporary pauses (Mohan Rao, 1994).
Banerjee (2000) studied regional RSL (Relative Sea Level) changes on
the east Indian coastline utilizing beach ridges and exposed Porites
coral colonies to indicate two late Holocene highstands (Woodroffe &
Horton; 2005). The relative sea-level curve of the east coast of India for
mid-Holocene period, on the basis of evidences & their 14C ages from
Cape Comorin to Godavari delta, proves that the sea-level stood
approx. at 3 m above present LTL around 7,300 +/- 130-110 yrs B. P.
and at above the present LTL around 5,500 yrs B. P. (Singh & Chadha;
2008).
Earlier studies suggested that the sea-level fluctuations all along the
east coast of India have been caused by a combination of neotectonic
movements and glacio eustatic sea-level changes. The first Holocene
highstand at approximately 3 m above present LTL was reached at
7,300 +/- 130-110 radiocarbon (calibrated) yr B. P. the ensuing
stillstand prevailed over nearly 1.7 kyr, thereafter a relative fall in sea-

23
level of approx. 2 m affected the stable segments of east coast of India
probably as a result of continuous hydro-isostatic adjustment of
increased load of melt water in the ocean basins. This was followed by
a second pulse of minor sea-level rise establishing near the earlier
highstand position and remaining stable from about 4,300 to 2,500 yrs
(calibrated radiocarbon age) B. P. (Banerjee, 2000).
The sea-level curve for east coast of India by Rana et al. (2007) suggests
and supports that the sea-level during 11,000 yr B.P. stood 80 m less
than present sea-level and there was sharp rise in sea-level between
11,000-7,500 yr B.P. upto -20 m. Sea-level curve for west coast of India
published by Hashimi et al. (1995), supports the facts stated in above
mentioned literature as well. The curve shows a high gradient between
11,000 - 7,000 years B.P. which could be related to the sharp rise in the
sea-level. The curve shows a low at 100 m depth around 14,500 years B.
P. and rise to 8- m depth around 12,500 years B. P. with the rate of ~10
m/ 1000 years. It was followed by a stillstand for about 2,500 years.
From 10,000 to 7,000 years sea-level rose at a very high rate (~20 m/
1000 years). After 7,000 years B. P. it fluctuated more or less at the
present level.
Fig. 2.7.1 Age-Depth relationship and sea-level values (after Rana et al., 2007)

24
Figure 2.7.2 Holocene sea-level curve for western Indian continental margin
relative to present sea leevel and generalized envelope curve (shown by dotted
lines) (after Hashimi 1995)

26
3.1 DATA
This study is based on interpretation of 55 km of high resolution digital
sparker seismic reflection data which were acquired onboard Coastal
Research Vessel (CRV) Sagar Sukti during 2010. The data was acquired
by Nearshore Group of National Institute of Oceanography, Dona
Paula, Goa in a 300 m wide corridor located northeast of Adams Bridge
and southeast of the maritime boundary between India and Sri Lanka
in the Palk Bay. The seismic surveyed tracks were spaced at 150 m
apart and designated as L01, L04 and L07. Each track is ~18.3 km long
(Fig 3.1.1).
Differential Global positioning system (DGPS) manufactured by
Trimble (Model 4000 SE) along with the differential beacons (operating
in 283.5 to 325 kHz band) was used for obtaining the positions during
the survey. The navigation data was logged using windows based
HYPACK® software during the survey. The recorded navigation data
were processed using the same software to edit/remove spurious data,
and generated position data track plot of the study area.
High resolution seismic system comprising of energy source CSP D700
& multi-tips squid spark array, data acquisition unit Octopus 760 and
20 element hydrophone array from M/s Applied Acoustics, UK was
used to acquire sub-bottom information in the survey area. The seismic
data was acquired along tracks L01, L04 and L07 using 300 joules
energy at a recording frequency band of 250-3000 Hz at a ship speed of
~4 knots. The seismic data was stored for a recording length of 500 ms.
The system was interfaced with the navigation computer to co-register
the position with sub-bottom data. The seismic data were processed for
re-sampling, filtering, gain correction, and muting to improve seismic

27
reflectors. In order to convert the Two Way Travel Time (TWTT) into
depth in meters, following formula is used:

