Introduction || Scope of Study || Methodology || Results

                    Recommendation for Dumping site for Dredge Spoil || Conclusion ||

                      Annexure - I  || Annexure - II (a) || Annexure - II (b)|| Figures

1.    INTRODUCTION

On behalf of the Ministry of Shipping, Government of India, the Tuticorin Port Trust (TPT) has taken up various studies and investigations on the implementation of Sethusamudram Ship Channel (SSC) project.

India has a long coastline of more than 6000 km and the maritime trade plays a vital role in the development of her economy. There are 12 major ports and over 150 intermediate and minor ports situated along the east coast and west coast of India.  But, there is no continuous navigable sea route within her territorial water around the Indian Peninsula between the Gulf of Mannar and the Bay of Bengal due to the geological formation of a shallow ridge Adam’s Bridge between India and Srilanka. Consequently, the entire coastal traffic from the east coast of the country to the west coast and vice versa has to go around Srilanka entailing additional distance of nearly 400 nautical miles and requiring additional navigational time of nearly 36 hours.  The proposed SSC project would reduce the navigational time from the east coast to the west coast and vice versa and would hopefully increase the volume of the coastal traffic.

The SSC has been presently proposed to facilitate movement of vessels of draught up to 10.0 m. Navigational Channel of 12.0 m depth w.r.t. CD (Chart Datum) and 300 m wide will be dredged for 20 km long near Adam’s bridge and 36 km long across Palk Strait (Fig.1.1).

Among the various investigations taken up, TPT has assigned the mathematical modelling studies related to SSC project to Indomer Coastal Hydraulics (P) Ltd., Chennai. The ship maneuvering and channel depth estimation studies were carried out by Alkyon Hydraulic Consultancy & Research bv, at Netherlands and the remaining hydrodynamic model studies were carried out by Indomer Coastal Hydraulics (P) Ltd., Chennai, India jointly with Alkyon Hydraulic Consultancy & Research, The Netherlands.

The scope of the study as per the TOR covers:

1.      The proposed channel alignment and dimensions have to be followed as shown in the NEERI’s EIA Report.

2.      The model boundaries are to be suitably selected to ensure that the entire flow regime is properly represented and the model should be calibrated using the available field data.

3.      The tidal currents velocity and direction along the alignment of the channel and anchorages are to be determined.

    iv.        The siltation pattern for the entire length of the channel, anchorages and wave breaking zones at Adams Bridge have to be studied and carry out the wave, flow and sedimentation studies Mathematical simulation / investigation to confirm the alignment, dredge depths, Channel dimensions quantum of dredging and work out technical parameters of the Channel.  The areas likely to experience excessive siltation also have to be studied.

5.     The quantum of annual maintenance dredging have to be determined for the entire length of the channel, anchorages.

    vi.        By conducting suitable mathematical studies, the impact of the said channel construction shall be evaluated considering the required parameters such as siltation in the channel, ship manoeuvring along the channel alignment, etc.

7.      The mathematical model study should account for influences of wind, waves, current, water depth, banks etc.

8.      The design vessels of bulk carriers of 65,000 DWT, 240 m (LOA), 33 m (B), 12.8 m (draught) and container vessels of 56,000 DWT, 290 m (LOA), 32.2 m (B), 12.8 m (draught) have to be considered for the mathematical model study.  The largest design vessel for 10.70 m (restricted draft) will be 237 m LOA, 33.5 MB. The size of the vessels etc. have to be suitably incorporated after consultation with TPT based on DPR.

