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(SSP2921)
DISTRIBUTION RESTRICTED
Marine Environmental Impact Assessment for Re-routing the 42
Sour Gas Pipeline Segment of ONGC in the Nearshore and Intertidal
Areas of Umbhrat, Navsari District SPONSORED BY OIL AND NATURAL GAS
COMMISSION, JANUARY 2015
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Marine Environmental Impact Assessment for Re-routing the 42
Sour Gas Pipeline Segment of ONGC in the Nearshore and Intertidal
Areas of Umbhrat, Navsari District Project Leader M.A.Rokade
Associate Project Leaders V.S.Naidu Sabyasachi Sautya B. R.
Thorat
JANUARY 2015
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CONTENTS
Project team i Executive summary ii List of tables xxi List of
figures xxiii List of plates xxiv Common abbreviation xxv 1
INTRODUCTION 1 1.1 Background 1 1.2 Objectives 1 1.3 Scope of work
1 1.3.1 Evolve prevailing marine environment off Umbhrat as follows
2 1.3.2 Assessment 2 1.4 Approach strategy 2 1.5 Studies undertaken
3 1.5.1 Sampling locations 3 1.5.2 Sampling frequency 4 1.5.3
Physical processes 4 1.5.4 Water quality 4 1.5.5 Sediment quality 5
1.5.6 Flora and fauna 6 2 PROJECT DESCRIPTION 8 2.1 Present
activity 8 2.1.1 Pipeline 8 2.1.2 Purpose for re-routing the
pipeline and EIA 8 2.13 Cargo: sour gas 9 2.2 Proposed activity 9
2.2.1 Installation of new pipeline 9 2.2.2 Submarine pipeline
segment 10 2.2.3 De-commissioning of redundant pipeline 11 3 STUDY
AREA 12 3.1 Tapi estuary 12 3.2 Mindhola Estuary 14 3.3 Umbhrat
coast 15 4 PREVAILING MARINE ENVIRONMENT 16 4.1 Bathymetry 16 4.2
Physical processes 16 4.2.1 Tides 16 4.2.2 Currents and circulation
16 4.3 Water quality 17
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4.3.1 Temperature 17 4.3.2 pH 18 4.3.3 Suspended Solids (SS) 20
4.3.4 Salinity 21 4.3.5 DO and BOD 23 4.3.6 Phosphorus and nitrogen
compounds 24 4.3.7 PHc 29 4.3.8 Phenols 31 4.4 Sediment quality 32
4.4.1 Texture 32 4.4.2 Metals 32 4.4.4 Phosphorus 35 4.4.5
Petroleum Hydrocarbons (PHc) 36 4.5 Biological characteristics 36
4.5.1 Mangroves 37 4.5.2 Bacteria 39 4.5.3 Phytoplankton 40 4.5.4
Zooplankton 45 4.5.5 Macrobenthos 48 4.5.6 Fishery 52 4.5.7 Corals
54 4.5.8 Birds 54 4.5.9 Reptiles and Mammals 54 4.5.10 Sand dunes
54 5 POTENTIAL ENVIRONMENTAL IMPACTS 55 5.1 Construction phase 55
5.1.1 Physical process 55 5.1.2 Water quality 55 5.1.3 Sediment
quality 56 5.1.4 Flora and fauna 56 5.2 Operational phase 57 5.2.1
Leak quantities 57 5.2.2 Fate of gas leak 58 5.2.3 Probable
movement of leaked gas 58 5.2.4 Impact on water quality 58 5.2.5
Impact on sediment quality 58 5.2.6 Impact on flora and fauna 59 6
MITIGATION MEASURES 60 6.1 Design considerations 60 6.2
Construction phase 60 6.3 Operational phase 61 6.4 Potential marine
environmental impacts 62 6.4.1 Construction phase 62 6.4.2
Operational phase 63
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6.5 Mitigation measures 63 6.5.1 Design considerations 63 6.5.2
Construction phase 63 6.5.3 Operational phase 64 6.6 Implications
CRZ Notification 64 7 SUMMARY AND CONCLUSIONS 65 7.1 Background 65
7.2 Project description 66 7.3 Study area 67 7.4 Prevailing marine
environment 67 7.5 Potential environmental impacts 69 7.6
Construction phase 69 7.6.1 Physical processes 69 7.6.2 Water
quality 69 7.6.3 Sediment quality 70 7.6.4 Flora and fauna 70 7.7
Operational phase 71 7.7.1 Leak quantities 72 7.7.2 Probable
movement of a leakage 72 7.7.3 Impact on water quality 72 7.7.4
Impact on sediment quality 72 7.7.5 Impact on flora and fauna 73
7.8 Mitigation measures 73 7.8.1 Design considerations 73 7.9
Construction phase 73 7.10 Operational phase 75 7.11 Potential
marine environmental impacts 75 7.11.1 Construction phase 76 7.11.2
Operational phase 76 7.12 Mitigation measures 77 7.12.1 Design
considerations 77 7.12.2 Construction phase 77 7.12.3 Operational
phase 77 7.13 Implications CRZ Notification 78 8 RECOMMENDATIONS
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PROJECT TEAM M.A Rokade V.S.Naidu Sabyasachi Sautya B.R.Thorat
S.N Gajbhiye Soniya Sukumaran Anirudh Ram S Jaiswar Rakesh P.S
Abhay S Fulke D.S Bagde Mohammed Ilyas Jairam Oza Tejal Vijapure
Suman Ghadigaonkar Nageshwar Rao Aditya Patwardhan Ajay Yadav Divya
Majithiya Shailesh Salvi Vaibhav Joshilkar Archana Kamble Parul
Chemburkar Gaurav Gavade Dipali Kanekar Edna Desouza Kiran M Manish
Stephen M Salazar Archana Dhumal
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EXECUTIVE SUMMARY 1 INTRODUCTION ONGCs Hazira Plant receives
sour gas from Mumbai offshore through
a 42 SBHT pipeline for processing. The pipeline segment of 769 m
at Umbhrat, District Navsari got exposed due to high tides and
cyclonic storms in 2011-12. ONGC carried out protection measures
using Geotextile tube, Geobags and Gabion boxes on the basis of
suggestions of EIL. Again in June 2014 its 500 m segment got
exposed. EIL then recommended relocation of the segment for which
Department of Environment & Forests, Govt. of Gujarat,
Gandhinagar instructed ONGC to conduct EIA study including impact
on mangroves. ONGC therefore contracted CSIR-NIO to conduct EIA
study with the following objectives.
2 Objectives a) To establish the prevailing marine environmental
status in respect
of water quality, sediment quality and biological
characteristics including mangroves for marine environment off
Umbhrat.
b) To assess probable impact of proposed pipeline laying on the
marine environment and ecology. c) To suggest Marine Environmental
Management Plan (MEMP) including
mitigation measures. 3 Studies undertaken Investigations were
conducted during October 2014 at 3 subtidal
stations each at Umbhrat and Umbhrat - Hazira coasts and at 2
intertidal transects in respect of physical processes, water
quality, sediment quality, flora and fauna including fisheries and
mangroves etc.
4 Project description Sour gas The gas (specific gravity 0.74)
mainly contains methane (C1) in 79%
while C2 and C3 hydrocarbons are 11.8%. Other high derivatives
are in trace quantities. H2
S is upto 200 ppm due to which the colourless gas has a typical
odour.
Proposed activity The proposed activity involves installation
and laying of a 42 dia, 1.9
km long pipeline between two hook-up points HK1 and HK2, and new
sectionalizing valve (SV) station.
A segment of 800 m between hook-up point : HK1 and the
landfall
point shall be installed through the marine environment using
open cut and trenching method in a 20 m corridor. The sediment side
cast during trenching will be used for back filling and the contour
of the intertidal area shall be
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restored after the construction activity is over. The re-routed
pipeline shall be hooked up with the existing pipeline. Afterwards
inertisation and nitrogen purging of the existing isolated pipeline
shall be done. All the materials shall be confirming NACE quality
as per specifications. Welding of the pipeline shall be carried out
as per specifications/standard API 1104/ASME B3.18. All the welded
joints shall be tested by radiography and other non-destructive
tests (NDT) as specified. The pipeline shall be subjected to
hydrostatic test pressure of 1.5 times of the design pressure. It
shall then be followed by dewatering and swabbing operations as per
specifications.
All the waste materials shall be disposed off properly. The
back-filling
of the excavated areas and clean-up/restoration of the site
shall be done on completion of the work. EGP and cathodic
protection shall be carried out as per specifications laid.
The redundant pipeline shall be retrieved and cut at welding
joints of
every 12 m and transported to ONGC storage yards. 5 Study areas
The nearshore coastal waters off Umbhrat i.e. the project area
located
10 km southward of Hazira includes Umbhrat beach, and coastal
waters of Tapi and Mindhola estuaries. They are described
below.
Tapi Estuary Tapi, a major perennial river of the west coast of
India, is an important
source of freshwater to the region. The 720 km long river
(Figure 3.0.1) originates near Multai in the Betoul District of
Madhya Pradesh and commands a catchment area of 65,145 km2 of which
around 4000 km2
is in Gujarat. During seaward course, the river meanders through
the hilly terrain of the Western Ghats before entering the coastal
alluvial plains of Gujarat to meet the Arabian Sea near Hazira. The
shallow and wide lower segment of the river exhibits
characteristics of a typical estuary with strong currents
associated with significantly high tidal influence upto 25 km
upstream. Further inland however, the seawater excursion is
restricted due to creation of a causeway at Rander which also
hinders the freshwater outflow.
The estuary and nearshore areas of Hazira exhibit a typical
character of South Gujarat coast with (a) vast intertidal regions
composed of poorly sorted sediment made up sand silt and clay with
isolated rocky outcrops, (b) supralittoral region either barren or
dominated by salt tolerant plant species of Prosopis, Acacia and
Zyphus, (c) gently sloping continental shelf with uneven seafloor
often strewn with sand bars, and (d) high tidal influence due to
the proximity to the Gulf of Khambhat.
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The riverine discharge to the sea is controlled by the Ukai and
Kakrapar dams constructed on the river at 141 and 115 km upstream
respectively. Mean runoff of Tapi was 1.7982 x 1010 m3/y in 1975.