28
Depth in meter = c*(TWTT/(2*1000)) (3.1.1)
where, c is the acoustic velocity (1500 meter/second in water, and 1585
meter/second in shallow sediments of the east coast of India (Krishna
et al., 1989). TWTT in milli second.
3.2 METHODOLOGY
3.2.1 Identification and mapping of paleochannels
Seismic analysis is based on: (1) identification of key seismic reflectors
showing erosion truncation; (2) internal configuration of seismic
reflections bounded by key reflectors; (3) angles and orientation of
reflectors; (4) seismic facies: amplitude, frequency, continuity.
Lenticular bodies interpreted as filled channels.
The flanks of buried channel systems are interpreted as inclined
surfaces that truncated stratal features. The observed transparent
seismic layers of the channel are considered channel-fills typically of
fine grained sediments of homogeneous acoustic properties.
The seismic sections are carefully analysed and initially channel flanks
are demarcated on the hard copy of each seismic lines. Considering
channel characteristics and its proximity from track to track, they are
joined together in the form of continuous channel systems.
3.2.2 Channel dimensions
Morphometric analysis of paleo-channels can provide estimates of
parameters needed to link channel morphology to paleo-hydrology.
Various morphometric parameters such as width, depth can be used to
derive hydrological equations and allow us to infer the environment of

29
their formation. In this section, the methodologies for determining the
channel dimensions are presented:
(i) Channel width
After marking channels on the seismic sections (Fig. 3.2.1), their extents
were calculated using fix number. The geographical location of the fix
numbers was taken from the navigation data file and the distance
between two fix numbers were computed using „Haversine‟ equation.
It is well known that the Haversine equation gives great-circle
distances between two points on a sphere from their longitudes and
latitudes.
(ii) Mean channel depth
Mean channel depth is calculated from the geometry of the channel
using following formula:
(3.2.1)
Where, dmean is mean value of the channel depth, and di is the channel
depth at channel point i, n is the total number of channel points where
channel depth is considered.

30
Fig. 3.2.1 Geometry of a paleo-channel and marking system for channel parameters
(dmax = maximum representative depth of the channel base, dmean = mean
depth of the channel)
3.2.3 Quantitative analyses of geomorphologic parameter of buried
channel systems
Morphometric analysis of any channel system is a major tool employed
in studies pertaining to channel morphology and channel discharge
estimates. This morphometric analysis can be applied to the paleo-
channel systems in the given study area; which includes applying
empirically derived hydraulic equations for the modern rivers and
estuaries to estimate former discharges on the basis of preserved paleo-
channel geometry.
Using this method we can estimate channel discharges for both fluvial
and marine channel systems. Sediment transport potential can be

31
estimated by using above mentioned method but requires
sophisticated techniques and tools for further analysis.
In this study measured parameters include width, depth, and
calculated parameters from these include width/depth ratio, paleo-
discharge, channel velocity and, channel slope % considering both
fluvial and marine environments and using appropriate hydraulic
equations for them.
(i) Cross-sectional area
Cross-sectional area of each interpreted buried channels are computed
using 2-dimensisional geometry of the channel in the seismic section.
The marked channels were referenced on a customized coordinate
system and areas were calculated using area measurement tool in
ArcMap of ArcGIS-10 software package. The areas computed using
GIS tool were validated with area computation by manual square-grid
method for selected channels.
(ii) Estimation of paleodischarge
As mentioned earlier, the study area lies within the shallow marine
environments in the Palk Bay, therefore, calculations regarding Paleo-
discharges were made for both fluvial conditions as well as tidal
conditions. Formulae for fluvial and tidal channel discharges are
different and are referenced from literature of worldwide studies and
are presented below:
Paleo-discharge for the presumed tidal systems can be estimated on the
basis of the power-law equation:
Aα (3.2.2)
where, Q is the paleodischarge, and A is the cross-sectional area
(Friedrichs, 1995). Mean value of α is taken as 0.96 (Nordfjord et al.,

