Physical Geography Lecture 17 - Oceans and Coastal Geomorphology 120716
CHAPTER 6 COASTAL GEOMORPHOLOGY -...
Transcript of CHAPTER 6 COASTAL GEOMORPHOLOGY -...
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CHAPTER 6
COASTAL GEOMORPHOLOGY
6.1 INTRODUCTION
The shape of coastal landforms is the response of the materials that
are available to the coastal processes acting on them. The geomorphology of
the coast can be examined in planform (also called shore-parallel, or long
shore) or in profile (also called cross-section, cross-shore, shore-normal, or
orthogonal). In planform, a coastal embayment can be divided into relatively
distinct landforms based on factors such as topography and lithology, or
sediment texture and resistance. The most resistant are rocky coasts,
characterised by cliffs. Sand and gravel are more mobile in the coastal zone
and form coastal barriers on which there are beach associated landforms.
Deltas or estuaries occur where rivers reach the sea. The coastal zone extends
upstream as far as the tidal influence is felt, which can vary over time in
response to tidal or river flow characteristics. Muddy substrates occur in the
more sheltered areas and can support coastal wetlands.
Availability of repetitive, synoptic and multi-spectral data from
various satellite platforms have helped to generate information on varied
aspects of the coastal and marine environment. The Indian remote sensing
satellite which provides high resolution data is useful for coastal zone studies.
The modern scientific tools of remote sensing, GIS and GPS play a valuable
role in coastal geomorphology studies.
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6.2 GEOMORPHOLOGICAL STUDIES – REVIEW
Geomorphic unit is defined by Fairbridge (1968) as an individual
and genetically homogeneous landform produced by a definite constructional
or destructional geomorphic process. Ahmed (1972) has done pioneering
work along the Indian coast by describing the geographic features of the
coastal region and evolution of the Indian coastline. Ahmed (1972) has
surveyed the shelf region of Andaman in detail. The shelf topography in the
region of Andaman and Nicobar islands is much more irregular than
anywhere in the continental shelf adjoining the mainland.
Worth mentioning among the studies related to remote sensing
applications along the Indian coast are that of Baskara Rao and
Vaidhyanathan (1975); Nageswara Rao and Vaidhyanathan (1978); Wagle
(1982); Sood (1983); Ganapathy et al (1984); Nayak et al (1985); Prabhakara
Rao et al (1985); Sinha and Kumar (1986); Anand et al (1987); Nair (1987);
and Sambasiva Rao (1990). Almost all these studies have a common term of
mapping geomorphology and interpreting geomorphic evolution of the
concerned areas. Wagle (1979) carried out a reconnaissance marine
geological survey of Gulf of Kutch in March 1976 and divided the sea bed of
the Gulf into 3 distinct morphologic units – even, uneven and rough.
Vaidhyanathan (1981) has stated that the coral reefs and a few terraces close
to Rameswaram Island between Tamilnadu and Sri Lanka must have been the
result of some emergence.
The geomorphic units have a specific set of characteristics that
determine this image signature. High resolution satellite data provide reliable
source of information to delineate and generate comprehensive and detailed
inventory of geomorphic units in an area (Mukerjee 1982). Rameswaram
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coastal area was investigated geologically by Foot (1883), showing the scope
of the investigation could be extended to mapping of coral reefs and
geological formations by latter researchers.
Loveson and Rajamanickam (1987 and 1988a) and Loveson et al
(1990) have discussed the changes in the shoreline of south Indian coast based
on deposition of landforms like beach ridges, patterns, occurrence of
backwater zone etc., through remote sensing based geomorphological
interpretation. Rajamanickam and Loveson (1988) have brought out very
informative studies on sea level variation in Tamilnadu coast. Loveson and
Rajamanickam (1988b) have also pointed out the possible fall in sea level.
Quaternary geological and geomorphological studies carried out by
Roy and Das (1988) along the coasts of the 24-Parganas (South) and
Medinipur districts of West Bengal between 1984 and 1988 were followed by
monitoring of the coastal process which was initiated in 1990-91 and
completed in 1992-93 and explained the dynamic changes of the coasts
between the two periods of studies.
