Sedimentation:

 

 

4.3.9. Rameshwaram Island sand spit

 

‘East of Rameshwaram, a long sand spit of about 20 km length is formed and it tends to grow longer and wider. The width of this sand spit is about 2 km near Uthalai, reduced to 1250 m at Mukkuperiyar, 750 m at Dhaushkodi and 150 m at just east of Arimunai and converges on the tip at Arimunai. The beach berm is found to be highly elevated along the sand spit bordering GOM, but very low and flat along the side bordering Palk bay. There is a marked depression in the sand spit level between Palk Bay and GOM, between Dhabushkodi and Arimunai. Due to such level difference, the water overflows during spring tide particularly from Bay carrying the fine sediment to the backshore regions. Most of the time, the water is stagnant and remains along the trough of the spit. This low lying region is fully occupied by water column during monsoon season.’ (NEERI EIA, p-1.12)

 

 

 

 

4.3.10. Sediment Load to Palk Bay and Gulf of Mannar

 

‘The sediments carried by the rivers and by the surf zone currents as littoral drift get partly deposited in permanent, semi-permanent and temporary sinks along the Indian coast. The Gulf of Kachchh, Gulf of Khambhat, Gulf of Mannar, Palk Bay and Sandheads act as major sinks. In order to identify the extent of their significance for the sediment deposition, a study was undertaken to evaluate the long-term sediment deposition in the above regions.

 

Vaigai, Vaishali and Valryar rivers and the littoral transport by various sources from the northern part of the Tamil Nadu coast are the major sediment sources entering the Palk Bay region. It is largely occupied by sand banks, numerous shoals, sand spits and islands. Occurrence of cyclonic storm during north-east monsoon is common in the Nagapattinam–Poompuhar region, which causes an erosion along this region. The sediments are transported southerly and deposited in the Palk Bay. Low wave action inside the bay and protection from the southerly waves encourages the deposition of sediment. In the present study, the estimation based on eq. (1) covering the area of 117 km × 105 km, shows that 0.3 × 10¹⁰ m³ sediment got deposited over a period of 51 years. Assuming that the rate of accumulation is uniform over the years, it is estimated that between the years 1931 and 1982, the sediment deposition has caused a reduction in water depth of about 0.32 m, i.e. 0.006 m per year. The depositional feature observed in the present study agrees with the formation of very shallow areas in the Palk Bay. The enlargement of the Manamelkudi sand spit and the emergence of sand banks between Point Calimere and Point Pedro (Sri Lanka) across the entrance of the Palk Bay are the evidences of the depositional features occurring in this region. Usha and Subramanian stated that the accretion pattern was observed in the Palk Bay at Ammapattinam, Mandapam and Rameswaram.  Loveson et al. have discussed that large amounts of sediments from the pediments are removed constantly by rainfall and carried by minor rivers and dumped into the Palk Bay.

 

Along the Gulf of Mannar, the sea–land boundary is almost uniform and regular but for inundation in a few places, where it is intercepted by rivers forming tidal inlets. Tambraparni, Vembar and Vaipar and some other inlets are present in this region. At present, Vembar river is not supplying sediments into the sea, except during the rainy days in the north-east monsoon period. Wave action is low during most of the time and hence, the formation of sand dune is common in this region. The estimation using equation (1) covering an area of 19.5 km × 13.5 km at the Gulf of Mannar, shows that 0.02 × 10¹⁰ m³ sediment got deposited over a period of 75 years. Assuming that the rate of deposition is uniform over the years, it is found that the deposition between years 1906 and 1981 has caused a reduction in water depth of about 0.72 m, i.e. 0.010 m per year. There are a number of coral banks and islands present in the Gulf of Mannar. Formation of sandy islands off Tuticorin region shows as sinks having accumulation of sand. Large beach storage of sands between Manapad and Tiruchendur, Vembar and Valinokkam and along Rameswaram island indicates depositional features of the littoral sediments. (P. Chandramohan, B. K. Jena, and V. Sanil Kumar, “Littoral drift sources and sinks along the Indian coast”, in CURRENT SCIENCE, VOL. 81, NO. 3, 10 AUGUST 2001, p-295)

 

 

 

4.2.11. Study of suspended sediments using remote sensing

 

Prakash Chauhan et al., have used remote sensing technique to study suspended sediments along the Tamil Nadu coast both during the monsoonal (ie., North East monsoon; imagery dated 17-12-1990) and the non-monsoonal periods (image dated 18-04-1990). The maps they have prepared will be useful here for us and hence they are reproduced here.

