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Bathymetric
survey
The
bathymetric survey has been carried out in an area of
approximately 1km wide and 150km long along the proposed channel
alignment using CRV Sagar Paschimi.
In total 8 lines were planned at a spacing of 150m i.e. 3
lines to the west of alignment and 4 lines to the east of
alignment. NIOT used ODOM make Echotrac MK-II survey grade
single beam dual frequency echo sounder for collecting
bathymetry data. The positioning system used was Leica 9250 DGPS.
The differential corrections were provided by Fugro seastar
(OMNI star).
The
output of the echo sounder was interfaced to a data acquisition
cum navigation software called HYPACK installed in a PC. Hypack
is capable of storing the depth data along with the position and
time output from DGPS system. The detailed specification of the
systems used are given in Annexure.
Of
the total planned length of 1469km, 1350km have been completed
in this phase, leaving segment G-A that is yet to be completed.
The completed survey tracks are shown in figure 1.
Fig
1 Map showing the completed survey tracks
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Vertical
Reference
All
depths are reduced to Chart Datum which is considered as 0.4m
below MSL for the present work. The tidal corrections were
derived from Numerical Modelling for the survey area. The survey
area was divided into 5 segments and the time series were
extracted for the survey period and used for reducing the
bathymetry to Chart Datum.
Tidal
Propagation Model
Since
the survey area extends to a length of 150km almost in
North-South direction encompassing the palk staright and gulf of
mannar, it was anticipated that the tidal characteristics would
not be similar through out the survey area. Hence, prior to the
commencement of survey operations, tide gauges were installed at
three locations along the coast at Nagapatnam, Sethubavachatram
and Pamban Pass. The observations were made for a period of 1
month from 4th October 2004. The observed time series
were compared and found that the range difference between these
places was less than 30cm but time lag was varying upto 5 hours.
To arrive at site
specific tidal corrections, a numerical model was setup using
Danish Hydraulic Institute’s MIKE 21 software. The bathymetry
for the model was extracted from the C-Map navigation chart. The
model area included the eastern coast of India from Nagapatnam
to Pamban Pass and the western coast of Srilanka as shown in
figure 2. The boundary condition applied was the observed time
series of tidal variation Nagapatnam. The calibration of the
model was done using the observed time series of tide at
Sethubavachatram. It was observed that the model result was in
agreement with the observed as shown in figure 3.
Fig
2 The model area
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Fig
3 Calibration of
time series at Sethubavachatram
From
this simulated results, the time series along the track was
extracted and compared. Based on the variation in tidal
characteristics, area was divided into 5 segments (Figure 4 and
5) and one representative time series was used for reducing the
bathymetry to chart datum in each segment.
Fig
4 Locations of extracted time series and the considered segments
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Fig 5 Variations of
extracted time series
Presentation
of bathymetry data
The
tide corrected xyz data was presented as a sounding chart. The
bathymetry of entire survey area has been presented in 6 charts
of scale 1:30000.
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