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:[email protected]]
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
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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
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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.
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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
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Unpublished Progress Report of Marine and Coastal Surveys
Division, Geological Survey of India, 2010, pp. 37.
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