After the construction of dams, it has reduced to an average of
7.301 x 109 m3/y during 1982-91. In 1995 a weir-cum-causeway was
constructed across the river at Rander that prevented seawater
incursion further inland. In recent years the river discharge of
8000 m3/s during monsoon decreases to 10 to 45 m3
/s during November-May leading to stagnation in the riverine and
inner estuarine segments, during the dry season.
Due to proximity of the Gulf, the region experiences
significantly high tidal influence with mean spring and neap tidal
ranges of 5.7 and 4.3 m respectively at Hazira. The tidal influence
however decreases with distance into the estuary with spring and
neap tidal ranges of 2.3 and 0.4 m respectively at Surat. During
the period of freshwater dominance the flood duration of 6 h in the
openshore reduces to 4 to 5 h in the mouth segment of the estuary
and decreases to barely 2 h at Surat with corresponding increase in
the ebb period. Decrease in flood period though to a lesser extent
occurs in the landward direction during the dry season also.
The Tapi River is subjected to sporadic floods associated with
heavy
rainfall in the catchment area during July-September. When the
flood coincides with spring tide, the water level rises
substantially inundating vast areas. A level of 10 m with respect
to CD has been recorded at Magdalla during one such flood in the
past. The construction of two dams on the river has however reduced
the flood fury to a great extent.
Currents are largely tide-induced during dry season and are
considerably influenced by the riverine discharge during
monsoon. At times when the runoff is heavy, the flow is
unidirectional in the estuary throughout, irrespective of the tide.
During dry season, the maximum flood and ebb speeds often exceed 1
m/s with the tidal excursion of 7 to 20 km within the estuary and
11 to 13 km along the open coast. The direction of flow is
predominantly decided by the channel geometry that reverses with
the change in the tidal phase. The u component of the current is
dominant throughout a tidal cycle and net negative u component
favours seaward transport. The v component is considerably weak as
expected for estuaries under high tidal dominance.
The lower segment of theTapi Estuary is well flushed with
flushing time
of less than 2 tide cycles calculated based on the tidal prism
method. Flushing time of 3 tide cycles during spring for both
freshwater flow conditions reveals the dominance of tidal influence
over freshwater flow during spring tide. However, during neap when
the seawater incursion in the estuary
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greatly reduces, the flushing time increases to 7 tide cycles
when the river discharge decreases to 10 m3
/s.
The water quality of the estuary has deteriorated after the
industrialization time though not severely. Considerable depletion
in DO with values decreasing to
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Umbhrat coast Purna River having a winding channel and shallow
depths is situated
10 km southward of Umbhrat which is a low plan of sand. This
Umbhrat beach is almost barren with absence of mangroves and
other vegetation. Umbhrat village is located eastward between
the beach and the Highway. The beach ends at Danti further
northward along the Mindhola Estuary which has confluence to sea
with Tapi estuary.
6 Prevailing marine environment The prevailing marine
environment was evolved in respect of physical
processes, water quality, sediment quality, flora and fauna etc
for the Umbhrat coast. It is the project areas which were
investigated during October 2014. The recent data generated for the
Hazira-Umbhrat coast during January 2014 (postmonsoon) and May 2014
(premonsoon) were also assessed for the purpose. The data generated
since 1983 and available with CSIR-NIO has been employed for
comparison with the present results so that variations, if any due
to natural or otherwise causes would be indicated for the
prevailing marine environment which has been described in detail
below.
Bathymetry
The Mindhola and the Tapi Estuaries which meet at the mouth
region is exposed during the low tide. Bathymetry shows two
channels namely east and west channels at the mouth region. Depths
of 2 and 10 m are available at a distance of 1.7 km and 2.7 km,
respectively. At the mouth, in the eastern side between Danti and
Bhimpur, there is a channel which leads to the Mindola River. The
width of the river at this point is around 3 km and the length of
the river is around 6 km.
Physical processes Due to the proximity to the Gulf, the region
experiences significantly
high tidal influence (1.7 - 7.4 m). The tides are semi-diurnal
type with two high and two low waters with unequal amplitudes
occurring or each tidal day. The maximum flood and ebb speeds
varied from 0.4 to 0.8 m/s. The u and v components of the currents
denote that the v component dominated the u component, showing the
currents are parallel to the coast. The tidal excursion indicates
an elliptical circulation pattern with the major axis with
excursion lengths varying from 6 to 19 km and the track parallel to
the shoreline and the minor axis perpendicular to the coast during
an average tide.
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Water quality: Temperature: Temperature of the Umbhrat-Hazira
coast varied 20 to 33oC
range indicating seasonal variations. However, spatial
variations were not marked. Temperature of the project area i.e.
Umbhrat coast varied from 29.0 to 32.2oC (av 31.1oC) during October
2014. It is comparatively higher than that of the Hazira-Umbhrat
coast during the period possibly because of low depths of the
nearshore waters. Hence, average temperature was 22.4 to 31.5oC
during 1983-2013 indicating clear seasonal variations. The present
data are in similar range and also indicate similar seasonal
changes. These temperatures are below 35o
pH: pH of the Hazira-Umbhrat and the Umbhrat coasts was 7.8-8.2
(av 8.1) and 7.8-8.2 (av 8.1) respectively during 2014 indicating
spatial as well as seasonal variations. The lower pH for the areas
could be possible because of drainage of contaminated waters
through the estuaries particularly during low tide. The data though
were in similar ranges that recorded since 1983, lower average
values upto 7.7 were recorded in several instances because of the
polluted outflow from the Tapi estuary.
C which is below the threshold for the aquatic biota to impart
any harmful effects.
SS: SS was highly variable from 92 to 2265mg/l (av 1170 mg/l)
and 250 to
1612 mg/l (av 763 mg/l) for the Hazira-Umbhrat coast and the
Umbhrat coast respectively during 2014. The content clearly
indicated spatial as well as seasonal variations particularly due
to rain runoff and erosion of banks in the estuaries. It is
comparatively low along the Umbhrat coast as the coast is not
erosion prone.
Average SS varied from 96 to 1433 mg/l for the Hazira region
during
1990-2013. The content recorded in the present study was
comparable with the earlier data as illustrated in the above
table.
Salinity: During 2014 salinity varied in the ranges of 25.9 to
33.9 ppt (av
31.1ppt) and 27.3 to 29.2 ppt (av 28.7 ppt) for the
Hazira-Umbhrat coast and the Umbhrat coast respectively indicating
comparable salinity for both the coasts. The lower salinities were
recorded during monsoon revealing the impact of rain runoff through
the estuaries. They were higher during premonsoon because of high
evaporation rates and negligible land drainage having low salinity.
Hence seasonal as well as spatial variable were marked for the
area. Average salinity of the Hazira-Umbhrat coast varied in the
range of 25.7 to 39.0ppt during 1983-2013. The data of 2014 possess
similar range as illustrated in the above table.
DO and BOD: DO was highly variable from 2.0 to 7.3 mg/l (av 5.5
mg/l) and
3.8 to 6.1 mg/l (av 5.8 mg/l) for the Hazira-Umbhrat and the
Umbhrat coast
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respectively during 2014. The lower content for the
Hazira-Umbhrat coast particularly during monsoon indicated negative
impact of the polluted drainage through the estuaries due to which
spatial as well as seasonal variations have been noticed. In
comparison the content was higher for the Umbhrat coast possibly
because of low influence of the drainage through the estuaries.
With the average DO of 4 mg/l since 1983, the coastal waters of
Hazira-Umbhrat region were well oxygenated indicating healthy
environment. The content observed during 2014 was comparable with
the earlier data as presented above. BOD was low (av 2.5 mg/l) and
2.7 mg/l) for the Hazira-Umbhrat as well as the Umbhrat coasts
during 2014. The seasonal and spatial variations were not
significant. High average BOD shows that organic load entering
through the estuaries is efficiently oxidized by incoming tidal
water in the coastal waters. It indicated good oxidising conditions
in the coastal waters.
Phosphorus and nitrogen compounds: Phosphate was highly variable
in
the Hazira-Umbhrat coast (1.6-15.2 mol/l) during 2014 possibly
attributed to the drainage of contaminated waters through the
estuaries, hence indicating spatial variations. The content along
the Umbhrat coast was comparable though in lower range. The content
also indicated seasonal variations. Phosphate was present in a low
range of 1.1 to 5.3 mol/l in the Hazira-Umbhrat coast during
1983-2013. The seasonal and spatial variations were not marked in
the content which is comparable with the records of the present
study.
Nitrate was highly variable in the study area of the
Hazira-Umbhrat
coast during 2014 (6.6-23.2 mol/l). The concentrations indicated
spatial as well as seasonal variations. The levels at the Umbhrat
coast were higher (25.6 - 38.9 mol/l) as compared to those for the
Hazira-Umbhrat coast.
Average nitrate content was 0.6 to 34.8 mol/l during
1983-2013.
These concentrations are in the similar range as that of the
present study. Nitrite levels in the Hazira-Umbhrat and the Umbhrat
coast were low
(0.3-3.8 mol/l (av1.2 mol/l and 0.1-2.5 mol/l, av 0.9 mol/l)
during 2014. It is known that Tapi and Mindhola estuaries possess
high nitrite content because of high organic load entering in the
estuary. However, it is negative influence on the coastal waters
was not evident because the waters are highly oxygenated. Hence
spatial as well as seasonal variations were not seen. Similarly the
levels at the Umbhrat coast were low during 2014 indicating good
oxidising conditions.
The nitrite content in the Hazira-Umbhrat coast was low of 0.2
to 2.4
mol/l during 1983-2013.
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Ammonia in the Hazira-Umbhrat and the Umbhrat coasts was in the
comparable (1.1 - 5.6 mol/l) low ranges during 2014. Some adverse
impact of drainage through the estuaries has been visible in the
coastal waters off the Hazira-Umbhrat. Spatial variable were
evident in the content. However, seasonal variations were not
marked in the content. The low values were attributed to the good
oxidising conditions in the waters.