32
2005; Friedrichs, 1995) for computation of paleodischarge in tidal
conditions.
Paleo-discharge of fluvial systems is calculated using following
empirical equation of Dury, 1976:
A1.09 (3.2.3)
(iii) Mean flow velocity
Mean flow velocities for both fluvial as well as tidal channel systems
are then calculated using the continuity equation:
(3.2.4)
where, Q is the Paleodischarge (m3/s), A is the cross-sectional area of
the channel (m2), V is approximate velocity of the channel-flow (m/s).
(iv) Channel slopes
Channel slope is defined as the steepness of the channel base along its
flow path. The channel slope is one of the major factors in determining
the velocity of water in the channel. The velocity and channel depth
determine the amount of sediment transport through the channel.
Channel slopes are generally measured as percentages. The slope or
gradient of the channel is computed as the vertical distance divided by
the length as it passes through 2 meanders or along a distance equal to
20-30 times bankfull channel width. It may be mentioned here that the
slopes calculated in this study are of exercise purpose as total surveyed
corridor is only 300 m wide whereas the observed width of the channel
is minimum of 470 m and therefore does not meet the requirement for
slope calculation as mentioned above. Following equation is used to
calculate slope percentages.
( ) ( ) (3.2.5)

33
where, S is the channel slope (%); d is the vertical distance and h is the
horizontal distance (150 m in present study).

CHAPTER 4
MORPHOLOGY, DIMENSIONS AND
PALEODISCHARGE ESTIMATES

35
INTRODUCTION
The study of coastal plain fluvial & tidal systems and their responses to
external controls are important for the reconstruction of past sea-level
fluctuations from sedimentary strata (Nordfjord et al., 2005). The
geomorphology of buried channel systems on continental shelves
provides a broader understanding of the variables affecting deposition,
erosion and their preservation.
High resolution seismic profiles of the present study resolved
subsurface channels in greater detail. In this study total 8 channels
have been mapped (Fig 4.1.1) and their morphometric (width, depth,
width/depth ratios) and hydraulic (channel slope, paleo-discharge,
flow velocity) parameters have been computed. Factors like channel
flank steepness, channel-fill characters and trunk widths have also
been taken into consideration during interpretation. The interpreted
paleochannel system is buried by sedimentary strata up to ~6 m thick,
whereas, the channel base is observed at an average channel depth of
~19 m. In this study mapping and analysis of various parameters of
this buried channel system confirms their complex nature in terms of
morphology, depositional environment and timing of formation.
4.1 Paleochannel morphology
The morphology of paleochannel defines the erosive conditions during
the time of its formation, filling & closure. In absence of dated seismic
reflector, current rate of sedimentation may provide approximate age
of the channel formation. The width of the channel in seismic section
range between few hundred meters to kilometer whereas, the depths
vary only within few meters; thus giving us high width/depth ratios.
The morphology of the channels observed in these sections is
predominantly V-shaped, whereas channel cross-sections display
symmetric to asymmetric character. Symmetrical, box-shaped

37
geometries represent larger channels whereas asymmetric and V-
shaped channels show differential channel flow direction in which a
particular flank of channel is under high bank shear. Smaller channels
are more triangular in morphology in cross section. A brief description
of the interpreted channel system is presented below:
(i) Channel-1
Channel-1 characterizes V-shaped morphology in all 3 sections L01
L04, L07 (Figs. 4.1.2-4.1.4). Channel in section L07 shows widened
channel base as opposed to channels in sections L01 and L04. The
flanks of the paleochannel are steeply dipping and dip increase from
L07 to L01.
(ii) Channel-2
Channel-2 characterizes V-shaped morphology in all 3 sections L01,
L04 & L07 (Figs 4.1.5-4.1.7). The channel base width shows increasing
trend as we move from section L01 towards L07. The channel flank
steepness also shows increasing trend as we observe from section L01
towards L07.
(iii) Channel-3
Channel-3 characterizes V-shaped morphology in all 3 sections L01,
L04 & L07 (Figs 4.1.5-4.1.7). The channel base width observed in line
section L01 is higher than other two sections in L04 & L07. Channel
flanks also show gentle trend as we observe from line section L07
towards L01.
(iv) Channel-4
Channel-4 characterizes V-shaped morphology in section L04 & L07.
Channel in section L01 shows no bifurcation which is distinctly
observed in line sections L04 and L07 (Figs 4.1.8-4.1.10). Also a part of
channel in section L04 shows box-shaped geometry rather than V-

38
shaped geometry. Channel in section L01 cannot be assigned any shape
character but it is just a wider and shallower channel. Channel flanks
characterize gentler trend as we observe towards L01 from L07.

