IJMS 43(1) 76
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IJMS 43(1) 76
Indian Journal of Geo-Marine Sciences INDIAN J MAR SCI VOL. 43(1), JANUARY 2014 76 Vol. 43(1), January 2014, pp. 76-81 Prospects for developing a minor port facility at Betul, Goa Thomas Mathai1, Satish Kumar1, K. N. Rajarama1, P. Praveen Kumar2 & M. Suresh Chandran3 1 Marine and Coastal Surveys Division, Geological Survey of India. East Coast-I, ER, Bhu Vigyan Bhavan, Salt Lake, Kolkata-700 091, India 2 Marine and Coastal Surveys Division, Geological Survey of India. West Coast-II, SR, Kendriya Bhavan, Kakkanad, Kochi-682 037, India 3 Marine and Coastal Surveys Division, Geological Survey of India. West Coast-I, SR, Pandeshwar, Mangalore -575 001, India [Email:thomasmathai4@gmail.com] Received 13 July 2011; revised 26 March 2013 Marine and Coastal Surveys Division of Geological Survey of India, initiated preliminary, integrated geological, geotechnical and geophysical surveys off Betul for making an appraisal of the developmental possibilities of setting up a minor port in this sector. Detailed surveys helped generate a bathymetric map of the area indicating a smooth and gently sloping seafloor without any hazards, ideal for port development. Sediment cores were studied to evaluate the subseabed sediment package and visualize the seabed sediment distribution. Mean size of the seabed surface sediments broadly signifies a sediment milieu encompassing variants from silt to clay, broadly classified as silty clay. Geotechnical analysis of the sediments helped to evaluate the engineering properties of the seabed and sub-seabed sediments quintessential for development of a port in this sector. Clayey sediments have a high degree of plasticity with a very high swelling potential. Shear resistance is almost unregistered at almost all levels of the sediment package. Geotechnical characteristics of the sediments are generally conducive for offshore structures. Side scan sonar surveys brought out a clear seafloor, generally monotonous and thus well suited for a navigational channel access. Current Observations carried out in the nearshore sector indicate low velocity currents and are not influenced by tidal variations. Environmental parameters including Temperature, pH, Salinity, Conductivity and Dissolved Oxygen were studied to record the conditions prevalent in this sector, for post-development comparison. [Keywords: Port, vessel, bathymetry, sea bed] Introduction Konkan coast is developing at an amazing pace in tune with the escalating industrial development abetted by the localization of special economic zones (SEZ). This obviously warrants the enhancement of infra-structural facilities especially in regard to shipping and cargo movement. Heavy shipping traffic has totally congested both the major ports of Goa and Karwar thereby affecting the movement of country cargo and iron ore transshipment all along the Konkan sector. Development of a minor satellite port at Betul (located in between Goa, to the north and Karwar to the south) could help divert some of the medium to small vessels and ease the congestion. Goa State Government and Maritime board had indicated development plans for the Betul sector. Proposed strategies for dredging out the existing Sal river channel would have afforded only short-term benefits (Photo-1). Photo 1—Sand bar across Betul bay impedes development of the minor port. MATHAI et al.: MINOR PORT FACILITY AT BETUL, GOA In the present study Marine and Coastal Surveys Division, Geological Survey of India, initiated integrated geological, geotechnical and geophysical surveys off Betul for an appraisal of the developmental possibilities of setting up a minor port here that could serve as a satellite port to Goa port for reducing the congestion. Materials and Methods An area of about 130 sq. km off Betul was covered by preliminary surveys by ship mainly with an objective for generation of data pertaining to geotechnical parameters of the seabed and sub-seabed sediments besides some basic data on environmental parameters. Detailed bathymetric surveys were carried out for preparation of a bathymetric map of the area. Shallow seismic surveys formed part of the geophysical underway surveys. Scanning of the seafloor in the proposed channel alignment sector by Side scan sonar was also undertaken. Studies also included collection of gravity/vibro cores for 77 determination of the sub-seabed sediment nature and for preparing a seabed sediment distribution map. Subsamples from the sediment cores were subjected to geotechnical analysis in order to delineate the geotechnical characteristics of the area. Basic data on pollution was also generated from the seawater samples that helped in determining various environmental parameters. Field study of the hinterland areas helped gather information regarding geological setting, drainage pattern and coastal morphological scenario1. Results and Discussion Bathymetric surveys recorded depths vary from maximum of 33.11 m to a minimum of 5.48 m and shows a closely spaced bathymetric contours that shows a generally smooth and gently sloping seafloor. Isobaths are without much inflection and are generally parallel to each other and also to the general trend of the coast (NW-SE) (Fig. 1). However, between the 6 m and 8 m isobaths, a relative steepness can be observed Fig. 1– Bathymetric map off Betul, Goa 78 INDIAN J MAR SCI VOL. 43(1), JANUARY 2014 with the gradient at around 1 in 600. A low level flat (gradient at around 1 in 1200) between depths of 8 m and 9m is apparently seen to separate the relatively steeply dipping nearshore (up to 8m isobath) and a gently sloping offshore, having a gradient of about 1 in 900 (beyond 9 m isobath). Coast