Ammonia concentrations in the coastal waters off Hazira-Umbhrat
were
ND to 6.5 mol/l during 1983-2014. These and present data were
comparable. PHc: The average PHc levels were 0.3 to 43.1 g/l in the
coast during 1983-
2013. The levels reported for the present study were in the
lower range. Phenols: The phenols content in the Hazira-Umbhrat and
the Umbhrat coasts
were in the low range of 63 to 210 and 55 to 119 g/l
respectively during 2014. The levels suggested that negative effect
of the runoff through the estuaries was not marked in the coastal
waters. Average levels of phenols in the coast were 2 to 129 g/l
during 1990-2013. These concentrations recorded since 1990 were
comparable with the present results.
Sediment quality: Texture of the Umbhrat coast during October
2014 was
mostly sandy (62.9 - 96.2%) with variable percentage of silt and
clay. Spatial variations were clearly evident. Texture of the
Umbhrat coast was also sandy (86.4 - 93%, dry wt) with meager
content of silt and clay. The variations in the content were
narrow.
The results of the present study were comparable with the
earlier data. Metals: The results of metal content indicated
considerable variation in the
trace metals of concern such as chromium, nickel, copper, zinc
and mercury along the coast making the assessment difficult.
Variations in lithogenic fraction of metals in sediment along the
west coast in space and time are common due to various factors such
as variable inputs of SS through land drainage, littoral transport,
sediment movement due to tides etc. The variations in the
concentration of trace metals are also because of changing levels
of aluminium and iron in sediment which generally influence the
concentration of trace metals. The results of monitoring are to be
addressed in this context. The metal content in the sediment of the
Hazira-Umbhrat coast was highly variable. Station 4 which was
located close to the estuarine mouth exhibited mostly higher
content indicating spatial changes in the content. The seasonal
variations however were not marked.
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Similarly, the metal content at the Umbhrat coast was also
highly variable but comparable with the content of corresponding
metal levels as illustrated in the above table.
The average concentrations of selective metals as given in
the
table also indicated variation in wide ranges. The results of
the present study however were within the range of levels of metals
recorded since 1997. They did not reveal any gross accumulation of
the metals in the sediment of the study areas.
The contents in the intertidal sediment were highly variable and
they
were comparable with accumulation in subtidal sediment of the
Hazira-Umbhrat and the Umbhrat coasts reported for 2014. They
indicated that the intertidal sediment of the project area was
grossly uncontaminated.
The build-up of Corg
in the sediment of the study areas was negligible indicating
uncontaminated sediment and low influence of organic matter
originating through the estuaries on the coastal sediment. The
concentrations of 0.1 to 3.7%, dry wt recorded for the Tapi Estuary
and the Hazira-Umbhrat coast during 1990 to 2013 also suggested
that the sediments were uncontaminated and compared well with the
present results.
Phosphorus: The phosphorus contents were low and compared well
with the contents of 238 to 1014 g/g, dry wt reported for
Hazira-Umbhrat during 1998 to 2013. The levels showed that the
organic load flowing through the estuaries did not impact the
coastal sediment of the study area.
PHc: PHc in subtidal and intertidal sediment of the Umbhrat
coast and
subtidal sediment of Hazira-Umbhrat coast during 2014 was low
upto 0.3 g/g, wet wt. PHc in the subtidal sediment of the Tapi
Estuary and the Hazira-Umbhrat coast was 0.1 to 4.8 g/g, wet wt
during 1999 t0 2013. These concentrations compared well with the
present results.
The above results revealed low PHc accumulation in the
sediment
though the area experiences traffic of barges and ships. In
areas receiving anthropogenic petroleum residues such as oil
terminals, the PHc concentration in sediment often exceeds 10 g/g;
wet wt.
Apart from changes in the physico-chemical characteristics of
water
and sediment environments that a coastal development may induce,
the ultimate concern is invariably the biological resources as
described below.
Mangroves: Mangroves are absent at the development site at
Umbhrat.
However small bushes and shrubs at the proposed site have been
observed.
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A large area of mangroves (about 27 ha) dominated by Avicennia
marina exists along the open coast outside the mouth of Tapi
estuary which is located about 12 km away from the present study
sites. These mangroves are under considerable stress due to heavy
sand deposition and inadequate tidal flushing due to human induced
changes in morphology.
Water: The TVC counts in water varied widely from 107102 to 5600
x102
CFU/ml with the highest count of 5600x102 CFU/ml recorded at the
stations during post monsoon period. No clear trend in their
distribution was observed. TC (NG-300 CFU/ml) (NG- No Growth) and
FC (NG-30 CFU/ml) also varied with nominal ranges without any
trend. Overall average of TVC counts (2300102 CFU/ml) during the
postmonsoon period were much higher that that during monsoon
(395102 CFU/ml at Hazira-Umbhrat segment and 1057102 CFU/ml at
Umbhrat segment) and pre monsoon (860102
CFU/ml) periods; whereas TC counts was highest during monsoon
(av120 CFU/ml) at Hazira-Umbhrat segment.
Bacterial counts viz; TVC, TC and FC were high in sediment than
water. No clear spatial trend in the distribution of TVC, TC and FC
was noticed. However, monsoon period sustained higher number of
pathogens at Umbhrat coast than any other season and area of the
year 2014.TVC which gives a quantitative idea about the presence of
microorganisms such as bacteria, yeast and mould was reported to
high throughout the coastal areas during the present study. Water
quality criteria set by Central Pollution Control board (CPCB)
require TC to be 500 MPN/100ml or less for outdoor bathing
(organized). Concentration of chl a ranged from 0.1 to 3.2 mg/m3
averaging at 0.8 mg/m3. Overall highest average values were
recorded during monsoon (av 1.2 mg/m3) followed by monsoon (av 0.9
mg/m3) and premonsoon (av 0.4 mg/m3
). The average values of chlorophyll a and phaeophytin were low
in the coastal area of Umbhrat-Hazira area. In most of the stations
average phaeophytin values were lower than that of chlorophyll
signifying that production was more than disintegration. The
temporal studies showed narrow tidal variation in chlorophyll
distribution. The phaeophytin also did not show marked temporal
variation. Seasonally, higher concentrations of phytopigments were
recorded during premonsoon season.
Pre-industrialized average concentration of chlorophyll a varied
in 0.3 to 2.5 mg/m3 ranges for the coastal water. These
concentrations have not changed significantly seasonally though
high values were occasionally observed. The concentrations of
chlorophyll a, however, increased dramatically during May 2003
indicating eutrophication and hence an environment under stress due
to increase in the concentration of nutrients probably associated
with sewage release. The average concentration of
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phaeophytin was often lower or comparable with the concentration
of chlorophyll a as commonly observed for coastal waters of South
Gujarat.
Highest average cell count as well as number of genera were
found
during October 2014 (monsoon) followed by May 2014 (premonsoon)
and January 2014 (postmonsoon). The decrease in cell count was
associated with low chlorophyll a. Genera like Cylindrothe
Caclosterium, Thalassiosira, Navicula and Nitzschia were present in
all the segments and during all the seasons. Premonsoon season
represented by mainly Cylindrothe caclosterium, Thalassiosira,
Nitzschia, Navicula, Thalassiothrix, Coscinodiscus, Guinardia by
contributing more than 78% of phytoplankton population. Overall 46
genera of phytoplankton were encountered during the monsoon periods
dominated by Skeletonema costatum, Cylindrothe caclosteri. During
postmonsoon season total 42taxon of phytoplankton were recorded
dominated by Cylindrothe caclosterium, Thalassiosira, Nitzschia
etc. Overall 64 species from 46 genera of phytoplankton were
encountered during 3 seasons of 2014 and compared well with the
earlier reports, although all of them do not occur at any single
station or in one season.
The seasonal variability and generic diversity of the
phytoplankton
population recorded during the present study compared well with
the other season. The populations of phytoplankton were generally
dominated by Cyclindrotheca, Thalassiosira, Skeletonema, Nitzschia
and Navicula.
Zooplankton: The average standing stock of zooplankton in terms
of
biomass and population was relatively higher during monsoon
period than the premonsoon period, whereas in terms of population
count, higher abundance was recorded during premonsoon period. The
table indicates a fluctuating trend in all the segments of the
coastal segment. The community structure revealed the dominance of
copepods (86.2 % in January), chaetognaths (8.1% in January),
gastropods (45.7% in October), lamellibranches (12.4% in October),
decapods(7.9% in October) and polychaetes (0.3% in January). The
composition was fairly diverse and the number of faunal groups
varied between 12 and 15 (av 13 groups). Overall, 19 groups of
zooplankton encountered, however all the groups were not present at
any given time or station. Other less common groups encountered
were foraminiferans, siphonophores, medusae, ctenophores, mysids,
ostracods, cumaceans, amphipods, Lucifer sp, stomatopod,
cephalopods, isopods and marine insects.
The average baseline biomass of zooplankton (4.1-8.0 ml/100 m3)
in
the coastal segment had increased in the period between April
1997 and November 2003 and decreased thereafter. The overall trend
suggested gross changes in the current study as compared with
pre-industrial baseline (9.1-9.5
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xiii
ml/100m3). In the coastal segment which receives industrial
effluents, on the contrary, the trend fluctuated and there was no
clear evidence for impact of the effluents on the zooplankton
biomass. In the off Umbhrat-Hazira coastal segments sea off the
mouth of the estuary, the post-industrial average biomass was
variable (0.1-9.5 ml/100m3
) and high during mid tide (Figure 4.5.20).
The results since 1984 indicated a fluctuating trend in the
coastal segments of Umbhrat-Hazira. The number of zooplankton
components (no) varied randomly. The baseline for the coastal
segment as a whole did not reveal any gross variations in the
zooplankton diversity with the number of zooplankton components of
the current study falling in the range of 7 to 15 no.
Macrobenthos: The results revealed not significant spatial
variations in the
intertidal macrobenthic standing stock. The intertidal
macrobenthic standing stock in terms of abundance (0-500, av 154
no/m2) and biomass (0 1.1 g/m2, wet wt av 0.4 g /m2
) varied widely. The faunal diversity varied between 0 and 3 (av
2) and mainly dominated by the polychaetes. Overall, about 3 groups
of intertidal fauna were recorded during the present
investigation.