Srinivasan and Srinivasan (1990) attempted to infer coastal
geomorphology of Tamilnadu using remote sensing techniques without
fieldwork. SAC (1991) has demarcated the shoreline changes of the Mahi
estuary and demarcated the paleocourse, changes in the island and erosion in
the area using Landsat MSS satellite imagery. Bannur et al (1991) and
Gangadhar (1995) have studied the changes in the geomorphology of the
Gurpur-Netravati, Pavanje-Mulki and Seetha-Swarna estuaries using Landsat
MSS data. They concluded that the Gurpur-Netravati estuarine mouth was
migrating to the north, whereas Mulki-Pavanje estuarine mouth was migrating
southwards. Wright (1993) has suggested that the geomorphological features
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are manifestations of underlying parent materials and the nature and duration
of geomorphic processes that have produced the associated geomorphic units.
The formation of coastal strandlines, petal shaped lakes in
Vedaranyam and Ramnad coastal areas clearly show the migration of
shoreline due to marine transgression during Cretaceous and regression from
Pleistocene to Recent period (Radhakrishnan et al 1993). Krishnamoorthy
et al (1995) carried out the analysis of shoreline change using GIS in
demarcating the areas of progradation, retrogradation and the geological
hazards due to shoreline erosion and accretion in Andaman and Nicobar
islands.
Verma (1995) has studied the erosional site near Coast Guard
Officers mess of South Andaman and recommended close spaced piling,
construction of a wall at sea level and transverse drains. A detailed
geomorphological survey was carried out by Haldar (1995) around
Beadonabad, South Andaman. Sabitha (1999) studied the geomorphology of
southeast coast of Tamilnadu using remote sensing. Hegde (2000) studied the
geomorphic history of the two spits of Mangalore using IRS–1A / 1C data, its
present status and their possible future in the light of the seawalls and
breakwaters built in the area. The geomorphological mapping of a terrain and
analysis of their processes help in soil resources mapping, groundwater
potential zones identification, landscape ecological planning, hazard mapping
and other environmental applications (Reddy et al 2001).
Tran et al (2002) presented the initial results of a case study using
remotely sensed data in recent years by combining traditional topographic
maps and field survey data with GIS database to monitor the coastal erosion
in Red River Delta of Vietnam during the last 70 years and postulated the
erosion status in the area. Kasinatha Pandian et al (2002) used integrated
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approach in shoreline monitoring and management with reference to Ennore
coast and suggested that it is essential that a long-term study should be carried
out in an integrated manner so that each and every aspect of shoreline
monitoring and management mechanisms can be identified.
6.3 COASTAL GEOMORPHOLOGY MAPPING
Satellite imageries are helpful in identifying the coastal
geomorphological features. Basic information like transport network, tanks,
rivers etc are transferred from SOI toposheet. The methodology of coastal
geomorphology study is given in Figure 6.1. Visual interpretation technique
was attempted for geomorphology classification. The classification system
developed by Space Application Centre, Ahmedabad for coastal
geomorphology was adopted SAC (1991) and is given in Table 6.1
Three types of approaches have been attempted to analyse the
satellite data for coastal geomorphological mapping.
1. Analysis and interpretation of optical remote sensing data
IRS-1D LISS-III 2003 for coastal geomorphology.
2. Analysis and interpretation of optical remote sensing data
IRS-1D LISS-III 2003 and Survey of India topographical
sheets for erosion and accretion.
3. Interpolation of bathymetry using Naval Hydrographic charts.
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Error!