9.      The various combinations of wind, wave and current field conditions have to be considered for the mathematical study and thereby examine the adequacy of the Channel dimensions, optimization of Channel dimensions and anchorage areas in the alignment of Sethusamudram Ship Channel Project as per nautical point of view considering relevant National and International standards.

      x.      Suggest if needed the required size of channel for the design vessels indicted in para (viii) above as per nautical point of view considering relevant National and International standards, work out the technical parameters of the Channel and under keep clearance for the Channel depth designs.

    xi.            Suggest if needed, the required protective measures, tug assistance navigational aid arrangement, placement of navigational aids i.e. buoys and other markings and effects of passing and manoeuvring ships on moored ships etc.

  xii.            The mathematical model study shall be conducted to determine the quantum of siltation / accretion / erosion, etc., likely to occur in the channel due to the prevailing oceanographical conditions in the respective region indicated.

xiii.            The study should establish the annual and seasonal wave climate throughout the length of navigational channel by transforming the available deep water wave data to take account of shallow water effects, using mathematical modelling techniques.

xiv.            The study should provide essential wave parameters for input to subsequent studies relating to ship manoeuvring in the channel, flow modelling and channel siltation for the entire length of the channel.

  xv.            The study should include both the short and long wave climate and provide information regarding significant wave height, significant wave period and wave directions for different seasons as well as for a complete year, including the extreme values of the parameters (with their return periods).

xvi.            The probability of exceedance of various significant wave heights and the probability of occurrence of different wave directions should also be evaluated.  Wave scatter diagrams and wave expedience curves should be prepared along with a wave spectrum.

This PART 1 report describes the results of various hydrodynamic model studies conducted in connection with the SSC Project. Part 2 covering the Figures for Part 1 report and Part 3 covering the ship maneuvering study are presented separately.

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2.    SCOPE OF THE STUDY

        i)            to carryout wave modelling study,

        ii)            to carryout the tide and current circulation modelling study,

        iii)           to carryout sedimentation modelling study,

        iv)           to carry out nautical study, and

        v)            to carryout channel depth estimation.

 
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3.    METHODOLOGY

Conventions and Definitions

Units:

All parameters and variables have units according to international SI conventions.

Coordinate system:

The coordinate system used for model grid generation and other horizontal positioning was UTM, based on the Everest 1830 (NHO) spheroid, with a central meridian of 81^O E, zone 44. All coordinates in this report are given in the above coordinate system unless specified otherwise.

Vertical reference level:

The depth information as used in the tidal flow models is relative to Mean Sea Level (MSL), which is a good representation of a horizontal plane. The use of Chart Datum (CD) as the reference would result in a non-horizontal reference plane since the difference between MSL and CD varies substantially over the area. For the project area around the Palk Bay, the MSL is defined 0.40 m above CD based on the Naval Hydrographic Charts published by Naval Hydrographic Office, Dehradun, India. All depths used in this report are in meters relative to MSL.  Depths below MSL are defined positive.

Bottom depths:

The bottom depths for the schematization of the wave and flow models were taken from the following source:

• The bathymetry as presented in various Naval Hydrographic Charts from the Naval Hydrographic Office, Dehradun, India; the data were digitized and transformed to UTM coordinate system and the depths were reduced to MSL. The following Naval Hydrographic Charts covering the study region were used: 262, 316, 317, 357, 358, 359.

• Bathymetric data for the deep ocean in Bay of Bengal model was derived from the GEneral Bathymetric Chart for Oceans (GEBCO) database published by British Oceanographic Data Centre, UK.  This database contains the heights and depths worldwide for one geographical minute resolution, based on satellite altimetry observations combined with shipboard echo sounding measurements.

In addition to the hydrographic charts, the data gathered from bathymetric surveys carried out by Naval Hydrographic office exclusively for the SSC project and National Institute of Ocean Technology (NIOT), Chennai were incorporated in depth schematization. All depth data were corrected to MSL and stored in one file for schematization of flow models.

Directions:

Flow:  Flow directions refer to the direction towards which the flow is going. Directions of the flow are always given clockwise w.r.t. North. The Unit is degrees, where 360 degrees cover the circle.

Wind: Wind directions refer to the direction from which the wind is coming. Directions of the wind are always given clockwise w. r. t. North. The Unit is degrees, where 360 degrees cover the circle.