The data indicated poor subtidal macrobenthic biomass (0-0.1
g/m2; av 0.1 g/m2, wet wt) and population (0-75 no/m2; av19 no/m2)
in all the stations of the Umbhrat-Hazira coastal segment during
all the three seasons while 0-175 no/m2; av44 no/m2
was observed at Umbhrat segment during monsoon season in the
study year. This revealed that the prevailing subtidal environment
was not conducive for the proliferation of macrobenthos due to the
instability of the substratum associated with strong tidal currents
and was probably the major factor for the low macrobenthic
productivity of many coastal segments of Gujarat. The faunal
diversity of macrobenthos was also poor throughout the study area.
Polychaetes were the dominant group followed by Polychaete and
pelecypods. In general, 5 groups of subtidal macrobenthos were
recorded during the present investigation.
The data since 1984 indicated poor macrobenthic biomass in the
coastal water in the pre-industrial (
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xiv
in macrobenthos. The overall trend suggested a stabilized poor
structure of faunal density except one abrupt increase during
October 2012, otherwise decrease in faunal components in the
Umbhrat-Hazira coastal segment over the years. The faunal diversity
was low and comparable with the earlier results indicating the
absence of gross changes in the subtidal ecology during the
post-industrialization period.
Fishery: The results of the experimental fishing operation
carried out during
February-March 2013, October 2013 and January 2014 Tables 4.5.19
indicated a moderate fish catch rate suggesting a poor fishing
potential off Umbhrat-Hazira.
The results confirmed low fishery potential in terms of catch
per unit
effort as well as low to moderate diversity in the Tapi Estuary
than the coastal segment off Hazira.
During the three seasons of 2013-14 (premonsoon, monsoon and
postmonsoon) these catches were represented by 47 species of
fishes, 9 species of Prawns and 9 other species which includes
Crabs, Squilla, Lobster and Cephalopod. Harpadon nehereus, Johnius
glaucus, Thryssa mystax, Lepturacanthus savala, Cynoglos susarel,
Coilia dussumieri, Arius caelatus, Hilsakelee, Pellonaditchela,
Parapenaeop sissculptilis, Macrobrachium rosenbergii, Acetes
indicus, Charybdis annulata and Scylla serrata occurred in most of
the catches during all the seasons. During October 2013 the fish
catch diversity was the highest and represented by 28 species of
fishes, 9 species of prawns and 5 other species. However,
composition was more or less comparable except for the exclusive
occurrence of Arius megaloptera, Arius sp, Ilisha megaloptera,
Johnius sp, Liza macrolepis, Lutjanus johnii, Opisthopterus
tardoore, Periopthalmus sp, Polynemus heptadactylus and Polynemus
tetradactylus. During January 2014 fish diversity was also high and
represented by 26 species of fishes, 9 species of prawns and 9
other species. Species like Boleopthalmus sp, Carangoides
malabaricus, Carcharhinus limbatus, Channa punctuta, Otolithes sp,
Pomadasys kaakan, Protonibea diacanthus, Sardinella longiceps,
Scomberoides commersonnianus, Thryssa hamiltoni, Trichiurus
lepturus, Charybdis cruciata, Matuta lunaris and Portunus pelagicus
were only encountered during the season. Overall, the study region
revealed low to moderate fishery potential and results are
comparable with that of the other areas of the Gulf. The low
fishery status of the Gulf could be related to high turbidity
associated with strong currents and the low biological potential at
different trophic levels.
Corals: The Umbhrat-Hazira region does not have corals as the
intertidal
area is largely sandy or muddy. Coral growth in the subtidal
region is unlikely
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xv
in view of the high suspended load in the water column, the
conditions under which corals do not thrive.
Birds: The Hazira coast offers different marine habitats like
rocky/sandy/muddy intertidal and mangroves for a variety of
resident and migratory birds (Table 3.4.20). The birds use these
habitats as their active feeding ground especially during low tide.
Hectic activities of Gulls, Heron, Terns, Egrets, Kingfisher,
Plovers, Avocets, Curlews, Whimbrels, Sand pipers, Shanks,
Spoonbills and Bitterns are frequently seen in these coastal
habitats. A large number of migratory species pass through Hazira
and a small population of them in the form of juvenile and
non-breeding adults take shelter in the coastal areas during
summer. In all, the region is represented by 81 species of
birds.
Reptiles and Mammals: Marine turtles are not common along the
Hazira coast and have not been sighted during the field studies.
The marine mammals are chiefly represented by Dolphin and Porpoise
in the coastal waters of Hazira but whales are very rare. They were
not observed during the field studies. Sand dunes: Sand dunes are
present along the coast of Umbhrat as well as around the project
site.
7 Potential environmental impacts The impacts during
construction phase would be associated with
trenching and laying of the pipeline in the shallow water depths
of 0 to 5 m to a small extent of 400 m and the remaining part in
the intertidal area. Impacts during the operational phase would be
the result of the sour gas transfer through the pipeline as
described below.
Construction phase
Physical processes: The time period to execute the work was
given as 3-4 months only and a very long trench would not be
feasible.
Water quality: Dredging, trenching and pipelaying activities
have a high
potential to increase turbidity due to increased rate of
dispersal of fine grained sediment in the water column. Apart from
affecting photosynthesis, DO as well as BOD levels may be altered
and concentrations of trace pollutant in water could increase due
because of the release of sediment interstitial water exposing the
biota to relatively high levels of pollutants. However, it will be
limited to a short period.
However, as discussed in Section 4 the sediment along the
pipeline
corridor is unpolluted and hence the interstitial water is
unlikely to have high
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xvi
levels of organic carbon and other contaminants. Thus DO and BOD
levels would not be grossly influenced and the concentrations of
trace pollutants would not enhance. Moreover, any transient
perturbations would be effectively smothered under the prevailing
dynamic conditions off Umbhrat and in the coastal waters. Hence,
the overall impact during construction phase on water quality would
be localised and minor. No adverse impact is envisaged.
Sediment quality: Backfilling and resettling of sediment may
lead to
changes in sediment texture in localised areas particularly
along the intertidal segments. However, these changes would be
minor and nonconsequential.
Flora and fauna: The most likely impact during construction
phase would
be on subtidal and intertidal habitats which could be
temporarily destroyed along the pipeline route due to dredging,
trenching and pipelaying activities. Moreover, if the dredge spoil
spreads over nearby areas under the prevailing high tidal excursion
(Section 4), the biota there would also suffer accordingly.
An increase in suspended load may marginally influence the
photosynthetic activity of phytoplankton, in localised areas.
Hence, the damage to phytoplankton in the region would be
localised, temporary and reversible and recovery would be fast once
the construction phase is completed.
Impact on zooplankton standing stock although a localised,
marginal
change in community structure and population counts may result.
Such changes are temporary, highly reversible and do not reflect in
the overall zooplankton productivity.
The trenching for the pipe laying would destroy the benthic
fauna of the
area. However, it will re-colonise when the pipeline is buried
and back filled. The process would however be slow in the trenched
area. Damage to the subtidal and intertidal benthic fauna along the
pipeline route would occur during construction period of 3 to 4
months.
The vegetation is scanty and shrubby in the nearshore belt of
Umbhrat
- Hazira and hence not very attractive for birds. Hence, an
increase in noise level during offshore construction would be of
little consequence, though the birds would be disturbed by the
noise of trenching operations. The impact however would be limited
to construction period only.
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xvii
Operational phase: Major concern during the operational phase of
the terminal would be the leakages of the sour gas under transfer
that might occur in the case of pipeline damage only.
Leak quantities: The possible failures resulting in leaks can be
attributed to
leaking valves and rupture of pipeline. Failure in the submarine
pipeline can also be another source of the
leaks. The leaks and rupture of sub-sea pipelines can result
from corrosion or due to gouge on the pipe during construction.
However, pipelines designed, fabricated, laid and periodically
inspected as per internationally accepted codes and practices are
relatively safe and spillage of POL are rare during the design life
of a pipeline. Nevertheless, in an unlikely event of pipeline
rupture due to unforeseen events such as a severe cyclone, an
earthquake or intentional third party intervention, the quantity of
sour gas that may be spilled can be 8.75 to 14 m3
(7.5 to 10.4 t) if the rupture occurs under the bed (since
sectionalising valves are provided on both the banks) which close
within 210 sec.
Since the pipeline is buried all this leaked will not surface
like a blob but leak slowly after gasification.
Fate of gas leak: Evaporation and dissolution are the major
processes mainly
responsible for changing the quantity and composition of the
spilled sour gas under water. The evaporation/gasification takes
place mainly due to the decrease of pressure on the pressurized
natural gas. Most of the leaked gas would escape from the shallow
water column as the processes exerted by the few metres of water
would not be sufficient to keep it in the pressurized state. Also
the dissolution would not be substantial as the water column
through which it travels is not much.
The transport of a leaked gas would be largely decided by wind
apart
from the physical and chemical properties of the gas as these
gasses are supported to be denser than the ambient air, it may
spread like a sheet on the water surface and the intertidal area
and eventually disperse catching fire is one of the
possibilities.
The gasses are traced when leaked due to the presence of H2
S in ppm level. However, as the gasses are not refined, they may
also contain traces of oil during the transportation and the oil
would form slicks on the water surface to indicate a possible
leak.
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xviii
Water quality: The solubility of hydrocarbons is low in water
and the depth through which this sour gas travels is limited as the
water depths in this case are < 5 m, the solubility is almost
negligible to impact the water quality.
Sediment quality: If a spill occurs in the nearshore area or
under the
intertidal area, traces of oil transported alongwith the sour
gas may remain trapped in animal burrows and may get mixed with the
sediment delaying its removal by tidal flushing apart from general
contamination of large segments of surficial sediment. The sediment
may sustain abnormal level of PHc for several days and even months.
The weathered residue sinking through the water column in the event
of a spill away from the shore may contaminate the subtidal
sediment also. This impact would be relatively minor for a given
product and given quantity since the weathered mass would be
transported widely before settling on the seabed.
Flora and fauna: Populations and community structure of marine
biota are
subject to considerable natural fluctuations due to changes in
climatic and hydrographic conditions as well as the availability of
food. Hence, it is often difficult to assess long term effect of a
sour gas spill and to distinguish changes caused by the PHc from
those due to natural variability. Moreover, the leakage of sour gas
which does not have any oxygen in it may produce anoxic conditions
in the oil depending on its pore city and the annihilation of the
benthic organisms that come in direct contact of the gas.