Figure 6.1 Methodology for Coastal Geomorphology Mapping
Coastal Geomorphologic map
Bathymetry map
Data
Base Map Remote Sensing
Data NHO Chart
Visual interpretation of
IRS LISS – III (2003)
GIS Input
Ground truth
Status of coastal geomorphology
SOI Toposheet
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Table 6.1 Coastal geomorphology classification
Origin Coastal Geomorphic features
Marine Origin Mud Flat
Beach
Spit
Beach Ridges
Swales and Backwater zone
Back swamp
Off-shore island and coral reefs
Wave cut platform
Sea cliff
Sea cave
Fluvio-marine origin Deltas / Deltaic plain
Fluvial origin Natural levee
Flood plain
Aeolian origin Sand dune complex
Teri dune complex
Others Flood basin
Salt pan / Salt flat
Strand line
Water logged land
Aquaculture pond
Source : SAC (1991)
6.3.1 Geomorphological Map
The image interpretation keys include tone, size, shape and pattern
(SAC 1991).
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Table 6.2 Image characteristics of the coastal geomorphic classes
Sl. No.
Category Tone Shape Texture Remarks
1. Mud flat Grey Irregular Smooth Made up of clay and silt
2. Sandy beach
White Linear / Crescent
Smooth Made up of fine sand particles, broken molluscan shells etc
3. Spit Whiter Linear, Curved at the end
Smooth Narrow embankment of land consisting of sand
4. Fringing reef
Greenish black
Irregular, Broad to narrow
Smooth except at the edge
Dead reef gives greenish black tone
5. Patch reef Greenish black
Oval to round, flat on the top
Smooth Forming island without lagoon, grows upward to the tide limit
6. Creek Blue Meandering Smooth Intricate network of narrow inlets of sea in tidal flats
7. Lagoon Deep blue Oval / linear Smooth Semi-enclosed body connected to sea by one or more opening
8. Deltaic plain
Red Triangular / Cuspate
Smooth Rapid deposition of stream born sediments in to still standing bodies of water
9. Flood plain Red Linear / meandering
Rough Lies parallel to the river course
10. Natural levee
Red / white Linear Rough Lies parallel to the river course
11. Coastal dune
Red / white Linear Smooth Detected only when vegetation present on it
12. Strand line White Linear Smooth Ancient shoreline
13. Sea cliff White --- --- Detected by the absence of beach on the coast
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Table 6.2 (Continued)
Sl. No.
Category Tone Shape Texture Remarks
14. Salt pan / plat
Dark blue / white
Rectangular Smooth Dry saltpan appears white, and shallow depression
15. Beach ridges
Yellow / white
Linear Rough Ancient beach
16. Teri sand Yellow Oval / rounded
Rough Undulating terrain having loose heaps of red coloured sand and silt dust of Aeolian origin
17. Back swamp
Greenish gray / bright red
Irregular Smooth Presently or periodically inundation by water and is characterized by vegetation
18. Swale Gray Linear Smooth Intervening depression found between beach ridges formed by marine process
19. Water logged
Blue Irregular Smooth Water stands for most of the year
Source : SAC (1991)
6.3.2 Bathymetry Map
Bathymetry map of North Andaman on 1:50,000 scale was
prepared manually using 1975 Naval Hydrographic chart. The prepared
bathymetry map was digitized using ARC-INFO and a digital elevation model
was prepared using ERDAS Imagine software (Figure 6.2).
Based on the interpretation technique the coastal geomorphic map
of North Andaman was prepared from IRS-1D LISS III 2003 (Figure 6.3).
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Figure 6.2 Bathymetry Map of North Andaman
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Figure 6.3 Coastal Geomorphology Map of North Andaman (2003)
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6.3.3 Ground Truth
Ground truth verification is one of the most important components
in the field of remote sensing applications. The validation of the informations
derived from remote sensing data is essential to estimate its accuracy by field
check. Many of the coastal mapping projects in India followed the
classification accuracy as discussed by Nayak (1991) and SAC (1992). In this
study, the coastal geomorphological map prepared using IRS-1D LISS III
2003 data was validated after incorporating the necessary corrections
observed during ground truth check.
About 132 random points related to coastal geomorphic categories
(sandy beach, fringing reef, patch reef, back swamp, mud flat, sea cliff and
cave etc.,) were selected with reference to SOI toposheet (1:50,000 scale) and
plotted on the interpreted maps. This validation work has been combined with
field check in order to verify more categories apart from the random points
selected for estimation of classification accuracy. An accuracy error matrix
was arrived based on the number of failures / successes and is shown in
Table 6.3. The accuracy matrix shows an overall accuracy of 84% in
geomorphology mapping using remote sensing data.