Waves: In the Nautical convention wave directions refer to the direction from which the waves are coming and directions of the waves are given clockwise w.r.t. North. The convention used always will be specified explicitly. The Unit is degrees, where 360 degrees cover the circle.

Wind and Wave Roses: Wind and wave roses provide a quick way of summarizing the directional wind and wave conditions. The number in the center of the rose represents the percentage of the time calm condition occurs. The direction, to which the arm points out, represents the direction from which the winds or waves come from. The width and pattern of a section of the arm indicate the corresponding height or speed class. The length of each section of the arm represents the percentage of time that winds / waves come from that direction and occur in a given speed or height class.  The bar scale in the figures presenting roses indicates the percentage represented by unit length.

3.1.  Waves

Information on the normal wave conditions is required for further studies on sedimentation and erosion and to establish the limiting conditions for navigation through the channel. Information on the extreme wave conditions is required for the design of various structures, like groynes and bank protection.

3.1.1.  General

It is observed that the availability of wind and wave data inside the Palk Bay and northern part of Gulf of Mannar is limited. Hence the wave climate along the ship channel alignment in this region is estimated by transforming the available data from the adjacent open sea region.  As the bathymetry near Adam’s Bridge is very shallow (» 2m), it behaves as a barrier for the propagation of waves towards Adam’s Bridge from the Palk Bay and also from the Gulf of Mannar. Therefore the wave fields north and south of the Adam’s Bridge are treated separately.

Palk Bay and Palk Strait:

The wave field in the Palk Bay generally consists of locally generated waves, which are strongly influenced by restricted fetch and by the propagation of offshore waves generated in the Bay of Bengal. Due to the limited information available on measured waves in the Palk Bay, the wave field in this region was estimated by transforming the available wind and wave data in the Bay of Bengal for the region close to Palk Strait.      

For the estimation of wave conditions in the Palk Bay, we used the wind and wave data in the Bay of Bengal north of the Palk Strait, from the Alkyon in-house database, hydrobase.net.  This database consists of time series (» 10,000 data sets, for 37 years from 1960 to 1997) of visually observed ship data on wind speed and direction, significant wave height, average wave period and propagation direction.  To transform this database to the observation points along the channel alignment in the shallow regions of the Palk Bay, the SWAN-module in the Delft3D suite was found suitable. 

SWAN-module

The swan (Simulating WAves Nearshore) module developed at the Delft University of Technology, is a fully spectral third generation wave model, which includes the following processes:

• Refraction and shoaling over a variable depth configuration

• Dissipation by bottom friction and by breaking

• Growth of wave energy by the action of wind

• Non-linear wave-wave interactions (quadruplets and triads)

swan represents the wave field on a regular grid using the spectral density at discrete frequencies and directions. In this project, SWAN was used in stationary mode. The following section describes the method used to translate the offshore waves to the locations of interest.

 The transformation matrices were generated by running the swan-model for a fixed number of combinations of wind speed, wave height, wave period (or steepness) and direction; a fixed relationship between wind speed and wave height was used in this computation. At each observation point along the ship channel trans­for­ma­tion matrices were constructed from the swan results.

The transformation matrices are then used to generate the wave conditions at the observation points for the combination of the observed wind speeds and offshore wave conditions from the database (» 10,000 data sets).  But, as the combination of the ship observed data, which is arbitrary, need not be the same as that chosen for the construction of the transformation matrices, suitably estimated weighing factors have been used to compute the transformed wave parameters at the observation points.

Gulf of Mannar:

Offshore waves from the southwest and south generated in the Indian Ocean and in the Gulf of Mannar, affect the navigation along the proposed shipping route south of the Adam’s Bridge.

For the wave field estimation south of the Adam’s Bridge, a similar approach as that used for the wave transformation matrices in the Palk Bay, has been used. In this approach the transformation matrices is generated by performing a (reverse) hind-cast for a fixed number of offshore wave conditions using the hydrobase-prob model. At each observation point a trans­for­ma­tion matrix is developed from the results of the (reverse) hind-cast.  The transformation matrix is subsequently used to transform the time series of ship observed offshore wind/wave data (» 30,000 data sets) to the near-shore observation points, where the resulting time series is analyzed to obtain the required tables and statistics.