Though sand dunes are present in the vicinity, the pipeline
route avoids
the sand dunes and no destruction to sand dunes is expected.
Mitigation measures: It is important that certain decisions are
taken and supplemented beginning with the planning stage itself so
that the risk factors during construction and operational phases
are reduced to a minimum to protect marine ecology from
anthropogenic shocks.
Design considerations: Major environmental concern at oil
terminals is
accidental leakages of the POL. No technology is available to
recover the sour gas once leaked. Hence, best approach is to
prevent the leakages. Evidently, the design and operating
philosophy of an oil terminal should be "No Leak" under normal
operating situations which if deviate beyond preset norms, the
pumping operations should automatically stop till normal conditions
are reset. It should be ensured that internationally accepted codes
and practices are followed for the sour gas transport through
proper inspection, frequent evaluation and intensive testing of all
critical components of the
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xix
pipeline system. Likewise, vulnerable units such as fittings,
valves, flanges, couplings etc should be rigorously tested and
certified for their reliability.
The pipelines should be buried below scour level to keep the
lines in
place as the intertidal areas often experience strong wave
forces particularly during monsoon and cyclonic condition can erode
the upper strata of the intertidal area considerably.
Construction phase: The key factors in minimising adverse
impacts would
be the reduction in construction period and avoidance of
activities beyond the specified geographical project area (50 m
corridor for pipelaying) which should be kept to a minimum. Hence,
as a part of the management strategy, it is important that various
activities are well-coordinated and optimised to avoid time
over-runs and to complete the project within an agreed time
schedule.
Pre-treatment to the pipes such as coating, concreting etc and
other
fabrication jobs should be undertaken in a yard on land located
sufficiently away from the high tide line and transfer of materials
to the site should be through a predecided corridor. Similarly, the
movement of construction barges, ships, machinery etc should be
restricted to the predecided operational area. However, the region
should not be crowded with too many vessels and construction
machinery to avoid accidents and subsequent spillages of materials
and fuel.
The pipeline in the intertidal and nearshore subtidal areas
should be
buried to a safe depth and the depth of burial should be
ascertained through reliable seismic surveys to guarantee its
safety.
The Umbhrat-Hazira region is prone to occasional storms with 6
severe
cyclones (wind speed 100 - 150 km/h) striking the region over
the past 100 y. At such wind speeds a wave generated offshore can
have pronounced effect on the coast as well as the nearshore
subtidal zone. This factor should be taken into account while
designing the subsea pipeline segment.
A large number of labourers and other personnel would be
involved in
the construction phase. Temporary colonies of the work force etc
should be established sufficiently away from the high water line
and proper sanitation including toilets and bathrooms should be
provided to the inhabitants to prevent abuse of the intertidal
area. Sewage and other wastes generated in these settlements should
not be released to the sea.
The operational noise level should be kept to minimum
particularly in
the nearshore region through proper lubrication, muffling and
modernisation of equipment.
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xx
It should be ensured that the intertidal and supratidal areas
are restored to their original contours after the pipe-laying
activities are completed. General clean-up along the corridor,
adjacent areas and intertidal and subtidal regions should be
taken-up and extraneous materials such as drums, sacks, metal
scrap, ropes, excess sediment, make shift huts and cabins should be
cleared from the site.
Operational phase: The major concern during operations is the
leakage of
sour gas. Another consideration in preventing spills is the
provision and regular
testing of not only emergency shutdown devices but also the
components vulnerable to fatigue or failure. Hence, it should be
ensured that auto-shut off valves, couplings, hoses, pumps, sub-sea
pipeline etc are periodically inspected for their integrity and to
ensure their proper functioning in an emergency. Accurate records
of all inspections, unusual findings, actions taken etc must be
scrupulously maintained as a part of the overall record system.
Operation should commence only after optimum conditions are
reset.
Provision for an effective and reliable communication between
the production platform, landfall and storage tanks should be made
to avoid ambiguities and time delays during pumping of POL.
Firefighting equipment should be readily available at the land
fall point
to mitigate any exigencies due to a leak in the intertidal areas
of the pipeline route during hook-up and welding jobs.
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xxi
LIST OF TABLES
2.1.1 Composition of cargo: sour gas (gas + condensate). 4.3.1
Water quality of Umbhrat-Hazira coast during October 2014. 4.3.2
Water quality of Umbhrat-Hazira coast during January 2014. 4.3.3
Water quality of Umbhrat-Hazira coast during May 2014. 4.4.1
Sediment quality of Umbhrat-Hazira coast during October 2014. 4.4.2
Sediment quality of Umbhrat-Hazira coast during January 2014. 4.4.3
Sediment quality of Umbhrat-Hazira coast during May 2014. 4.5.1
Taluka-wise status of mangroves (ha) of Surat District. 4.5.2
Microbial counts (CFU/mL) in water of Umbhrat-Hazira coast during
2014. 4.5.3 Microbial counts (CFU/g) in sediment of Umbhrat-Hazira
coast during
2014. 4.5.4 Range and average (parenthesis) of phytopigments off
Umbhrat-Hazira
coast during 2014. 4.5.5 Range and average (parenthesis) of
phytoplankton off Umbhrat-Hazira
coast during 2014. 4.5.6 Percentage composition (%) of
phytoplankton population off Umbhrat-
Hazira coast during 2014. 4.5.7 Range and average (parenthesis)
of zooplankton off Umbhrat-Hazira coast
during 2014. 4.5.8 Percentage composition (%) of zooplankton
population off Umbhrat-Hazira
coast during 2014. 4.5.9 Range and average (parenthesis) of
intertidal macrobenthos of Umbhrat
during October 2014. 4.5.10 Composition (%) of intertidal
macrobenthos of Umbhrat during October
2014. 4.5.11 Range and average (parenthesis) of subtidal
macrobenthos off Umbhrat-
Hazira coast during 2014. 4.5.12 Composition (%) of subtidal
macrobenthos off Umbhrat-Hazira coast
during 2014. 4.5.13 Marine fish landings of Gujarat State, Surat
and Navsari Districts during
1977-2013. 4.5.14 Species-wise composition of marine fish
landings of Gujarat State, Surat
and Navsari Districts during 2011-2012 4.5.15 Centre-wise marine
fish production (t) for Surat District during 2012-2013. 4.5.16
Monthly calendar of marine fish landings (t) at Hazira in Surat
District
during 2012-2013.
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xxii
4.5.17 Monthly calendar of marine fish landings (t) at Bhimpur
in Surat District 2012-2013.
4.5.18 Fishing villages, fishermen, boats and fishing gears of
Surat District (2007).
4.5.19 Fish catch in coastal water off Umbhrat-Hazira coast
during 2013-2014. 4.5.20 Check list of birds around Umbhrat-Hazira
coast.
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xxiii
LIST OF FIGURES
1.1.1 Map of project area showing Umbhrat, Mindhola and Tapi
estuary. 1.1.2 Schematic diagram of re-routing and redundant
pipeline at Umbhrat. 1.4.1 Subtidal and intertidal sampling
locations along Umbhrat-Hazira coast
during 2014. 3.0.1 Map showing marine environment of Umbhrat,
Mindhola and Tapi estuaries. 4.2.1 Current speed (a) and components
(b) observed at station 2 (Umbhrat-
Hazira coast) from 10 to 14 April 2010. 4.2.2 Drogue trajectory
conducted at station 2 (Umbhrat-Hazira coast) (Fl-Eb-Fl)
on 8 November 2009. 4.2.3 Drogue trajectory conducted at station
2 (Umbhrat-Hazira coast) (Eb-Fl) on
13 April 2010. 4.2.4 Drogue trajectory conducted at station 2
(Umbhrat-Hazira coast) (Fl-Eb) on
8 April 2010. 4.3.1 Water quality at station 1 (Umbhrat-Hazira
coast) on 9 October 2014. 4.5.1 Temporal variation in phytopigments
at station 2 (Umbhrat-Hazira coast) on
2 October 2014. 4.5.2 Temporal variation in zooplankton at
station 2 (Umbhrat-Hazira coast) on 2
October 2014.
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xxiv
LIST OF PLATES
Plate 1 Pipe protection measures at Umbhrat Geobags (Figure a
& b).
Plate 2 Pipe protection measures at Umbhrat - Geotextile mats
and tubes.
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xxv
COMMON ABBREVIATIONS Av - Average B - Bottom BOD - Biochemical
Oxygen Demand (mg/l) Corg
- Organic carbon (%)
DO - Dissolved Oxygen (mg/l) Eb - Ebb tide Fl - Flood tide Max -
Maximum Min - Minimum ND - Not Detected NH4+
-N - Ammonium nitrogen (mol/l)
NO3-
-N - Nitrate nitrogen (mol/l)
NO2-
-N - Nitrite nitrogen (mol/l)
PHc - Petroleum Hydrocarbons (g/l) Phenols - Total phenols (g/l)
PO43-
-P - Reactive phosphate phosphorus (mol/l)
S - Surface SS - Suspended Solids (mg/l)
NA - Not Analysed
-
1 INTRODUCTION 1.1 Background
Oil & Natural Gas Commissions Hazira plant (ONGC) receives
sour gas and condensate from Mumbai Offshore through 36 and 42
South Bassein Hazira Trunkline (SBHT) pipelines (Figure 1.1.1) for
processing and there after supplies sweet gas to HBJ line through
Gas Authority of India (GAIL) spanning about 3000kmacrossthe
northern states for downstream industries including CNG supply in
Delhi and to local consumer industries like KRIBHCO, RIL, ESSAR,
GGCL and GSPL.ONGC also produces and supplies products such as LPG,
naphtha, HSD, and ATF and kerosene to consumers like IOCL, BPCL,
HPCL, RIL etc. The 42 dia SBHT p i p e l i n e s e g m e n t w h i
c h t r a n s p o r t s g a s got exposed due to high tides and
cyclonic storm occurred to an extent of 769 m in 2011-12 at
Umbhrat, Navsari District. ONGC carried out protection measures in
2012 with geotextile tube, Geobags and Gabion boxes on the basis of
suggestions given by Engineers India Limited (EIL). The temporary
protection measures taken have resulted in change of the erosion
pattern in the vicinity of the pipeline and the geobags used for
this purpose have moved into lower intertidal areas (Plates 1 to
2). Again in June 2014, the 42 dia pipeline got exposed to an
extent of about 500 m due to high tides and cyclonic storms.
