Ocean & Coastal Management 22 (1994) 187-200

Analysis of Space-Borne Data for Coastal Zone Information Extraction of Goa Coast, India

P. D. Kun te & B. G. Wagle

National Institute of Oceanography, Dona Paula, Goa-403 004, India

(Received 7 January 1993; accepted 22 April 1993)

A B S T R A C T

Space-borne data covering the coastal zone of Goa State were processed using digital and visual image-processing techniques to extract informa- tion about the coastal zone. Digital image processing of thematic data included Principal Component analysis and Intensity-Modulated Pseudo Biomass imaging. SPOT image and aerial photographs of the coastal zone o f Goa were analyzed, and fluvial, estuarine, marine, vegetational, and nearshore features were identified and mapped. This study reveals that the coastal, sandy tracts are accreting and stable, whereas rocky cliffs and promontories are subject to constant erosion. Four turbid water masses were identified on the basis o f tonal variation; the pattern of sediment plumes indicated a southerly littoral drift. Finally, this study demonstrated the utility o f extracted coastal informa- tion to coastal zone managers.

1 INTRODUCTION

India, with its long coastline, cannot ignore potential threats to its coastal zone, which may have long-term economic consequences. Interactions between various natural processes and man-made activities are important factors in the coastal area. Marine scientists and coastal engineers recognize the coastal zone as a highly dynamic physical system.

187 Ocean & Coastal Management 0964-5691/94/$07.00 Q 1994 Elsevier Science Limited, England. Printed in Northern Ireland

188 P. D. Kunte, B. G. Wagle

Because coastal changes are very dynamic in nature, accurate, comprehensive, and timely scientific data are required for coastal environmental protection. Space-borne data, because of their temporal and synoptic nature, have been used extensively worldwide in studying various components of the coastal environment, such as: erosion and inundation,' coastal resources management, 2 shallow-water mapping, 3 aquaculture potential, 4 coastal geomorphology,~ 7 reclamation, suspended sediment concentration, 9 ,2 near-shore environment, '3 and coastal zone management. ,4

The Goa State situated along the central-west coast of India, has a coastline of about 105 km, composed of alternately situated headlines, estuaries, bays, and world-famous beaches. Its geographical position is marked by 15°48'00"N and 14°53'54"N latitude and 74°20'13"E and 73°40'33"E longitude between Western Ghats escarpment and the Arabian Sea (Fig. 1). The coastal zone of Goa offers a host of attractions to tourists from India and abroad, as well as ideal sites for fishery activities and natural ports for transportation. The need to protect the coastal environment while ensuring its development is therefore clear. In the Goa area, detailed studies of coastal and nearshore geomorphology, geology, lineaments, mangrove delineation, shoreline drift, etc. have been carried out using remote sensing techniques. '5-23 Thematic data were digitally processed, and SPOT images and aerial photographs of the same area were analyzed and interpreted.

2 MATERIALS AND METHODS

Multi-Spectral Sensor (MSS) data pertaining to the scene of Landsat 5 (Path 49, Row 142, dated 31 March 1986) were used to create and photograph feature images for four spectral bands (4, 5, 6, 7) for visual interpretation. Because the influence of oceanic features on each band is highly correlated, compressing all bands into a single plane improves the identification of oceanic features. For this reason, Principal Com- ponent Transformation--digital processing of four spectral bands--was performed to produce four new Principal Component (PC) images. The first image (PC1) exhibited enhanced coastal features, whereas the second image (PC2) was suitable for studying turbidity patterns in nearshore areas (Plate 2). An Intensity Modulated Pseudo Color Biomass Index image was produced 24 for studying coastal vegetation. The processing of digital data was carried out on a DIPIX image processing system working under a VMS operating environment.

Analysis of space-borne data for coastal zone information 189

a) Mesa top Mesa slope

b) Promontory c) Offshore island d) Tidal fiat e) Tidal flat (submerged) f) Open beach g) Bay beach h) Shelter beach

i) Salt marshes 1 Panaji j) River terrace 2 Calangute k) Channel bar 3 Aguada 1) Estuary island 4 Marmagao m) Old dunes 5 Colva n) Recent dunes 6 Mandovi o) Reclained area 7 Zuari p) Mangroves

Plate 1. SPOT image showing interpreted geomorphic features.