 

 

They have noted that ‘during the non-monsoon season the sediment boils were observed near Vedaranyam and Mandapam.’ This they say ‘may be attributed to the difference of the density of the different water masses and the prevailing current direction.’ ( Prakash Chauhan, Sailesh Nayak, R.Ramesh, R.Krishnamoorthy, S.Ramachandran, “Remote Sensing of Suspended Sediments along the Tamil Nadu Coastal Waters”, in Photonirvachak, Vol.24, No.2, 1996, p-105-114).

 

 

An attempt to study the sedimentation dynamics using remote sensing off the Vedaranyam coast has been made by Usha Natesan recently. (‘Role of satellites in monitoring sediment dynamics’, in CURRENT SCIENCE, VOL. 86, NO. 8, 25 APRIL 2004, p-1068-1069). Let us note the salient points from her article:

 

“Vedaranyam, the study area is located in the southeast coast of India and has a peculiar projection, which is of interest for geologic and geomorphic studies. Tamil Nadu coast experiences a climate strongly influenced by the southwest (June–September) and northeast (October–December) monsoons. Formation of Point Calimere projection is due to two constantly opposing wave directions, such as northeast and southeast, with one set of waves predominant over the other. Further, the foreland formation is determined by the rate at which the stream delivers the sediment and the rate at which the waves can winnow and move the sediments in either direction away from the mouth. The coastline is consequently affected predominantly by waves from the northeast. This is clearly reflected by the shape of the foreland, which has veered windward.

 

Sediment distribution at Vedaranyam during different seasons is depicted in Figure 2 a–c. From the figures, the direction of sediment movement can be clearly identified. Sediments move towards north during southwest monsoon (Figure 2 a, b) and vice versa during northeast monsoon (Figure 2 c). As the sediments are transported by the longshore currents, the directions indicate the movement of currents. The above observations from satellite data agree with the longshore current direction perceived through conventional methods.

 Fig. 2.a. Sediment distribution at the onset of SW monsoon

 

 

Fig. 2. b. End of SW monsoon

 

Fig. 2. c. Sediment distribution during NE monsoon

 

 

 

 

With the onset of southwest monsoon, sediments were disturbed considerably (Figure 2 a). Tidal flats at Vedaranyam acting as a source for sediment is clearly identified from the image. Daniel observed that the sediments are discharged from tidal flats due to soil erosion. The retreat of seawater eroding the banks of tidal flats contains high amount of sediments in suspension. This appears bright red in colour around the mouth of tidal flats. As the sediments move towards the north due to the longshore currents, their concentration is reduced which is indicated by yellow colour in Figure 2 a. At the end of the southwest monsoon, initial disturbances are very much reduced. Sediments moving towards the north are obstructed by the projection at Vedaranyam (Figure 2 b). Therefore, it takes a turn near that tip and then follows the coastal configuration.

 

In contrast, during the northeast monsoon season, sediments transported from north are unable to take a bend around the Vedaranyam tip and hence dissipate a greater part of it (Figure 2 c). A portion of the sediment is moving towards the east and the rest of it moves downsouth of Vedaranyam along with the longshore currents.

 

From the conventional data seasonal sediment distribution patterns in near-shore/ offshore are almost impossible to obtain. Satellites are the only source of providing such information. The present study proves that the satellites play a significant role in monitoring the sediment dynamics.”