39
Fig 4.1.2 Seismic image of Channel-1 along trackline L01

40
Fig 4.1.5 Seismic image of Channel-2 & Channel-3 along trackline L01

41

42

43

44
(v) Channel-5
Channel-5 characterizes V-shaped morphology in all 3 line sections
(Figs 4.1.11-4.1.13). Channel base widths show increasing trend as we
observe from L07 towards L01. Channel flanks also show gentle trend
as we observe from L07 towards L01. For line section L07 channel
shows wider flank width on left side (NW in the trackline) of the
channel, whereas channels for line sections L04 & L01 shows more or
less equal distribution of channel flank width. This can infer that in line
section L07; the channel experienced more boundary shear on the SE
flank than that of NW flank and more or less equal boundary shear on
both channel flanks of line sections L01 and L04.
(vi) Channel-6
(Figs 4.1.14-4.1.16) but channel in line section L01 shows very distinct
box shaped channel morphology. Channel base widths show
increasing trend as we observe from L07 towards L01. Channel flanks
also show similar trend of incrementing as we observe from L07
towards L01. These observations can primarily denote that the channel
growth and development is from L07 to L01 as the base widths are
increasing.
(vii) Channel-7
(Figs 4.1.17-4.1.19). Channel in line sections L07 and L01 are deeply
carved channels whereas channel in line section L04 is very shallow
and narrow channel. Channel shows bifurcation in case of line section
L07. Seismic expressions of this bifurcating channel are absent in line
sections L04 and L01 and it appears as a single channel. Channel flanks
show steeper gradient for all 3 sections.

45
(viii) Channel-8
This channel does not show any particular channel morphology in all 3
sections (Figs 4.1.20-4.1.22). Channel bases in all three sections show
very undulating morphology. The undulating base morphology can
suggest turbulent flow conditions.

46

47

48

49

50

51

52
4.2 Channel dimensions and paleodischarge estimates
4.2.1 Mean channel depth
The mean channel depth (Table 4.2.1) from channel top varies between
9.0 metres to 23.7 metres. The depth depicts that the channels are
shallow buried paleochannels.
4.2.2 Channel width
Channel width (m) = 354.8 to 2399.3
The paleochannel width varies from 354.8 to 2399.3 m (Table 4.2.1). The
channel width coupled with the paleochannel morphology can help
interpret the channel dynamicity in the past.
4.2.3 W/D ratios
Width/dmean ratio = 24.8 to 129.0
Width/dmean ratio varies from 24.8 to 129.0 (Table 4.2.1). All 8
interpreted channels in the present study reveal more than 40 w/dmean
ratio. The w/dmean ratio does speak about the intensity of the flow.
High w/dmean ratios are characteristics of tidal paleochannel system
(>50), whereas low w/dmean ratios depict estuarine system. Also lower
w/ dmean ratios generally correspond to smaller, V-shaped tributaries
and higher w/dmean ratios correspond to box-shaped trunk channels.

53
Table 4.2.1 Paleochannel depth, width and w/dmean ratio
Channel
ID
Line
ID
Channel depth
dmean (m)
Channel
Width (m)
w/dmean
1
L01 17.4 1906.6 109.5
L04 18.6 2399.3 129.0
L07 23.7 2082.2 87.7
2
L01 17.4 1211.9 69.6
L04 17.0 1156.8 68.0
L07 21.4 1437.8 67.3
3
L01 18.6 1073.3 57.7
L04 19.8 1026.7 51.9
L07 17.4 970.2 55.7
4
L01 10.3 1265.2 123.0
L04 15.4 1382.2 89.6
L07 18.2 1748.1 96.0
5
L01 17.4 1126.4 64.7
L04 20.2 811.9 40.2
L07 16.6 856.6 51.5
6
L01 19.8 1035.1 52.3
L04 16.2 861.1 53.1
L07 19.0 470.2 24.8
7
L01 15.8 472.5 29.9
L04 9.9 655.5 66.3
L07 12.5 849.3 68.1
8
L01 16.6 1798.4 108.2
L04 12.1 1177.3 97.5
L07 17.4 880.8 50.6
4.2.4 Cross sectional area and paleodischarge estimates:
The paleo-discharges are calculated for fluvial as well as tidal
conditions. The analyses of the computation (Table 4.2.2 and 4.2.3)
reveals that:(i) cross-sectional area (m2) varies from 1787.25 to 21199.14,
(ii) paleo-discharge (m3/s) using tidal equation varies from 1324.7 to
14232.0, and (iii) paleo-discharge (m3/s) using fluvial equation varies
from 2910.5 to 43128.6.
As will be discussed later in chapter-5, channel formation due to fluvial
discharge is not conceivable in the present study area due to high