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Table 6.3 Accuracy error matrix of coastal geomorphology map of
North Andaman
Verified on Ground
Verified on
Imagery
B M S FR BS PR C Total
B 19 5 2 1 2 29
M 15 3 1 19
S 18 2 20
FR 26 26
BS 4 20 24
PR 6 6
C 8 8
Total 19 19 23 30 24 9 8 132
B = Beach, M = Mudflat, S = Sea cliff, FR = Fringing Reef
BS = Back Swamp, PR = Patch Reef, C = Creek
Total sample points = 132 Total no. of failures = 20 Accuracy = 84.8 %
6.4 RESULTS AND DISCUSSION
The coastal land form in North Andaman has various geomorphic
units with different types of configurations. The geomorphic units, interpreted
from remotely sensed data and checked subsequently through fieldwork, have
been categorized into three classes - depositional, erosional and other land
forms. The areal distribution of various coastal geomorphic features of North
Andaman are shown in Table 6.4.
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Table 6.4 Areal distribution of Geomorphic features of North
Andaman (2003)
Sl.No. Geomorphic Classes Area in ha.
1 Fringing reef 3923.20
2 Patch reef 1147.28
3 Mangrove swamp 21630.47
4 Sandy beaches 539.93
5 Creek 1880.32
6 Mud Flat 124.26
6.4.1 Coral Reef
Coral reefs are unique among the marine associations or
communities in that they are built up entirely by biological activity. The reefs
are essentially massive deposits of calcium carbonate produced primarily by
corals with minor additions from calcareous and other organisms that secrete
calcium carbonate. Although corals are found throughout the oceans of the
world, it is only in the tropics that reefs are well developed. This is because
there are two different groups of corals, one called hermatypic and the other
ahermatypic. Hermatypic corals are those that produce reefs; ahermatypic do
not form reefs. Ahermatypic corals distributed world wide, but hermatypic
corals are found only in the tropical regions (James 1996).
Reefs are dynamic geomorphologic systems demonstrating a
complex interplay between physical and biological processes. They form
solid limestone, simultaneously producing, breaking down and redistributing
sediments of different sizes to construct a range of landforms. As a result of
their ability to build rigid, wave-resistant structures, corals modify the
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environment in which they live, as expressed by Darwin. Reefs contain a
variety of interacting subsystems operating over a broader range of time
scales than generally seen on rocky coasts, comprising construction,
destruction and various responses to extreme perturbations, such as storms.
Coral reefs are geologically complex and ecologically diverse. The
coral reefs in North Andaman are fringing reefs. Some patchy corals are also
observed in the western coast between Paget Island to Hoare bay and covers
an area of 1147.28 ha. Normally fringing reef is observed on the eastern coast
from Cape Rice to Bond harbour it is narrow and the western side is broader.
It covers an area of 3923.20 ha. Broad fringing reef with reef flat has been
identified in the Table group of islands covering an area of 76.21 ha. Long,
narrow and broad fringing reefs have been observed along the Smith and Ross
coasts between Bayet, mangrove point and Back Bay and cover an area of
194 ha. Coral reefs are exposed during low tide period (Figure 6.4).
Figure 6.4 Exposed coral reef near Ross Island (North Andaman)
during low tide
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On the western coast, fringing reefs are well developed with reef
flat. Along the shore between Clough point to Kwang tung and Jub Jub island,
Micky point, Sugar loaf, Shearme island and Cape thorn hill, long and broad
fringing reef has been observed covering an area of 1206 ha. A broad fringing
reef has been identified near the Beele bay, Cape Horning and the Pine bay
covering an area of 569.69 ha.
6.4.2 Coral Reefs in Small Island Sanctuaries
The Land fall and East islands are situated north of North
Andaman. Their eastern shore is marked by a fringing reef with an extent of
336.34 ha. North Reef Island has a broad fringing reef with reef flat occuring
along the shore. It covers an area of 673.51 ha. Fringing reef has also been
identified on the western side of Latouche Island covering an area of
53.14 ha. Long, elongated and broad fringing reef with reef flat has been
observed in Mayo and Paget Islands covering an area of 367.76 ha and 186.80
ha respectively. The fringing reef in Reef Island covers an area of 37.34 ha.