A SPOT analogue image of the study area dated 16 February 1986 was analyzed visually to identify additional geomorphic features (Plate 1). A random mosaic of aerial photos of the study area (1960, 1" 15 000 approx, and 1973, 1:25 000 approx.) was prepared and examined. This was followed by a longitudinal study of the area (Nor th -Sou th strips) under the mirror s tereoscope, after which the coastal geomorphic

190 P. D. Kunte, B. G. Wagle

Plate 2. First and second principal component images showing coastal and near shore features.

details were transferred onto Kodatrace to be compiled later into a photo-geomorphic map and l ineament map (Figs 1, 2). Field checks were carried out at selected sites to confirm the findings.

3 R E S U L T S

After analyzing the above data, coastal and nearshore geomorphic features were identified and studied in terms of their relation with other

Analysis of space-borne data for coastal zone information 191

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associated features (Fig. 3). Location, geomorphology and image characteristics of the identified features are described below.

3.1 Coastal features

3.1.1 Fluvial forms

(a) Drainage. The images revealed that most of the rivers are flowing straight; sharp bends and meanders are rare phenomena. The drainage network is predominantely in two directions: NW or NNW, which is the regional trend, and E-W, which coincides with major

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Analysis of space-borne data for coastal zone information 193

I Fluvial

I Coastal Features

I I) Estuary islands

2) River terraces

3) Tidal flats 4) Estuaries

I ) Drainage 2) Channel bars

I I Wave cut twraces

21 Beaches

3) Paleo-beach ridges

4) Cl | f fs

5] Headlands

Geomorphlc I Features

J I Vegetational

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I I) Active Dunes

2) StabiLized Dunes

I) Man roves

2) Dune Vegetation

3) Beach Vegetation

I Near share Features

I I) Near shore islands

2) Turbidity

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I I1 Lineaments

2) Inselbergs

3) Laterite mesas

Fig. 3. Flow chart of geomorphic features identified.

faults, fractures, or joints (Fig. 1). The exception is the Sal River, which runs parallel to the coast and finally opens into the sea after touching the hard rocky exposure. None of the rivers have large deltas. The Mandovi and Zuari Rivers flow into a wide and deep bay. At several locations along the Mandovi and Zuari Rivers, large-scale reclamation projects have narrowed the course of the rivers.

(b) Channel bars. The deposits along the river channels are marked as channel or meander bars, depending on their position and origin. On imageries they appear as streaky, multiple channels, while stereomodels reveal some areas of elevation interspersed with silty clays. Such deposits are located along the channels of the Terekhol, Chapora, Sal and Mandovi rivers.

3.1.2 Fluvio-marine forms

(a) Estuary islands. On images, several islands were located as spots within river courses. These islands are probably rocky exposures, and are extending peripherally, because of heavy siltation. The islands of Tiswadi and Divadi are typical examples of this type of alluvial island. That they are connected to the mainland via alluvial and sandy extensions is demonstra ted by the island of St. Jacinto just north of the Dabolim plateau. Many of these alluvial islands are not obvious

194 P. D. Kunte, B. G. Wagle

features of the landscape because they are now linked by roads and bridges.

(b) River terraces. The horizontal or nearly horizontal alluvial terraces on either side of the river are depicted as such on aerial photos and images. These terraces are covered with vegation in places, and so are also visible on Biomass images.

(c) Tidal fiats. In all rivers, sea water encroaches during high tide and submerges the low-lying areas adjacent to these rivers, forming tidal flats. These are 2-3 metres above the present sea level. They are extensive, marshy, and barren tracts of land. These tidal flats appear as wide stretches along the Terekhol, Chapora, Mandovi and Zuari rivers. The agricultural lowlands in the coastal tract seem to be old tidal flats formed by sediments deposited along river banks.

(d) Estuaries. Out of seven major rivers in the state of Goa, only the Mandovi and Zuari rivers have developed large estuaries. The estuarine mouth of the Zuari river is abutted by the prominent plateau heights of Bambolim and Cabo in the north, and Cortalim, Dabolim and Marmagoa in the south, with a few small enclaves of alluvia. The Mandovi estuarine mouth is abutted by the plateau heights of Aguada in the north, and to the south by the reclaimed portion of Panjim City, Miramar Beach and Cabo.