54
values of fluvial discharge (2910.5-43128.6 m3/s) in a low gradient
coastal area. Therefore, paleochannel formation in the study area due
to tidal/marine environment is favored. Sümeghy (2011) suggested
that high paleodischarge values depict high energy regime and reflect
drastic environmental parameters in action.
Table 4.2.2 Cross-sectional area for paleochannels of the study area
Channel
ID
Line ID
Cross sectional
area (m2)
Ch - 1
L01 15078.4
L04 13514.1
L07 21199.1
Ch - 2
L01 12177.0
L04 8351.9
L07 16572.8
Ch - 3
L01 9791.9
L04 8003.7
L07 10742.0
Ch - 4
L01 9572.9
L04 7530.5
L07 9095.5
Ch - 5
L01 15595.5
L04 8678.8
L07 8462.5
Ch - 6
L01 13806.6
L04 8297.4
L07 5174.5
Ch - 7
L01 4498.8
L04 1787.2
L07 4425.10
Ch - 8
L01 10433.1
L04 4264.0
L07 12135.1

55
Table 4.2.3 Computed paleodischarge for fluvial and tidal environment
of paleochannels of the study area.
Channel ID Line ID
Paleodischarge (m3/s)
Tidal Equation Fluvial Equation
Ch – 1
L01 10261.8 29749.9
L04 9237.6 26402.0
L07 14232.0 43128.6
Ch – 2
L01 8358.3 23567.8
L04 5819.9 15625.1
L07 11236.3 32977.8
Ch – 3
L01 6780.1 18583.3
L04 5586.8 14916.6
L07 7410.4 20557.1
Ch – 4
L01 6634.4 18130.8
L04 5269.3 13957.9
L07 6316.5 17147.5
Ch – 5
L01 10599.4 30863.8
L04 6038.5 16293.0
L07 5893.9 15850.9
Ch – 6
L01 9429.4 27025.5
L04 5783.5 15514.2
L07 3675.5 9272.6
Ch – 7
L01 3213.5 7960.8
L04 1324.7 2910.5
L07 3162.9 7818.7
Ch – 8
L01 7205.7 19913.6
L04 3052.3 7509.0
L07 8330.7 23479.4
4.2.5 Channel flow Velocity
The channel velocity is a resultant of channel width, influx of water,
channel bank morphology and most importantly channel slope in
downstream direction. The high velocity channels cause deep cuts and
steeply dipping channel banks.
In this study, channel flow velocity using tidal equation varies from
0.67to 0.74 m/s, whereas, the velocity using fluvial equation varies
from 1.63 to 2.04 m/s (Table 4.2.4).

56
Table 4.2.4 Computed channel flow velocity using equation for fluvial
and tidal environment for paleochannels of the study area.
Channel
ID
Line
ID
Channel flow velocity (m/s)
Tidal Fluvial
Ch – 1
L01 0.68 1.97
L04 0.68 1.95
L07 0.67 2.03
Ch – 2
L01 0.69 1.94
L04 0.70 1.87
L07 0.68 1.99
Ch – 3
L01 0.69 1.90
L04 0.70 1.86
L07 0.69 1.91
Ch – 4
L01 0.69 1.89
L04 0.70 1.85
L07 0.69 1.89
Ch – 5
L01 0.68 1.98
L04 0.70 1.88
L07 0.70 1.87
Ch – 6
L01 0.68 1.96
L04 0.70 1.87
L07 0.71 1.79
Ch – 7
L01 0.71 1.77
L04 0.74 1.63
L07 0.71 1.77
Ch – 8
L01 0.69 1.91
L04 0.72 1.76
L07 0.69 1.93
4.2.6 Channel slope
The slopes of the channels (from L01 to L07) in this study are gentler,
and vary from 0.5 to 4.2 (Table 4.2.5). It may be noted here that slopes
calculated in this study are of exercise purpose and it does not meet the
requirement of slope calculations as mentioned in section 3.2.3 of the
chapter -3.