In White cliff Island, the fringing reefs are narrow and cover an area of 7.4 ha.
The shore in West island is noticed by broad and well developed fringing reef
with reef flat with an area of 224.99 ha.
6.4.3 Beach
Beach is a temporary or short-lived depositional landform
encountered along the coast (Ahmed 1972). It consists partially of
unconsolidated materials such as sand and silt. It may also be strewn with
cobbles of shingles (Fairbridge 1968). Sand accumulated along the beaches is
due to the effect of waves and transported materials through rivers from the
continent (Thornbury 1969).
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Beaches are deposition landforms, and are the result of wave action
by which waves or currents move sand or other loose sediments of which the
beach is made of these particles are held in suspension. Alternatively, sand
may be moved by saltation (a bouncing movement of large particles). Beach
materials come from erosion of rocks offshore, as well as from headland
erosion and slumping, producing deposits of scree.
Beaches can be classified on the basis of a wide variety of materials
from which they have been derived (Keithstowe 1983). Beaches are
characterized by the distribution of different colours of sands. The geometry
of the beach is dependent on coastal history and there is a close relationship
between beach characteristic and type of the coast (Lapedes 1977). Long and
straight beaches are typical of low sandy beaches. Small pocket and shorter
crescent shaped beaches are common along the rocky coastline. On the basis
of the characteristics of the beach, North Andaman beaches can be divided
into two types namely sandy beach and rocky beach.
6.4.4 Sandy Beach
Sandy beach consists of accumulated, unconsolidated sediments
transported to the shore and moulded into characteristic forms by wave
generated water motion. The origin of sand in Andamans involves both the
fluviomarine and the weathering of parent rock from the land. Sand takes
geologic years for its formation. The presence of sand in North Andaman is
noticed as a thin strip all along the coastal region. Sandy areas are very clearly
visible through IRS-1D LISS - III 2003 satellite data (Table 6.5).
In the northern portion of North Andaman near Cape Rice, a small
pocket beach that is gently sloping has been observed covering an area of
1.38 ha. To the western side of East island, another pocket beach was also
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identified covering an area of 17.21 ha. The beach is oriented in the north to
south direction with a width of 221 m. To the eastern side of Bond harbour, at
Bayne point, a linear beach is observed covering an area of 3.4 ha and a
length of 700 m. Its lies in the north to south direction. To the eastern side of
Smith Island, a curvilinear beach has been observed at Back Bay, covering an
area of 8 ha. and with a length of 1,548 m.
Table 6.5 Details of sandy beaches in North Andaman
Location Type Area (Ha.) Cape Rice Small pocket beach 1.38 Western side of East Island Pocket beach 17.21 Command point Small pocket beach 3.76 Bond harbour Small, narrow and Curvilinear
sandy beach 3.41
Smith island at back bay Curvilinear sandy beach 8.00 Between Ross and Smith Linear sandy beach 14.00 Durgapur Curvilinear sandy beach 10.00 Kalipur Pocket beach 3.20 Ramnagar Curvy beach 3.21 Eileen bay Curvilinear beach 20.36 Cliff bay Curvilinear beach 12.34 Macpherson Bay Pocket beach 72.81 Pembroke Bay Narrow linear beach 11.31 Coffine bay Narrow linear beach 15.29 Kwang tung Small pocket beach 19.37 Casurina Bay Narrow linear beach 50.21 Casurina bay to Jub Jub Island Pocket beach 48.02 Hudson bay Linear beach 6.70 Mickey Point Small pocket beach 7.81 South of Sugar loaf Island Linear beach 34.53 Paget Island Linear beach 16.00 Point Island Linear beach 30.65 Duncan Bay Linear beach 20.63 Beele bay Linear beach 109.01
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A linear sandy beach that connects Smith and Ross islands, has an
area of 14 ha. And a length of 500 m. A small curvy pocket beach has been
observed at Durgapur covering an area of 10 ha. Kalipur has a small pocket
beach. It is oriented in the north to south direction and covers an area of 3.2
ha. This sandy beach is seen mixed with coral rubble (Figure 6.5). Ramnagar
has a curvy beach, situvated at Taralait bay, covering an area of 3.2 ha. and
having a length of 782.14 m. A curvilinear beach has been identified at Eileen
bay having an area of 20.36 ha. Cliff bay has a curvilinear beach, covering an
area of 12.34 ha.