In the reverse hind-cast for several fixed combinations of wave parameters south of the Gulf of Mannar, the storm duration has been computed that would give the offshore wave height for a given wind speed and offshore fetch. For this purpose for each class (directional sector, wave height class) a mean wind speed was calculated from the combined wind and wave statistics.  For the given wind speed and wind direction, the computed storm duration and the local (restricted) fetch a normal hindcast is performed to determine the local (deep water) wave climate. Using a one-dimensional approach that takes into account the processes of refraction, shoaling, wave breaking and bottom friction, the local nearshore wave climate is determined at each required output location.

hydrobase-prop package in the hydrobase suite.

hydrobase-prop models the effects of:

• wave growth due to the action of wind;
• refraction over a bottom of uneven depth with parallel depth contours;
• wave breaking, dissipation due to bottom friction and shoaling over a bottom of uneven depth with parallel depth contours;
• diffraction past a semi-infinite breakwater;
• diffraction past a straight breakwater of finite length.

hydrobase includes a program representing the wave conditions according to the amount of energy in a number of direction segments. The duration and fetch limited growth of waves due to the action of wind is modeled following Hurdle and Stive (1989), adapted to account for directional spreading by considering wave growth to be directionally decoupled. The wave growth model can be used to compute either the significant wave height and period (input: fetch lengths, water depth along the fetch, wind speeds and storm duration) or the storm duration (input: fetch lengths, water depth along the fetch, wind speeds and resulting wave height).

Refraction and diffraction of each directional component of the resulting wave condition are accounted for using analytical models. The shore protection manual (1984) describes these models. It gives expressions for refraction over a prismatic bottom and for diffraction past a semi-infinite breakwater in water of uniform depth. It is up to the user to schematize the wave propagation process as a series of diffraction and refraction steps for each output point. The energy in each directional component is summed after each step to obtain the total energy and thus the resulting significant wave height. Similarly, the main wave direction is obtained by weighing each directional component according to its energy.

3.1.2.  Available Data

Offshore Wind and Wave Data:

The measured data available for the enclosed region covering the Adam’s Bridge, Palk Bay and Palk Strait are limited. The wind conditions at Gulf of Mannar and  at Palk Strait in  Bay of Bengal were derived from ship observations on wind and waves from Alkyon’s in-house database hydrobase.net. These data consists of observed wind and wave data along shipping routes all around the world’s oceans for the period 1960 to 1997.  The database for the area south of the Gulf of Mannar consists of » 30,000 wind and wave observations and for the area northeast of the Palk Strait, in the Bay of Bengal, consists of » 10,000 data values. 

The observations on wind at northeast of Palk Bay (in Bay of Bengal) have been considered representative for the wind conditions in Palk Bay north of Adam’s Bridge.

It is likely that the wind in the Palk Bay also will be influenced by the local geometry of the landmasses surrounding the bay. As we have applied a uniform wind over the entire model area, wave directions (with respect to the North) may differ slightly in the central area of the Palk Bay (in the clockwise direction) and in the area close to the Palk Strait (in the counter clockwise direction).

The wind roses at Palk Strait show a good comparison with the wind roses south of the Gulf of Mannar with respect to wind directions and speeds for the northeastern directions.  During the Southwest monsoon the area south of the Gulf of Mannar is very much influenced by the trade winds south of India and Sri Lanka and therefore the wind takes a more westerly direction.

Seasons:

The analysis will be performed for the following three seasons:

• SW-Monsoon: June, July, August, September;

• NE-Monsoon: November, December, January, February;

• Fair Weather: March, April and May.

As can be seen from the wave and wind roses the month of October is a transitional period between the two monsoon seasons.

Water Levels:

As the tidal variation in the area of interest is relatively small, the wave computations are performed for the mean sea level.