57
Table 4.2.5 Computed values of channel slope %
Channel ID Slope % Channel ID Slope %
Ch-1 2.1 Ch-5 0.3
Ch-2 1.3 Ch-6 0.3
Ch-3 0.4 Ch-7 1.1
Ch-4 2.6 Ch-8 0.3
Morphometric as well as hydraulic parameter computation is summarized in
Table (4.2.6).
Table 4.2.6 Computed values of various parameters using fluvial and tidal
environment equations of paleochannels of the study area.
Parameter Min Max
Channel Width (m) 354.8 2319.8
Channel base height (dmean) (m) 9.0 17.5
w/dmean 21.2 164.5
Cross-sectional area (m2) 1787.3 21199.1
Paleodischarge (m3/s)
Tidal equation (Eqn 3.2.2) 1324.7 14232.0
Fluvial equation (Eqn 3.2.3) 2910.5 43128.6
Paleochannel velocity
(m/s)
Tidal equation 0.67 0.74
Fluvial equation 1.63 2.04

CHAPTER 5
DISCUSSION
&
CONCLUSIONS

59
5.1 DISCUSSION
INTRODUCTION
Buried channels represent the depositional environment in which they
are formed. Also their existence is a proof of dynamicity of the region
and the conditions of their occurrence. Since the interpreted buried
channels represent paleo-drainages it is important to link these features
with one or more hydraulic paleo-environments. Understanding the
genesis and evolution of these buried channel system will enable us to
glance through the paleo-environmental regimes of the region. In the
following sections, a discussion on its development, depositional
environment, and time of formation are presented:
5.1.1 Development of observed channel system
Considering the measured width, depth, width/depth ratios & other
calculated parameters, development of the observed channel system
can be explained. These seismically observed channels can be thought
to have formed as incision on the sedimentary strata of the region.
Rapid influx of water must have caused channels to be carved onto the
sediments with deeper and wider geometries. The morphology of the
observed channel of the present study suggests high energy conditions
during formation of the channels which can be linked to sea-level
changes in the region.
Another causative agent for the channel formation can be thought to be
structurally controlled as active faulting or folding. The regional
lineaments are almost perpendicular to general orientation of the
channel systems in the Palk Bay (Vaigai lineament) (Lal et al., 2009).
The only neotectonic activity reported in the vicinity occurred in 1948-
49 AD near Dhanushkodi township. Thus development of these
channel systems under tectonic influence can be straightly ruled out.

60
5.1.2 Depositional environment
There are two possibilities of development of the interpreted channel
system of the present study with varying environmental conditions; (i)
fluvial development and (ii) tidal development. Though the study area
falls under coastal segments, fluvial channel development cannot be
conceived because, the trend of seismically observed channel systems
is seen in East-West direction which is perpendicular to the existing
and paleo river courses surrounding the study area. Also the region in
the vicinity of study area is low gradient and low energy beach facies.
This definitely excludes any possibility of extension of any river
channel from the proximity of the study area.
The observed buried channel system can be inferred of tidal/marine
origin which is supported by the fact that incisions are not indicative of
sluggish channel development. Further, all the seismic sections display
steep channel flanks, deeper cuts and high width/depth ratios which
are indicative of prompt channel cutting and widening. This can occur
only if the channel development had been sudden, rapid and by
increased influx of water, which can be thought to be related to rapid
rise in sea-level. The paleodischarge estimates of the present study
yield high value if fluvial equation is considered. Such high values do
not support channel development due to fluvial system in this coastal
region characterized by low gradient of seabed and sub-seabed strata.
5.1.3 Timing of formation and filling of channels
The formation of Palk Bay channel system is geologically complex, a
result of rapid sea-level changes during early Holocene and abrupt
spatial and temporal changes in depositional environments. These
results and inferences from geomorphologic and paleo-flow analyses
presented in this study, can allow us to propose the following