Figure 6.5 Kalipur Beach
Many sandy beaches are observed on the west coast of North
Andaman. A broad beach has been identified at Macpherson bay covering an
area of 72.81 ha. A narrow pocket beach has been observed north of
Macpherson bay with an areal extent of 72.81 ha. A narrow linear beach has
been identified to the north of Pembroke bay that has an area of 11.31 ha.
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This stretches to a length of 1,245 m. A narrow linear beach observed south of
Coffrie bay in the north west direction has an areal extent of 15.29 ha and
length of 1,915 m. A small pocket beach has been identified at Kwang Tung,
covering an area of 19.37 ha. A narrow linear beach is observed at Casuarina
bay at the mouth of Gudi Nallah, which has a length of 2,514 m. It covers an
area of 50.21 ha. Small and medium range pocket beaches have been
identified between Casuarina bay to Jub Jub Island covering an area of 48.02
ha. A linear beach has been observed at Hudson Bay with an area of 6.7 ha. A
small pocket beach has been identified at Mickey Point which has an area of
7.81 ha. A narrow linear beach has been observed south of Sugar Loaf Island
and it covers an area of 34.53 ha and it has a length of 3,777 m. The beach is
oriented in the north to south direction. Paget island has a narrow linear beach
covering an area of 16 ha. At Point island, three sides of the island are
covered by linear sandy beaches to an extent of 30.65 ha. A narrow linear
beach observed near Duncan bay, covers an area of 20.63 ha. To the eastern
side of Beele bay, a big sandy beach has been observed having an area of
109.01 ha.
6.4.5 Mudflat
At low tide, the intertidal mud is exposed as a mudflat leaving
water only in permanent channels. At high tide the mudflat is covered with
water. Mudflats are formed by the deposition of fine inorganic material and
organic debris in particulate form. They are wide expanses of deposit of clay,
silt and ooze, etc. Davies (1972).
The mud flats were clearly visible at Durgapur shore (Figure 6.6)
They were also clearly visible at Pine bay, Durgapur, Elizabeth bay and
Aerial bay covering an area of 124 ha.
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Figure 6.6 Mudflat near Durgapur
6.4.6 Mangrove Swamp
Mangrove swamp is an easily recognized habitat along tropical and
subtropical coastlines and brackish estuaries and deltas, where evergreen trees
and shrubs thrive in tideland mud or sand flats inundated daily with sea water.
These flats are found mostly along bays and inlets protected from heavy
waves.
Mangrove swamps occur in marshy areas along the coast. They
particularly occur at the edge of tropical or sub-tropical seas, in bays lagoons
and estuarine regions (Gerlech 1973). The mangrove swamps of North
Andaman have been observed all along the coast. Most of them are fringing
mangrove (Figure 6.7). The larger mangrove swamps are observed at Blair
bay, Kalpong Creek, Terapa river, Kalighat Creek, Shearme Island, Elizabeth
bay and Cape Thornhill island.
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Figure 6.7 Mangrove swamp at Kalighat Creek
6.5 COASTAL EROSION AND ACCRETION
Shoreline is one of the most rapidly changing landforms of the
earth. The geomorphic processes of erosion and accretion are periodic storms,
flooding and sea level changes which continuously modify the shoreline. The
rate of shoreline change varies depending upon the intensity of the causative
forces, warming up of oceanic water and melting of continental ice sheets etc.
According to Ahmed (1972), the erosional coast is the coastal
interior; the land immediately behind the shore zone (whose inner limit is
defined by the highest storm and tide waves) has an indeterminate inner limit.
