58

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.

59

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

60

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

61

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

62

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.

63

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

64

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).

65

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

66

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).

67

Figure 6.2 Bathymetry Map of North Andaman

68

Figure 6.3 Coastal Geomorphology Map of North Andaman (2003)

69

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.

70

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.

71

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

72

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

73

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).

74

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

75

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

76

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.

77

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.

78

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.

79

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.

80

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.

81

Figure 6.8 Erosion and Accretion map of North Andaman (1967-2003)

82

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

83

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

84

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).

85

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).

86

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.

87

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

88

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.