EIL has proposed relocation of the pipeline segment as shown as
in Figure 1.1.2 and ONGC has engaged L&T for laying of the
pipeline and to carry out execution of re-routing scheme of 42 SBHT
line at Umbhrat within the CRZ. Now Department of Environment &
Forests (DOEF), Govt. Of Gujarat, Gandhinagar has instructed ONGC
to conduct EIA study includingmarineaspectsand impactof theproject
activityon themarine environment and mitigation measures, details
of mangrove patches a n d impact of proposed activity on mangroves.
ONGC therefore, contracted Council of Scientific & Industrial
Research-National Institute of Oceanography (CSIR-NIO) to conduct
the EIA study with the following objectives:
1.2 Objectives
a) To establish the prevailing marine environmental status in
respect of water quality, sediment quality and biological
characteristics including mangroves for marine environment off
Umbhrat.
b) To assess probable impact of proposed pipeline laying on the
marine environment and ecology.
c) To suggest Marine Environmental Management Plan (MEMP)
including mitigation measures.
1.3 Scope of study CSIR-NIO has conducted several site-specific
investigations in and around the Hazira-Umbhrat coast over the
period (1984-2013) thereby generating an
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2
extensive database for the area. Considering the availability of
these data, the following studies are proposed:
1.3.1 Evolve prevailing marine environment off Umbhrat as
follows: a) Physical processes: Physical processes like tides,
currents and circulation also play a crucial role by controlling
the extent to which many of the perturbations can influence the
ecology. Hence, information available on currents, tides will be
assessed to evaluate current and tidal fluctuations of the water
level.
b) Water quality: Water quality at 3locations will be assessed
based on salinity, temperature, pH, Suspended Solids (SS),
Dissolved Oxygen (DO), Biochemical Oxygen Demand (BOD), nitrate,
nitrite, ammonia, phosphate, Petroleum Hydrocarbons (PHc) and
phenols. c) Sediment quality: Sediment from the intertidal and
subtidal regions will be studied for texture, organic carbon,
phosphorus, aluminium, chromium manganese, iron, cobalt, nickel,
copper, zinc, mercury and petroleum hydrocarbons.
d) Biological characteristics: The status of flora and fauna in
the subtidal and the intertidal zones will be established based on
microbiology, phytoplankton pigments, population and generic
diversity; zooplankton biomass, population and group diversity;
macrobenthic biomass, population and group diversity; fisheries
status; mangroves and their species diversity etc.
1.3.2 Assessment The impact assessment for the proposed
projecton the marine ecology will be carried out based on the
database generated and available with CSIR-NIO as well as other
information made available by ONGC. Suitable mitigation measures
will be suggested for adverse impacts, if any, in the form of
Marine Environmental Management Plan (MEMP).
1.4 Approach strategy
Severity of negative impacts of developments in the coastal zone
on associated marine ecology vary widely depending on many factors
such as the extent, period and type of disturbance, anthropogenic
perturbations, capacity of the receiving water to assimilate
contaminants and extent of ecological sensitivity. Hence, the
primary requirements for assessing adverse impacts due tore-routing
the pipeline at Umbhrat are the baseline information for the
coastal area off Hazira-Umbhrat in general and intensive site
specific data for the nearshore waters off Umbhrat, in
particular.
CSIR-NIO has been conducting general and site-specific studies
for the
coastal waters off Hazira-Umbhrat region since 1983 with more
frequent investigations in recent years due to several proposed and
ongoing developments bordering the region. These data are adequate
to describe general environmental
-
3
setting of the study areas and field data acquisition in the
coastal waters off Hazira-Umbhrat and Umbhrat in the present
investigationsis considered sufficient to meet the objectives.
CSIR-NIO monitors the coastal waters of Hazira at a few
locations in the
vicinity of the proposed project area once every year since 1990
that has generated long term database for water quality, sediment
quality and biological characteristics. This available information
is assessed to plan field data acquisition for the present study.
Accordingly, 6subtidal stations covering an area of 9.0 km2 of the
openshore segment of Umbhrat-Hazira were sampled. Intertidal area
has been also considered for the study and samples at 2 intertidal
transects were collected. The sampling locations are illustrated in
Figure 1.4.1.
This report is based on the field studies conducted off
Hazira-
Umbhrat and Umbhrat coasts during October 2014 and the available
immediate data of Jan 2014 and May 2014 for selected areas. Apart
from this the available data of 1983-2014 with CSIR-NIO also used
for long term comparison.
MEIA for the proposed project was conducted on the basis of
information
provided by ONGC and that available in the literature.
Mitigation measures including EMP were suggested for the adverse
impacts, if any due to the proposed project.
1.5 Studies undertaken
Details of studies undertaken during October 2014 are as
follows:
1.5.1 Sampling locations Investigations were conducted at 3
subtidal stations (1-3) and 3 subtidal
stations (4-6) in the Umbhrat coast i.e. the project site and
Hazira-Umbhrat coast respectively. Sampling was done at 2 transects
(I-II) at Umbhrat coast. The locations are illustrated in Figure
1.4.1 and their positions are given below.
Coast Area Station/
transect Position
Umbhrat Subtidal
1 21o0048.3N, 72o4349.4E 2 21o0033.7N, 72o4346.5E 3 21o0119.0N,
72o4332.6E
Intertidal I 21o0102.5N, 72o4342.3E II 21o0110.2N,
72o4340.1E
Hazira-Umbhrat Subtidal 4 21o037.7N, 72o4055.56E 5 21o029.90N,
72o4040.55E 6 21o0113.7N, 72o4122.38E
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1.5.2 Sampling frequency Sampling was done at all stations in
duplicate for water quality and
biological characteristics. Temporal measurements of 1 h
frequencies over a tidal cycle were conducted at station 1.
Intertidal stations were sampled once along each transect in the
area
between the Low Tide Line (LTL) and the High Tide Line
(HTL).
1.5.3 Physical processes a) Tide: Available information on tides
for Gulf of Khambat was assessed. b) Currents: Available
information on Currents for Gulf of Khambhat was assessed c)
Circulation Available information on circulation for Gulf of
Kambhat was assessed
1.5.4 Water quality a) Sampling procedure: Surface water samples
were collected using a clean polyethylene bucket while an
adequately weighed Niskin sampler with a closing mechanism at a
desired depth was used for obtaining subsurface water samples.
Sampling at the surface and the bottom (1m above the bed) was done
when the station depth exceeded 3 m. For shallow regions, only
surface samples were collected. A glass bottle sampler (2.5 l) was
used for obtaining samples at depth of 1 m below the surface, for
the estimation of PHc. For shallow areas only surface water was
collected.
b) Methods of analyses: Majority of the water quality parameters
were analysed within 24 h of collection in the field laboratory.
Colorimetric measurements were made on a Shimadzu (Model 1201)
spectrophotometer. Shimadzu (Model 5301) fluorescence
spectrophotometer was used for estimating PHc. The analytical
methods for the measurements were as follows:
i) pH: pH was measured on a Cyber Scan (pH 510) pH Meter. The
instrument was calibrated with standard buffers just before use.
ii) SS: A known volume of water was filtered through a preweighed
0.45 m membrane filter paper (Millipore), dried and weighed again.
iii) Salinity: GUILDLINES Autosal, Model 8400b-50 Hz, Sr. No.
71463, laboratory salinometer used for measuring salinity with the
accuracy better than 0.002 equivalent practical salinity units
(psu/ppm) was used for estimating salinity. iv) DO and BOD: DO was
determined by Winkler method. For the determination of BOD, direct
unseeded method was employed. The sample was filled in a BOD bottle
in the field and incubated in the laboratory for 3 d after which DO
was again determined.
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5
v) Phosphate: Acidified molybdate reagent was added to the
sample to yield a phosphomolybdate complex which was then reduced
with ascorbic acid to a highly coloured blue compound which was
measured at 882 nm.
vi) Nitrite: Nitrite in the sample was allowed to react with
sulphanilamide in acid solution. The resulting diazo compound was
reacted with N-1-Naphthyl-ethylenediamine dihydrochloride to form a
highly coloured azo-dye. The light absorbance was measured at 543
nm. vii) Nitrate: Nitrate was determined as nitrite as above after
its reduction by passing the sample through a column packed with
amalgamated cadmium. viii) Ammonia: Ammonium compounds (NH3 + NH4+)
in water were reacted with phenol in presence of hypochlorite to
give a blue colour of indophenol. The absorbance was measured at
630 nm. ix) PHc: Water sample (1 l) was extracted with hexane and
the organic layer was separated, dried over anhydrous sodium
sulphate and reduced to 10 ml at 30o C under low pressure.
Fluorescence of the extract was measured at 360 nm (excitation at
310 nm) with Saudi Arabian crude residue as a standard. The residue
was obtained by evaporating lighter fractions of the crude oil at
100oC for 30 min. x) Phenols: Phenols in water (500 ml) were
converted to an orange coloured antipyrine complex by adding
4-aminoantipyrine. The complex was extracted in chloroform (25 ml)
and the absorbance was measured at 460 nm using phenol as a
standard.
1.5.5 Sediment quality
a) Sampling procedure: Subtidal surfacial bed sediment from all
locations was obtained by a van Veen grab of 0.04 m2
ii) Metals: Sediment was brought into solution by treatment with
conc HF-HClO4-HNO3-HCl and the metals were estimated on a Perkin
Elmer (ICP-OES Model
area. The sample after retrieval was transferred to a
polyethylene bag and preserved for further analysis. Intertidal
sediment was sampled using a hand shovel. b) Methods of analyses:
(i) Texture: Dried sediment (25 g) mixed with deionised water and
10 ml sodium hexameta phosphate (6.2 g/l) was sieved through 63 m
sieve to retain sand and the passed material was dispersed in
deionised water (1 l). The fraction (20 ml) picked up at 20 and 10
cm depth immediately and after 2 h 30 min respectively were
considered as silt and clay after drying and weighing.