Features like cliffs, sheets and gullies are seen along the erosional coast.
While in the case of an accretion coast, there is a seaward migration of the
coastal tract. Depositional features like deltaic sedimentation, estuarine,
marsh and sand dunes are characteristic of the coast.
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The coastal erosion and accretion processes are not a new
phenomena. Generally the main causes of these processes are (i) beach
configuration; (ii) neotectonic movements in the coastal belt and the near
shore region; (Stoddard and Pillai 1972; Ahmed 1986; Loveson and
Rajamanickam 1987; Ramasamy 1989) (iii) presence of mud banks,
(iv) gradual climate changes (Loveson and Rajamanickam 1988a and 1988b;
Bruckner 1988 and 1989), (v) reduction in the discharge of sediments from
rivers (Joshi 1995) and (vi) manmade activities along the coast.
The study area has been affected by cyclonic storms at least
50 times during the period 1877-1977. Apart from these natural phenomena,
anthropogenic activities (construction of harbour, sand mining and coral
mining etc) have affected littoral drift. Consequently, nourishment on the
opposite side of the breakwaters is reduced resulting in net erosion (Joshi
1995). Kaliasundaram et al (1991) studied the coastal behaviour and
processes in 30 representative sites over the entire coastline of Tamilnadu
based on an extensive ground truth observation.
Different layers of multi-date shoreline maps (1969 and 2003)
were overlaid using ARC-INFO software, to identify erosion and accretion
areas along the coasts of North Andaman. The shoreline change map derived
from the overlay is shown in Figure 6.8.
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Figure 6.8 Erosion and Accretion map of North Andaman (1967-2003)
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Coastal erosion sites have been identified and the details are given
in Table 6.6.
Table 6.6 Coastal erosion sites in North Andaman
No. Site Length of
eroded coastline (m)
Area (Ha)
Width (m)
1. North East Shore 677 4.72 69.72
2 Cadell Bay 1600 34.91 218.21
3 Smith Island 2700 14.37 53.24
4 Mangrove Point 782 6.37 81.43
5 Hood Point 540 3.64 67.49
6 North to Hood Point 435 1.31 30.00
7 Minerva bay 488 6.25 128
8 Podra Torak 1800 17.59 97.7
9 Atlanta Bay 824 22.58 274
10 East to Shibpur 933 10.82 116
11 Kalipur 1500 18.60 124
12 Ramnagar 1200 17.76 148
13 Cliff Bay 1100 34.98 318
14 Mangrove Bay 1000 25.60 256
15 Khalighat creek Mouth 1900 159.98 842
16 Western Side of Mangrove Bay
4500 85.50 190
17 East of Austin Harbour 3000 47.40 158
Total 24979 512.38 3171.79
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Coastal accretion sites identified are given in Table 6.7.
Table 6.7 Coastal Accretion sites in North Andaman
No. Site
Length of Accreted coastline
(m)
Area
(Ha)
Width
(m)
1 Bayne Point 510 2.09 41.00
2 North Island 256 1.14 44.58
3 North to Tralait Bay 908 5.36 59.00
4 Eileen Bay 1200 20.88 174.00
5 Cliff Bay 630 3.36 53.39
6 South of Macpherson Bay 2415 52.41 217.00
7 North of Macpherson Bay 419 3.29 78.54
8 South of Hudson Bay 1220 12.07 98.95
9 South to Casuarina Bay 2400 21.50 89.58
10 Mickey Point 607 4.51 74.32
11 North of Duncan Bay 1800 21.24 118.00
Total 12365 147.85 1048.36
6.5.1 Shoreline between Aerial Bay to Kalipur (Eastern North
Andaman)
The shoreline between aerial bay to Kalipur shows both erosion and
accretion. The erosion has been estimated to cover an area of 47.55 ha. With a
length of 2.51 km and a width of 274.32 m. The accretional shore occupies an
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area of 16.34 ha with a length of 4.21 km and a width of 276.45 m
(Figure 6.9).