normal profile sections reveal the slight differences in the offshore seabed morphology at places though in general it tends to be monotonous in nature. Seabed and sub-seabed sediment characteristics of Betul (nearshore and offshore) are based on the study of gravity and vibrocores. Seabed surface sediment is seen to comprise of only silty clay. It is quite obvious that the sluggish nature (silting up and meandering nature of the channel at near the confluence) of the Sal River and the almost complete blocking of its outflow channel into the sea has completely cut of the fluvial sediment input into the nearshore sector. Mean size of the sediments broadly signifies silty clay. Sorting index implies a rather poorly sorted to very poorly sorted sediment package which is nearly symmetrical to negatively skewed wherein an excess of fine fraction is implied with a peaking in the coarser fractions. Obviously a low energy domain is indicated. Kurtosis denotes thoroughly mixed sediments having a population comprising almost equal proportions of coarse fractions and fines. Apparently, the fluvial sediments debouched by the Sal River into the nearshore have been reworked and subsequently deposited in a low energy environment ensuring extreme levels of mixing. Sediment blockade from the hinterland, in the recent past, has ensured that a nearly 5 m sediment blanketing occurred in the Betul nearshore during a marine dominant regime, although with reworking and mixing of the older fluvial sediments. Engineering properties of the seabed and subseabed sediment gain much significance in the context of port development and port related onshore and offshore constructions besides erection of any nearshore structures. A detailed study of the geotechnical characteristics of the cohesive clayey sediments off Betul show that the Mass Physical Properties, except for water content, are within a restrictive range and do not have any marked downcore variations nor lithological control (Fig. 2). Fig. 2–Downcore Variation of Sediment Texture and Geotechnical Parameters of Vibrocore VC-13 MATHAI et al.: MINOR PORT FACILITY AT BETUL, GOA Water content generally displays an apparent depletion towards deeper levels mainly due to compaction and consolidation effected at the lower levels. Atterberg Limits and Indices for the clayey sediments from various levels show that the liquid limit tends to be quite high (150 to 183%) while plastic limit is within the moderate to high range (40 to 100%)2. A dominance of montmorillonite with little admixtures of illite is indicated for the clay component of the sediment package. Plasticity Index shows a very high degree of overall plasticity while the Liquidity Indices have a majority of values closer to, but lesser than one and this more or less is indicative of a slightly stiff to soft state of consistency for the clay; the clayey sediments would not tend to flow when subject to any disturbances. Plasticity Chart (Fig. 3A)3 (USAWES, 1967) shows that the clayey sediments are organic clays having a high degree of plasticity barring a few exceptions identified as clays having a high degree of plasticity yet with inorganic affinities. 79 Fig. 3B–Activity Chart for the core sub-samples sediments reiterate the dominance of montmorillonite as the most prolific clay component amongst the clayey admixtures; only a couple of plots fall in the illitekaolinite fields. From the Swelling Potential Chart5 it is obvious that a majority of the clayey sediments have a very high swelling potential though a lesser number has high swelling potential (Fig. 3C). Shear resistance is almost unregistered at almost all levels of the Fig. 3A–Plasticity Chart for the core sub-samples Clayey sediments are seen to have high levels of activity in the Activity Chart (Fig. 3B) and these are montmorillonite in nature4, which corroborates the observation made on the basis of LL and PL values indicating the clay in the sediment to be mainly of montmorillonite type. Only very few sub-samples have medium activity signifying zones with relatively lesser proportions of clay. Proportions of various constituent clay minerals in their respective fields relating to the Fig. 3C–Swelling Potential for the core sub-samples 80 INDIAN J MAR SCI VOL. 43(1), JANUARY 2014 sediment though at some depths where silt/sand components are seen to occur, some low to medium values are registered; they are seen to broadly fall in the category of sediments having very soft consistency. Geotechnical characteristics of the sediments, though having slightly compact, soft clayey sediments at shallow levels are conducive towards ensuring stability of the navigation channel sides through the very cohesive nature of the clayey sediments. High swelling potential of the clays would have to be taken into account from the engineering point of view when planning any structures at shallow levels. Shear resistance offered by the sub-seabed sediments at relatively lower levels, although quite low, augers well for any shallow nearshore constructions envisaged during development of the port and also for erecting nearshore/offshore foundation structures. Shallow seismic reflection surveys were carried out along twelve East-West traverses (10 km long with two km interval) nearly perpendicular to the coast and along a North-South tie-line covering a total of 181 line km. The surveyed area shows a smooth seafloor and in general deciphers three sub-bottom reflectors (R1, R2 and R3) below it. Reflector R1 is nearly parallel to the seafloor at 5.2 m below it. Reflector R2 at a depth of about 13m below seafloor is also nearly parallel to the seafloor though not continuous because of the truncations at many places due to extrusions formed by a