3.1.3.  Method applied for Palk Bay and Palk Strait

Wave Propagation:

Various physical processes have to be taken into account while transforming the wave data from the open sea region to observation points along the channel alignment. Refraction, shoaling, bottom friction, depth induced breaking, white-capping, wind growth and non-linear interactions affect waves propagating from offshore to nearshore.  The swan wave model has been used to simulate the local generation of waves by wind and the propagation of waves from offshore (Bay of Bengal) to nearshore. This was done for a fixed number of offshore conditions. Each offshore condition is defined as a combination of wind speed and direction, and offshore wave height, steepness and direction. The wind speed and wind direction is assumed to be uniform over the computational area.

Computations were made for the following combinations of parameter values:

Wind and wave direction

q             : 0, 30, 60, 90, 120, 150, 180, 210, 240, 270, 300 and 330 (°N)

Wind speed

U_w         : 4.5, 6.5, 8.5, 12.5, 17.0 (m/s)

Significant offshore wave height

H_s,0      : 1.25, 1.75, 2.25, 3.25, 4.25 (m)

Wave steepness

S₀           : 0.005, 0.02, 0.05

A fixed relationship is used between wind speed and wave height and direction.  This relationship is based on the analysis of the wind speed and the wave height from the northeasterly sectors (Table 3.1).

Table. 3.1. Fixed combinations of wave height and wind speed for northeasterly sectors

For modeling the waves in the Palk Bay for the northeast wind in combination with the waves entering the area from the Bay of Bengal, the offshore wave boundary condition was defined according to the relation given in Table 3.1.  For winds from south-west direction, the wave condition specified at the open sea boundary near the Palk Strait will have no effect on the wave field inside the Palk Bay and the wave field will be affected only by the local wind.

Wave Diffraction:

The waves propagating inside the Palk Bay is influenced by depth induced refraction and bottom friction. Since the bathymetry is uniform and no vertical obstruction is observed, the effect of wave diffraction is considered negligible.

Output locations:

Twelve output locations (observation points) were selected along the channel alignment, numbered P01 to P12 starting from the Palk Strait (Fig. 3.1). The points P11 and P12 are also affected by the wave propagation from the Gulf of Mannar and have also been modeled following the HYDROBASE-PROP approach as it has been applied for the Gulf of Mannar.

Transformation Matrices:

Two types of transformation matrices were constructed. The transformation matrices relate the offshore wind condition and the wave condition at the boundary to the wave conditions at the site of interest.

The first type of matrix transforms the offshore wave climate to local wave conditions; the simultaneously observed wave heights are not considered.   The dimensions of the transformation matrices depends on the number of class intervals considered for the following variables while constructing the matrices:

• wind speed and

• wind direction.

The second type matrix transforms the offshore wave climate to nearshore wave conditions; the simultaneously observed wave heights are considered. These transformation matrices have the dimensions of the following variables considered in the construction of the matrices:

• wave height /wind speed,

• wave steepness and

• wave direction.

The transformation matrices can be used to transform an observed offshore wind / wave condition to the observation point by multi-linear interpolation within the class intervals.  The discretization of the wave or wind conditions should satisfy the following requirements, while constructing the transformation matrices:

1.      It should cover a wide range of offshore conditions and be able to take into account all combinations of observed offshore data.

2. The matrix grid resolution should be sufficiently fine, so that the values between the nodes can be obtained within 5% accuracy by linear interpolation.

The transformation matrices are used to compute the wave penetration for a large number of observed offshore wave data considered in the study (37 year time series, » 10,000 data sets).

Weighing factors:

By means of weighing factors the wave energy determined by the transformation using local wind conditions and the wave energy determined by the transformation using the offshore wave conditions have been combined in the fashion (m²+n² =1), where, m is the weighing factor for wind and n is the weighing factor for waves. The weighing factor has been assumed equal for all northeastern (on-shore) wave directions (Table 3.2). For the southeastern (offshore) directions the local wave conditions will be based only on the transformation using local wind conditions.