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6
no OPTIMA-7300DV). Mercury was estimated by flameless AAS
technique after digesting the sediment with aquaregia. iii) Corg:
Percentage of Corg in the dry sediment was determined by oxidising
organic matter in the sample by chromic acid and estimating excess
chromic acid by titrating against ferrous ammonium sulphate with
ferroin as an indicator. iv) Phosphorus:Dried and powdered sediment
(20.5 g) was digested using HF, HC104, HNO3 and HCl. It was used
for estimating total phosphorus as described under Section
1.5.3(b)(v). v) PHc: Sediment after refluxing with KOH-methanol
mixture was extracted with hexane. The residue was subjected to
clean-up procedure by silica gel column chromatography. PHc content
was then estimated by measuring the fluorescence as described under
Section 1.5.3 (b) (ix).
1.5.6 Flora and fauna
a) Sampling procedure:For microbial analysis, surface water was
collected directly in a sterilised glass bottle. Sediment sample
was obtained using van Veen grab and transferred directly into
sterilised polyethylene bag.
Polyethylene bucket and Niskin sampler respectively, were used
for sampling surface and bottom waters for the estimation of
phytoplankton pigments and population. Samples for phytoplankton
cell count were fixed in Lugol's solution.
Zooplankton samples were collected by oblique hauls using a
Heron
Tranter net (mesh size 0.3 mm, mouth area 0.25 m2) with an
attached calibrated TSK flow meter. All collections were of 6 min
duration. Samples were preserved in 5% buffered formaldehyde.
Sediment samples for subtidal macrobenthos were collected using
a van-
Veen grab of 0.04 m2
i) Microbes: Samples were analysed by plating and membrane
(Millipore 0.22 m) filtration techniques for Total Viable Counts
(TVC), Total Coliforms (TC), Escherichia Coli like Organisms
(ECLO), Faecal Coliforms like Organisms (FCLO), Shigella like
Organisms (SHLO), Salmonella like Organisms (SLO), Proteus,
Klebsiella like Organisms (PKLO), Vibrio like Organisms (VLO),
Vibrio
area. Intertidal collections between the HTL and the LTL were
done with a hand shovel for muddy and silty substratum while
quadrants of different sizes were employed for sampling rocky,
sandy and areas of compact sediment. Samples were preserved in 5%
buffered formaldehyde - Rose Bengal.
Experimental bottom trawling and gill netting were undertaken
wherever
feasible using a high opening bottom net of 20.7 m (637 meshes
of 50 mm) length and locally procured gill net.
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7
Parahaemolyticus like Organisms (VPLO), Vibrio Cholerae like
Organisms (VCLO), Pseudomonas aerugenosa like Organisms (PALO) and
Streptococcus faecalis like Organisms (SFLO). Colonies of TC, ECLO,
VLO, VPLO and VCLO were counted separately. The media employed for
growth of colonies were as follows: Nutrient agar for TVC, Mac
Conkey agar for TC and ECLO, M-Fc agar for faecal coliforms,
Xylose-lysine deoxycholate agar for SHLO, SLO and PKLO,
thiosulphate citrate bile salts medium for VLO, VPLO and VCLO,
cetrimide agar for PALO and M.Enterococcus agar for SFLO. ii)
Phytoplankton pigments:A known volume of water was filtered through
a 0.45 m membrane filter paper (Millipore) and SS retained on the
filter paper were extracted in 90% acetone. For the estimation of
chlorophyll a and phaeophytin the extinction of the acetone extract
was measured at 665 and 750 nm before and after treatment with
dilute 0.1 N-HCl. The concentrations of phytopigments were
expressed as mg/m3. iii) Phytoplankton population:The cells in the
sample preserved with Lugols solution were allowed to settle and
transferred into a Sedgwick-Rafter slide. Enumeration and
identification of phytoplankton were done under a microscope. The
cell counts were expressed as number/litre. iv) Algae, seaweeds,
mangrove and sand-dune ecosystem: Algal, seaweed and mangrove flora
were assessed from upper to lower intertidal region along
predecided transect. v) Zooplankton:Volume (biomass) was obtained
by displacement method. A portion of the sample (25-50%) was
analysed under a microscope for faunal composition and population
count. Biomass and population were expressed as ml/100 m3 and
nox103/100m3 respectively. vi) Fish eggs, fish larvae and decapod
larvae:These groups were sorted out from zooplankton samples and
counted. Frequency of occurrence and their percentage composition
were then determined. vii) Benthos:The sediment was sieved through
a 0.5 mm mesh sieve and animals retained were preserved in 5%
buffered formaldehyde-Rose Bengal. Total population was estimated
as number of animals in 1 m2 area and biomass on wet weight basis
(g/m2). viii) Fishery:After trawling fishes were sorted out into
different groups, weighed,and catch rate (kg/h) and composition
were determined. A part of the catch was preserved in 5%
formaldehyde for identification at species level. Fish landing data
were obtained from the Department of Fisheries, Government of
Gujarat, for assessing the fishery status of the region.
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2 PROJECT DESCRIPTION 2.1 Present activity 2.1.1 Pipeline
ONGC Hazira plant receives sour gas and condensate from Mumbai
Offshorethrough36and 42 SBHT (South Bassein Hazira Trunkline)
pipelines for processing (Figures 1.1.1 and 1.1.2) and thereafter
supplies sweet gas to HBJ line through GAIL spanning about 3000km
across all northern states for downstream industries including CNG
supply in Delhi and to local consumer industries like KRIBHCO, RIL,
ESSAR, GGCL & GSPL. Hazira plant produces and supplies products
of LPG, Naphtha, HSD, and ATF and Kerosene to consumers like IOCL,
BPCL, HPCL, RIL etc. Thus 36 and 42 dia SBHT lines from Mumbai to
Hazira is lifeline of India and it is essential to secure safety,
stability and security of the pipelines.
The 42 dia SBHT line t r a ns po r t i n g th e ga s got exposed
to the extent
of 769 m (approx.) in 2011-12 at Umbhrat, District Navsari.
Based on design and engineering by their consultant (EIL), ONGC
carried out protection measures in 2012 with geotextile tubes,
geobags and gabion boxes which has been approved by Gujarat
Maritime Board (GMB).
2.1.2 Purpose for re-routing the pipeline and EIA
Meanwhile, due to high tides and cyclonic storm on 15-16 June
2014, about another 500 m of the 42 gas pipeline has got exposed,
its previous protection measures got damaged and the line got
shifted by 25 to 30 m towards seaside. ONGC carried out protection
measures in 2012 with geotextile tube, Geobags and Gabion boxes on
the basis of suggestions given by Engineers India Limited (EIL).
The temporary protection measures taken have resulted in change of
the erosion pattern in the vicinity of the pipeline and the geobags
used for this purpose have moved into lower intertidal areas
(Plates 1 to 2). Again in June 2014, the 42 dia pipeline got
exposed to an extent of about 500 m due to high tides and cyclonic
storms.In view of this, ONGC has engaged Engineers India Limited
(EIL) as consultant for permanent remedial measures of the
pipeline. EIL has submitted scheme of re-routing the line at
Umbhrat as a permanent solution.
ONGC will implement re-routing the pipeline in about 1.9 km
stretch which includes the exposed position on fast track for
permanent solution to the problem of its exposure at Umbhrat beach
and therefore has engaged L and T to carry out execution of
re-routing scheme of 42 SBHT line at Umbhrat. ONGC has submitted
application for permission to DOEF, Gandhinagar. DOEF has asked to
submit EIA study report including marine aspects and impact of the
project activity on marine environment and mitigation measures,
details of mangrove patches and impact of the proposed activity on
mangrove and CRZ Map in 1.4000 scale alongwith super imposition of
proposed activity and technical recommendations/suggestions.
Accordingly ONGC requested CSIR-NIO to carry
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out EIA studies along with details of mangroves and impact of
pipeline laying on mangrove vegetation.
2.1.3 Cargo:sour gas Composition of the sour gas gas+condensate
which is transported through
the pipeline (Table 2.1.1) is as follows: The gas with molecular
weight of 21.42 and has specific gravity of 0.74 to
0.7418 under the pressure condition on the 60 - 80 kg/cm2. It
mainly contains(%,mole) methane(C1) with 79%, other gases (C2 + C3)
are 11.8. The higher hydrocarbon derivatives in the gas are lower
of 2.6 while C6+ fraction is1%.
The condensate contains (% volume) (4:8.6, C5:12.9, C6:21.6,
C7:25.2,
C8:13.4, C9:80, C10:4.5, C11:2.1, C12:31 and polynaphthalene:
0.7 hydrocarbons.
The gas (density 676 kg/m3) flows at a normal rate of 20 to 22
MMSCMD
under a operating pressure and temperature of 60 to 80 kg/cm.2g
and 20 to 40oC respectively through the 42 O.D. pipeline. The
condensate flows at the rate of 5000 to 8000 m3
The 400 m long pipeline portion prior to the entry of HDD having
concrete weight coating shall be installed by open cut and
trenching method. Another 400 m long section without concrete
coating after HDD exit shall also be installed by the same method.
The 1100 m long portion (HDD) between these sections shall be
installed using Horizontal Directional Drilling (HDD)
technique.
/d.
2.2 Proposed activity 2.2.1 Installation of new pipeline
Now ONGC has proposed to re-route the dislocated/exposed
pipeline segment. Accordingly EIL has prepared the engineering plan
for re-routing of the pipeline as shown in Figure 1.1.2.
The proposed activity includes installation and laying of the 42
dia, 1.9 km
long pipeline between hook-up points HK1 and HK2 (Figure 1.1.2).
It also includes installation of new SV station having a 42, 900#
ball valve; gas actuated; buried; remote and manually operated
sectionalising valve (SV). Installation of a new insulating joint
of 42 dia, 900 # (buried) will be near HK1. Hot tapping with
stappling (double line stop with temporary by-pass) shall be
carried out at HK1 and another at HK2 in the existing pipeline. The
gas in the isolated section between HK1 and HK2 shall be flared off
safely through the existing SV station.
HK1 shall be located minimum 100 m upstream of the existing land
fall
point (LF2, (Figure 1.1.2). The re-routed pipeline section of
400 m length from HK1 to HDD entry point shall be gradually lowered
to HDD entry point matching the depth of 6 m at HDD entry
point.