Figure 6.9 Erosional and accretional sites on the eastern North Andaman
6.5.2 Shoreline between Austen harbour to Clough Point (Western
North Andaman)
The shoreline between Austen harbour to Clough Point shows both
erosional and accretional coast. The erosion has been estimated to cover an
area of 39.54 ha. with a length of 4.84 km and a width of 394.78 m. The
accretional shore occupies an area of 22.56 ha with a length of 5.54 km and a
width of 172.37 m (Figure 6.10).
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Figure 6.10 Erosional and accretional sites on the western side of North
Andaman
6.6 CONTINENTAL SHELF MORPHOLOGY
Continental shelf is the marine floor between the normal shoreline
and the submarine contours of 100 fathoms (Ahmed 1972). The continental
shelf is marked by an abrupt break of slope near its outer edge. There is a
remarkable difference in the extent of continental shelf on the east coast of
India when compared to the west coast of India. The shelf on the west coast
is broad with a thin layer of sediment, while the eastern shelf is narrow with a
thick layer of sediment (Agarwal 1990).
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The shelf around North Andaman is much more irregular and the
outer margin of the shelf is very irregular frequented by several spurs and re-
entrants. The upper surface of the shelf is also marked by frequent rises
supporting coral reefs and depressions known as ‘Passages’ and ‘Straits’. It is
probably composed of the same rocks as Arakan Yoma, whose geology itself
is a terra incognita.
The total width of the shelf is around 1.26 km on the eastern side
and 15.25 km on the western side having a slope of about 21º. The slope near
the shore is about 3º-10º. Shelf morphology of North Andaman has been
described in detail by dividing the study area into the following two segments.
1. Eastern shelf region
2. Western shelf region
6.6.1 Eastern Shelf Region
In the shore between northeastern sides, East Island to Cadell bay,
the sea floor slopes gently down to 30 m depth. This gentle slope extends to
an average distance of 4.3 km and then slopes very steeply. In the shore
between Smith and Ross Islands to Sound Island, the sea floor depth of 5 m to
10 m, which extends to an average distance of 350 m from the shore. Here the
sea floor is very steep and coral reef and submerged rocks are observed along
the coast. An extensive well-developed fringing reef has been identified
around Ross Island and the eastern side of Smith Island extending upto 3 m
depth and it steeply increases towards the east.
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6.6.2 Western Shelf Region
To the Northwest side of North Andaman, the sea floor is at a depth
of 20 m and extends to an average distance of 3 km from the shore. At an
average depth of 11 to 19 m there is continental slope with 8.6 km. In
between 10 m and 20 m depths, the sea floor is broad and has a very gentle
slope.
Many elevated islands namely Point, Paget, Reef, Thorn Hill and
West Island are situated in the west coast. The sea floor is covered by fringing
coral reef and submerged rock. In this region, the sea floor gently slopes. The
north and northwestern part of North Andaman area is very broad and gentle.
6.7 CONCLUSION
Remote sensing and GIS are very much useful for mapping
different coastal geomorphic features like beach, mudflat, mangrove swamp
and reef area. All the coastal geomorphic features reveal the erosion and
depositional features in North Andaman. Remote sensing and GIS are useful
for mapping the erosion and accretion sites along the coast and islands of
North Andaman. Erosion and accretion was observed in North Andaman over
a period of 36 years. The western coast of North Andaman has witnessed
accretion which is in contrast to the eastern coast. Such a dynamic coast can
be analysed using GIS was found to be a useful tool for mapping
geomorphology and calculating the extent of various geomorphic features.
GIS software ArcGIS is useful for bathymetry mapping in North
Andaman. Various shelf morphological features like channels, continental rise
and islands and their slopes and extents were identified. It was observed that
the eastern side of North Andaman was narrow and the western side broad
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and gently sloping. The maximum length of the shelf on the eastern side is
4.3 km and on the western side is 8.6 km. The shelf’s slope gradient increases
further seawards. In general, the continental shelf is irregular in North
Andaman.
The studies on coastal geomorphology and shoreline changes imply
that North Andaman experiences retro gradation or sea level rise.