61
hypothesis for timing of formation and filling of the buried channel
systems of the Palk Bay.
The timing of formation of Palk Bay paleochannel system can be
estimated by calculating the age of strata onto which first incision
started to occur. In all three seismic line sections, channel incision
initiated at more or less same depth and thus can be considered to have
formed during similar geological ages.
Since valid data regarding age of the formations is not available for the
study area, an attempt has been made to calculate age of the stratal
formations using rate of sedimentation which can be variable during
past geological times. Considering current sedimentation rate of
6m/kyr in the Palk Bay (Chandramohan et al., 2001), the ages for the
channel base and channel top have been calculated as 3203 yrs and 837
yrs respectively. The total thickness of the sediment strata in seismic
section is on an average 19.2 m & the water depth in the study area is
~16 m. In order to form the observed channel system, sea-level should
have been ~35 m lower than that of present sea-level (~19 m of
sediment thickness and ~16 m of current water depth). Such sea-level
was present at an age between 9,000 to 8,000 yrs B.P. (Figs 2.7.1 &
2.7.2), which 3 times more than the calculated ages of channels with
current sedimentation rate. Thus, the sedimentation rate in the
geological past must have been considerably lower (at least 1/3rd of the
current sedimentation rate) in this area.
The sea-level during early Holocene was 30 m below present sea-level
(Fig 2.7.1). During 11,000 to 7,000 yrs B.P. there was rapid sea-level rise.
Therefore it is suggested that the channel might have formed during
the rapid rise in sea-level and channel carving was a result of the
increased water influx. This is supported by the seismic expression of
channel morphology in the section.

62
Later the conditions were not so dynamic. The sedimentation was quite
uniform over the years as the strata‟s above channel-fill sediments
show horizontal disposition and lateral continuity in all three seismic
sections. It may be mentioned here that the sea-level from 7,000 to 2,000
yrs B. P. was 3 m above present mean sea level. (Woodroffe & Horton
2005; Singh & Chadha 2008). This sea-level facilitated stable conditions
for sediment deposition in the study area, resulting stratified
sedimentary layers. The high amplitude, continuous, and parallel
reflectors above channel fill observed in the high resolution seismic
sections can be attributed to have been formed during stable high-
stand duration mentioned above.
If the abovesaid consideration of -30 m sea-level during channel
formation is taken, then the study area was not submerged by any
water body during early Holocene and thus, it was exposed as a land
portion. The only connection of Arabian Sea with the Bay of Bengal
could have been through these channels. The exchange of water mass
as well as sediments must have been through these shallow but wider
channel systems.
5.2 CONCLUSIONS
The quantitative geomorphological study, based on high resolution
seismic mapping of buried channel systems, allows us to link the
hydrological properties of bedding surfaces of these channels and their
fills to specific mechanisms of formation and evolution. The method of
applying empirically derived hydraulic equations to estimate former
discharges and measuring width, depth and cross-sectional area
facilitates to study process of formation, environment of deposition
and rough estimates of age of formation.
The Palk Bay channel systems were likely to have been formed as
incision onto older strata and chaotic seismic units consistently

63
observed at the bases of channel fills. However, channel morphologies
may have been subsequently and quickly modified and partially
overlain by erosion and deposition imparted by tidal currents and
waves. Thus paleo- flow values using both fluvial and tidal
assumptions have been estimated in this study. The range of mean
paleo-flow velocities under the fluvial assumption is generally
considered too high for the presumed low hydraulic gradients of
mapped channel systems. This may indicate that tidal energy modified
the channel geometry and preserved fill deposits during subsequent
transgression.
5.3 FUTURE WORK
High resolution seismic profiling surrounding the study area in the
Palk Bay is required to trace the complete extent of inferred
paleochannel system. Further, deep core sampling (about 40 m)
investigations at selected locations is suggested to infer ages of
sedimentary strata and detailed information concerning channel
submergence and filling processes. This in turn will help to understand
paleoenvironment and channel formations.

xi
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