3.1.3 Marine forms

(a) Wave-cut platforms. These are found at the bases of cliffs and promontories (Aguada, Dona Paula). In some areas they are covered with a thin veneer of recent alluvium. At least two or three levels of platforms can be seen on aerial photographs. They are indicators of recent sea level variations.

(b) Beaches. Beaches are typically represented by bright or very light bands along the coast. The images reveal the unprotected areas facing the sea as long stretches of straight, sandy beaches; semicircular bays are marked by invariably crescent-shaped beaches, also called bay beaches. Beaches also can be identified at bay heads, on the seaward side of sand bars, and between the promontories.

Analysis of space-borne data for coastal zone information 195

(c) Old beach ridges. These are one to three metres high and are roughly parallel to the present shoreline. The beach ridges can be distinguished from dunes because of their continuous linear forms, each of which marks the position of an ancient shoreline. Four or five major beach ridges occur far inland from the present shoreline.

(d) Cliffs. Active and abandoned cliffs are important indicators of marine erosion, and can be identified on aerial photographs. They range from five to forty metres in elevation, and are formed by under-cutt ing and subsequent falls of large rock masses. Such cliffs are located along the nor thern and southern coasts of Goa (Anjuna, Vagator, Marmagoa and Cape Rama). Abandoned cliffs found a few hundred metres inland indicate the retreat of the sea.

3.1.4 Aeolian features A series of sand dunes occurs parallel to the coast. Within these dunes a set of old or inactive and recent or active dunes can be discerned. The old dunes display a dark tone from the sand-loving vegetation that covers them; this tonal contrast distinguishes them from recent dunes, which exhibit a very light tone in both aerial photos and images, and are located close to the coast. These dunes extend about 1-2 km inland, and indicate the accretion of the shorelines.

3.1.5 Vegetational features The biomass index image, along with aerial photos, is used to identify vegetational features. Mangrove vegetation is extensive in the estuaries of the Mandovi and Zuari rivers, particularly in the silted-up fringes of the Cumbarjua Canal; it also appears in the minor estuaries to the north and south. Above the tidal limits, wherever the elevation allows beach formation, typcial beach flora occurs with shrubs and maritime grasses along with coconut plantations. This beach flora appears as a thin red line alongside the beaches, which appear as yellow stripes on biomass images. This natural vegetational growth now covers ancient dunes, stabilizing their movement . Low-lying areas made up of alluvial deposits are occupied by agricultural fields. Mesa tops are covered by shrubs and grasses, whereas their slopes are covered by trees.

3.1.6 Structural features

(a) Lineaments. Lineaments observed in aerial photos and images are depicted as incised fractures, stream courses, or abrupt topographic and vegational changes (Fig. 1). It was observed that the lineaments in

196 P. D. Kunte, B. G. Wagle

Goa are not randomly distributed but appear to be grouped in sets. The majority of l ineaments along coastal Goa follow a N W - S E or N E - S W direction, with a few running ENE-WSW. The N W - S E direction, which corresponds with the regional Dharwarian trend, is predominant; secondary N E - S W linears occur due to the cross-folding of Dharwar Orogeny. Based on the configuration of the coast and associated linear features, it is clear that the present shape of the coast is controlled by lineaments.

(b) lnselberg. Because of differential weathering due to the varying degree of resistance to erosion, many outcrops of hard rock stand out predominatly in the form of an inselberg located near Panaji.

(c) Mesas. Laterite mesa tops are covered with coarse grasses and scrub. They are fringed on the slopes with some natural forest cover and abundant cashew plantations. These mesas, therefore, are clearly depicted on biomass images as yellow patches, which represent the barren flat tops of the plateaus, encircled with red fringes, which indicate vegetation on the slopes. The height of mesas varies from 40 to 100 metres above mean sea level. Along the shore, these mesas end in headlands; the Aguada, Cabo and Marmagoa promontories are typical examples which have counterparts along the coast to the north and south.

3.2 Nearshore features

3.2. 1 Nearshore islands Many islands along the coast (St. George's, Pequene, Grande, Kam- bariam, St. Jacinto, etc.) can be located through the use of space imagery. The erosional islands consist mostly of rocky islands such as those off the Marmagoa coast, which are the protrusions of a drowned topography separated from the mainland because of faulting. These islands are 1-2 km offshore.