highly eroded underlying layer (R3). Erosion of this strong reflector R3 (lateritic layer? exposed along the coast) is so severe that it has given rise to many erosional buried channels. Erosional channel fills seem to comprise sediments of low reflectivity (sand?) and possibly represents sediment deposited during a transgressional phase of the sea. Top layer lying between seafloor and reflector R1 comprises clayey sediments of relatively high reflectivity as compared to the layer between R1 and R2 (Fig. 4). Fig. 4–Part of Seismogram along Line L-1 Side scan surveys off the mouth of the Sal river, failed to bring out any rocky outcrops, reefs or rock fall debris giving an indication that the nearshore areas are quite monotonous and generally without any prominent features. Typical offshore features like sand ripples and waves are discernible at a few places in the Betul nearshore at the outflow sector of the Sal river. A few selected scan records show diffuse mounds on the seafloor probably denoting the sediment-blanketed extensions of the promontories and lateritic rock fall/debris scattering out into the sea (Fig. 5). Fig. 5–Side Scan Sonar Record Current Observations made at three selected locations in the nearshore sector, at water depths ranging from 9 m to 14 m, show comparable low current velocities (within a range of 3 to 10 cm/sec and averaging at 5 cm/sec) though the current directions were found to be quite at variance. In the northern part of the area and at the central outflow sector of the Sal river at Betul bay, South-South Easterly (in the N140o to N160o - Vector mean 123o S57oE) and South-South Westerly (in the N150o to 200o - Vector mean 186o - S6oW) current directions are seen to dominate. A distinct variation is seen in the southern sector, within the Rama bay, probably caused by the reflecting currents from the onshore cliffs or due to interference current patterns giving rise to a dominantly Northerly current direction (variant between N 230o and N 60o - Vector mean 319o N40oW). Current directions are apparently not much affected by the tidal variations and the low current velocities would apparently not prove detrimental to the navigational movements of the vessels when negotiating this nearshore sector. Shallow port related installations or constructions likely to be set up in this MATHAI et al.: MINOR PORT FACILITY AT BETUL, GOA sector would also be quite unaffected by the low velocity currents. Some environmental parameters including Temperature, pH, Salinity, Conductivity and Dissolved Oxygen were studied to obtain a record of the conditions prevalent in this sector prior to any development so as to serve as background values for comparison at a later date, post development and increased human interference. Seawater o temperatures average at 27.89 C while pH values average at 8.22. Salinity values are seen averaging at 32.31 ppt; the marginally lower values of salinity may be due to mixing of fresh water from the on-land fluvial influx. Conductivity values averages at 50.95 mS/cm and Dissolved Oxygen at 6.44 mg/L. Almost all the values recorded are seen to conform to the standard values for ocean waters along this part of the coast (United Nations Environmental Programme6. Very well developed proximal beaches at Canaguinim Bay and Rama Bay have proven to be excellent tourist attractions. Surveys reveal that both bays could provide excellent anchorages for Cruise liners and pleasure craft; the Rama Bay, in particular, is well protected on the south by a huge natural promontory in the form of a gabbroic ridge jutting into the sea (Photo-2). Photo 2—Gabbro dykes are natural promontories for safe anchorages. 81 Conclusion Dual utility of this sector in the backdrop of enhanced impetus to eco-tourism and the imminent need to cater to the pleasure sailing crafts that frequent this area with its high tourist potential could thus be well addressed. Coastal landforms and the seabed morphology are very conducive for setting up a port in this sector and the capital expenditure in this regard would be quite minimal given the inherent natural setting afforded by the Betul nearshore. Overall assessment therefore ascribes an immense potential for the establishment and development of a minor port and related facilities in the Betul sector. This minor port would prove to be immensely useful in catering to small and medium vessels and thus help divert some of the heavy traffic from the heavily congested adjoining major ports. Acknowledgements Authors thanks the Deputy Director General, Marine and Coastal Surveys Division, Geological Survey of India, Mangalore for kind encouragement and permission for this study. Participant Geophysicists of the cruise are acknowledged for the shallow seismic data. References 1. Mathai, Thomas., Chandran, M.S., Satish Kumar, Praveen Kumar, P., Rajarama, K.N., Das, P.C., Sahoo, N.C. and Premakumar, P., Geotechnical Appraisal off Betul, Goa, Unpublished Progress Report of Marine and Coastal Surveys Division, Geological Survey of India, 2010, pp. 37. 2. Mitchell, J.K. , Fundamentals of Soil Behaviour. (Wiley & Sons Inc., New York) 1976, pp. 422. 3. USAWES, Unified Soil Classification System Technical Memoir No. 3, 1967, pp. 357. 4. Skempton, A.W., The colloidal activity of clays, Proc. 3rd International Conference on Soil Mechanics & Foundation Engineering Switzerland. I (1953) 57-61. 5. Seed, H.B., Woodward, R.J., Lundgren, R., Prediction of swelling potential for compacted clays, Journal of the Soil Mechanics & Foundation Engineering Division, ASCE, 88 (SM-4, 1962b), 53-87. 6. UNEP, United Nations Environmental Programs, Industry and Environment. Report of an UNEP/IEO Workshop, Technical Report Series No - 2, Paris, 1990.