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The re-routed pipeline segment shall be hooked up with the
existing pipeline by cold field bands at both the ends using manual
cutting and welding. Afterwards inertisation and nitrogen purging
of the existing isolated section shall be done.
The pipeline shall be 42 O. D. X 32.4 mm W.T., APISL Gr x 60
PSL2,
NACE, LSAW. The 1100 mm portion (HDD) shall be installed minimum
12 m below lowest natural ground level.
All the materials shall be confirming NACE quality as per
specifications.
Welding of the pipeline shall be carried out by semi
automatic/automatic/manual (including Auto UT) method as per
welding specification. Fresh WPS/PQR/EQT and other procedure
qualifications shall be carried out as per specification/ standard
API 1104/ASME B3.18. Production test also be carried out at
acceptable limit. All the welded joints including cutting edge
preparation, fit-up, bending, preheating etc. shall be tested by
radiography and other non-destructive tests (NDT) as specified.
All the waste materials like gabions, stones, boulders, geobags
and
geotubes etc lying on the seashore shall be disposed of
properly. Surplus excavated soil trench outside ROU and bentonite
shall be disposed suitably with the permission from the concerned
authorities. The back-filling of the excavated areas and
clean-up/restoration of the site shall be done on completion of the
work. EGP (Calliper Run) and cathodic protection (CP) shall be
carried out as per specifications laid.
The pipeline and the new SV stations shall be subjected to
hydrostatic test
pressure of 1.5 times the design pressure. It is then followed
by dewatering and swabbing operations to be carried out as per
specifications.
2.2.2 Submarine pipeline segment The existing pipeline segment
between hook-up point HK1 and landfall
point LF2 in the marine environment shall be re-routed in the
proposed project (Figure 1.1.2) of the total length of 800 m of the
segment, the 500 m long portion shall be in the subtidal area while
the 300 long portion shall be in the intertidal area (Figure
1.1.2).
The 42 O.D. 800 m long segment which shall have concrete
weight
coating for 400 m length shall be installed by open cut and
trenching method in a 20 m wide corridor. The sediment side cast
during trenching shall be used for back-filling. The excess
sediment is expected to be dispersed by the tidal action. The
contour of the intertidal area shall be restored after the
construction activity is completed.
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2.2.3 De-commissioning of redundant pipeline De-commissioning of
the redundant pipeline between hook-up points HK1
and HK2 and existing SV station valves shall be carried out. The
old pipeline shall be retrieved and cut at welding joints at every
12 m. All the materials including the valves shall be transported
to the ONGC storage yards. The dismantled area in the existing SV
station including the valve pit shall be restored to the levels of
the adjoining areas.
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3 STUDY AREAS The coastal waters off Umbhrat located 10 km
southward of Hazira forms
the project area (Figure 3.0.1). The Hazira-Danti region is the
confluence of Tapi and Mindhola estuaries with area (Figure 1.1.1).
Hence Tapi Estuary, Mindhola Estuary and Umbhrat coast are
described below.
3.1 Tapi Estuary Tapi, a major perennial river of the west coast
of India, is an important
source of freshwater to the region. The 720 km long river
(Figure 3.0.1) originates near Multai in the Betoul District of
Madhya Pradesh and commands a catchment area of 65,145 km2 of which
around 4000 km2 is in Gujarat. During seaward course, the river
meanders through the hilly terrain of the Western Ghats before
entering the coastal alluvial plains of Gujarat to meet the Arabian
Sea near Hazira. The shallow and wide lower segment of the river
exhibits characteristics of a typical estuary with strong currents
associated with significantly high tidal influence upto 25 km
upstream. Further inland however, the seawater excursion is
restricted due to creation of a causeway at Rander which also
hinders the freshwater outflow.
The estuary and nearshore areas of Hazira exhibit a typical
character of
South Gujarat coast with (a) vast intertidal regions composed of
poorly sorted sediment made up sand silt and clay with isolated
rocky outcrops, (b) supralittoral region either barren or dominated
by salt tolerant plant species of Prosopis, Acacia and Zyphus, (c)
gently sloping continental shelf with uneven seafloor often strewn
with sand bars, and (d) high tidal influence due to the proximity
to the Gulf of Khambhat.
The riverine discharge to the sea is controlled by the Ukai and
Kakrapar
dams constructed on the river at 141 and 115 km upstream
respectively. Mean runoff of Tapi was 1.7982 x 1010 m3/y in 1975.
After the construction of dams, it has reduced to an average of
7.301 x 109 m3/y during 1982-91. In 1995 a weir-cum-causeway was
constructed across the river at Rander that prevented seawater
incursion further inland. In recent years the river discharge of
8000 m3/s during monsoon decreases to 10 to 45 m3
/s during November-May leading to stagnation in the riverine and
inner estuarine segments, during the dry season.
Due to proximity of the Gulf, the region experiences
significantly high tidal
influence with mean spring and neap tidal ranges of 5.7 and 4.3
m respectively at Hazira. The tidal influence however decreases
with distance into the estuary with spring and neap tidal ranges of
2.3 and 0.4 m respectively at Surat. During the period of
freshwater dominance the flood duration of 6 h in the openshore
reduces to 4 to 5 h in the mouth segment of the estuary and
decreases to barely 2 h at Surat with corresponding increase in the
ebb period. Decrease in flood period though to a lesser extent
occurs in the landward direction during the dry season also.
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The Tapi River is subjected to sporadic floods associated with
heavy rainfall in the catchment area during July-September. When
the flood coincides with spring tide, the water level rises
substantially inundating vast areas. A level of 10 m with respect
to CD has been recorded at Magdalla during one such flood in the
past. The construction of two dams on the river has however reduced
the flood fury to a great extent.
Currents are largely tide-induced during dry season and are
considerably
influenced by the riverine discharge during monsoon. At times
when the runoff is heavy, the flow is unidirectional in the estuary
throughout, irrespective of the tide. During dry season, the
maximum flood and ebb speeds often exceed 1 m/s with the tidal
excursion of 7 to 20 km within the estuary and 11 to 13 km along
the open coast. The direction of flow is predominantly decided by
the channel geometry that reverses with the change in the tidal
phase. The u component of the current is dominant throughout a
tidal cycle and net negative u component favours seaward transport.
The v component is considerably weak as expected for estuaries
under high tidal dominance.
The lower segment of theTapi Estuary is well flushed with
flushing time of
less than 2 tide cycles calculated based on the tidal prism
method. Flushing time of 3 tide cycles during spring for both
freshwater flow conditions reveals the dominance of tidal influence
over freshwater flow during spring tide. However, during neap when
the seawater incursion in the estuary greatly reduces, the flushing
time increases to 7 tide cycles when the river discharge decreases
to 10 m3/s.
The water quality of the estuary has deteriorated after the
industrialization
time though not severely. Considerable depletion in DO with
values decreasing to
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14
3.2 Mindhola Estuary Mindhola River which originates in the
Western Ghats follows a westward
but meandering course before meeting the Arabian Sea at Danti
adjacent to the mouth of the Tapi Estuary (Figure 1.1.1). The river
though narrow, broadens into a wide estuary below Nanod, which is
about 25 km inland from the sea. The estuarine region has extensive
sandbanks and mudflats that get submerged during flood tide. The
network of tributaries that get periodically filled with tidal
water harbour a variety of edible mudskippers, crabs, mullets and
prawns. Intertidal region, which is generally muddy, supports a
rich growth of Typha and other plants except in the mouth region
where no vegetation except halophytes occur.
Industries located in Palsana and Choryasi talukas discharge
their treated
wastewater through Kadodara/Kakra/Bhedwad, Gabheni/Unn and
Hanuman/Lajpore Khadis in the Mindhola estuary. Besides, majority
of sewage from in and around Surat City is also discharged in the
estuary through some natural creeks. Baleshwar/Gangadhar/Itarva
Kankarakhadis also carry effluents from nearby areas to the estuary
apart from direct release of effluents from industries of Maroli
Udyognagar in Navsari District.
A variety of industries release their effluents in Bhedwad and
Kakra
Creeks; the tributaries of the Udhna Khadi, which in turn
transports this load to the Mindhola estuary near Budiya roughly 15
km from the estuary mouth.
The Mindhola Estuary is under considerable tidal influence with
the spring range at the mouth of 5.5 m decreasing to 1.9 m at 32 km
inland. The respective neap ranges are 2.5 and 0.5 m. The flood
period decreases from 4 to 5 h at the mouth to 2 to 3 h at 32 km
upstream with the ebb extended over a period of 7 to 10 h. High
tidal influence generates strong currents with maximum speeds of
0.8 to1.9 m/s in the mouth segment. The excursion length during
spring decreases from 17 km at the mouth to 12 km at Budiya where
Udhna Khadi meets Mindhola. During neap the respective distances
are 11 and 8 km. The excursion lengths of 3 to 5 km further inland
indicate the possibility of accumulation of pollutants in the inner
estuary during periods of weak riverine flows.
Maximum intrusion of salinity occurs upto 25 km inland during
April (lean period of freshwater flow) and decreases to 15 km
during September (when the riverine flow is substantial). The water
is generally well mixed vertically though occasional stratification
is recorded in the mouth area when the freshwater flow is
significant. The flushing time of the estuary is estimated to vary
between 4 and 8 tide cycles in April, based on salinity
distribution. Based on this, the effluent load retained in the
estuary has been estimated to be 2.2 to7.3 times the load
introduced per tide cycle.
The SS entering the estuary through river discharge during dry
season is
below 160 mg/l. There is a marked increase in the estuarine zone
and the
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15
concentrations are widely variable. The anthropogenic organic
load entering the estuary influences the DO balance in the system.
There is a general tendency for DO to increase with flood tide and
decrease as the ebb progresses even during September when the
riverine flow is good. The values around 2 ml/l are commonly
recorded in the mid-estuarine zone during April. BOD increases
markedly during low tide and values of 4-8 mg/l are common in the
mid-estuary. No marked increase in the levels of phosphate and
nitrate as compared to other estuaries of South Gujarat is evident
though the concentrations of nitrite and ammonium are often
high