3.2.2. Turbidity Second Principal Component image has provided a synoptic view of discrete turbid water masses in the nearshore region. Depending on the type of particulate matter, stratification, depth of settling, and bottom conditions, turbid waters will be more reflective than clear water and will appear as different shades of grey. On satellite images, the sharp contrast between various sediment-laden waters is clear and mappable. Tonal variation is considered as a measure of turbidity concentration.

Analysis of space-borne data for coastal zone information 197

Texture and pattern help in monitoring the distribution and movements of turbid water masses. Based on the tonal variations, four distinct types of water masses were identified:

• highly turbid water off the Terekhol coast; • moderately turbid water off Calangute Beach and Cape Rama,

and in central portions of the Mandovi and Zuari Bays; • less turbid water near the circular current pattern off Marmagoa;

and • virtually clear water off Colva Beach.

On satellite images, current directions are indicated by sediment plumes that become elongated and pointed in the direction of the flow. In the area off the Mandovi and Zuari estuary region, the plumes travel first in a westerly, then a southerly to south-easterly direction. Off the Sal, Baga and Terekhol river mouths, the plumes, though dispersed, are moving south. Less turbid waters appear near the circular pattern in current-borne sediments off Marmagoa. Another eddy pattern was also discovered off the Baga river mouth.

4 DISCUSSION AND CONCLUSIONS

The synoptic view of space images provides an opportunity to study the coastal areas in an integrated manner. This study led to discoveries which have a direct bearing on coastal zone development. Old dunes, ancient beach ridges, abandoned cliffs and strand lines observed along the coast indicate that the coast has been progressing seaward and is fairly stable. Features such as nearshore islands, wave-cut terraces, estuaries, overhangs, and caves, on the other hand, indicate that the cliffs and promontories of the coast are being constantly eroded. The rate of erosion depends upon their orientation, lithography and structure of the rocks. The intensive removal of coastal material for construction purposes as well as the filling of coastal wetlands may also contribute significantly to erosion.

The lineament study indicates that regional drainage and the present shape of the coastline is controlled by the lineaments. Moreover, these rock fractures tend to accelerate erosion when they run parallel to the coast. Consequently, ancient faults and weak zones may be reactivated in the future. A close analysis of these features is therefore essential before beginning construction on any coastal project.

The highly turbid water seen in the northern region of the study area indicates that active sedimentation is taking place along this part of the coast. The diffused textural and tonal patterns of turbid water indicate

198 P. D. Kunte, B. G. Wagle

that these sediments consist mostly of clay particles, which remain in suspension for a longer period. Long-shore currents, which are unable to bend around the Marmagoa headland, discharge the greater part of their energy in a large eddy, resulting in less turbid water occurring near the circular patterns of sediment flows along the central coast. Little turbidity occurs near the coast, and the water off Colva Beach is virtually clear. This is probably due to a combination of a fairly straight coastal track, absence of sediment-discharging rivers, and long-shore current diversion by the Marmagoa headland north of this beach. Since this area is thus a protected zone, it promotes the deposition of sediments, and hence is identified as a prograding coast. The move- ments of the sediment plumes seem to suggest that littoral sediment transport direction during the month of March is towards the south.

SPOT images (dated 1986) and aerial photographs (dated 1960, 1973), when examined for temporal changes, indicate that heavy siltation is causing estuarine islands to accrete, which in turn causes a shift in drainage. The banks of the Zuari and Mandovi rivers show significant alluvial deposition, which is fast converting the bays into swamps. These facts indicate that the rivers may be carrying sediment loads beyond their capacity. Reclamation projects along the rivers are depicted as meander-controlling structures which form straight lines along the river channel. These projects result in tidal flooding, further accelerating the erosion of river banks, which ultimately has adverse impacts on fish nurseries and salt pans. These revelations demonstrate that remote sensing with spatial, spectral, and temporal capabilities provides quick, reliable and accurate information on coastal and nearshore processes. Such information can be of vital importance to coastal zone planners in managing the coastal zone.

A C K N O W L E D G E M E N T S

The authors thank Dr B. N. Desai, Director, NIO, for permission to publish this material. Authors are grateful to Dr R. M. S. Bhargava, Head, Data and Information Division, Shri. R. R. Nair, Head, Geological Oceanographic Division, and Dr L. V. G. Rao, Head, Physical Oceanographic Division for encouragement and interest in the study.

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