 

 

 

 

4.2.12. Physico-chemical parameters of Palk Bay

 

“This area is under the spell of both southwest and northeast monsoon. Southwest monsoon contributes little towards the total annual rainfall of this area. Rain is moderate to heavy during October to January (northeast monsoon). The mean annual rainfall varies from 820 to 1650 mm. The monthly average temperature of the waters of the Palk Bay ranges between 24.6° and 29.1°C (minimum during December and the maximum during May). The tidal elevation is around 1m. Palk Bay is practically calm except during the northeast monsoon when turbulent condition prevails (Gopinadha Pillai, 1969). The salinity of the water decreases gradually along an axis in the southwest direction running from the strait. High saline water is ‘pocketed’ in the south-west corner of the bay. This may be due to the incursion of the Gulf of Mannar water through Pamban pass. The density of the water also decreases along an axis on the southwestern direction from the Strait.

 

The North-east wind velocity at Nagapattinam is about 8 to 10 knots (North of Palk Bay) and at Pamban the wind strength reduces between 2 and 4 knots (South of Palk Bay). Temperature, salinity, density and dissolved oxygen values of the surface waters of the Palk Bay would indicate that the Bay of Bengal waters entering into the Palk Strait influence the hydrographic condition of the Palk Bay, unlike the Gulf of Mannar waters whose influence on the hydrological parameters of the Palk Bay is only minor (Murty, A.V.S. and P. Udayavarma, 1964. The hydrobiological features of the waters of Palk Bay during March. J. mar. biol. Ass. India, 6(2):7-216.).

 

The bottom sediments of Mandapam consist of silt and clay, clayey silt and sand, fine to medium sand, coarse sand and coarse sand with gravel. Distribution of various size classes indicates that the offshore sediment in this area is usually unimodal with the primary mode around 1.5 to 2 (medium sand) and a secondary mode around 3.5. Beach samples have prominent mode around 2.25, 1.75, 2.75 and 3.25 suggesting the polymodal nature of the sediments. Grain size parameters of the Palk Bay samples near Mandapam are shown in Table 28. The distribution patterns of heavy minerals are shown in Table 29 (Mallik, 1983). Most of the offshore areas contain a high amount of opaques (16-80%) with maximum concentration in small patches. Majority of opaques consist of ilmenite. Other minerals are magnetite, rutile, hydroxide of iron and minor amounts of pyrite (Mallik, 1983). (p-58)

 

Inshore waters of the Palk Bay during the monsoon become muddy due to the presence of suspended sand and silt stirred up from the sandy shore by wave action. The large degree of silt settlement has a remarkable effect especially during the northeast monsoon. Cyclonic winds during monsoon season, with high velocity, cause enough mechanical damage to the corals of this area. Huge quantity of silt settlement during the northeast monsoon has a remarkable effect on the distribution and diversity of the coral reef associated plants and animals. (p-66)” (‘CORAL REEFS OF INDIA’ - State-of-the-art report, ENVIS Publication Series : 4/2001, ENVIRONMENTAL INFORMATION SYSTEM CENTRE, Centre of Advanced Study in Marine Biology, Parangipettai - 608 502, Tamil Nadu, India)

 

4.2.13. Longshore currents and sediment transport

 

“The objective of the work was to study the variations in wave climate based on the data collected in two different years, longshore currents and sediment transport along the Nagapattinam coast.

 

The daily variations of longshore current estimated based on Longuet–Higgins equation (VL) and Galvin’s equation (VG) with the measured current at three stations shows that the average longshore current speed was 0.25 m/s at all stations, and it was predominantly towards north during March–October and towards south during November–February. This matches with the earlier study carried out in the region.

 

During 1998–99, strong longshore currents (> 0.5 m/s) were observed in November and December. Longshore current was relatively weaker (< 0.5 m/s) during the rest of the year. During 1995–96, strong longshore currents (> 0.6 m/s) were noticed in May, June, November and December. Longshore current was relatively weaker (< 0.3 m/s) from January to April. The computed longshore current speed estimated based on Galvin and Longuet–Higgins was about 50% of the measured one. The average correlation coefficient between measured and computed data was around 0.75.

 

The annual longshore sediment transport was 0.175 × 10⁶ m³ in the northerly direction (March to October) and 0.273 × 10⁶ m³ in the southerly direction (November to February). The annual net sediment transport was 0.098 × 10⁶ m³ in the southerly direction and the annual gross sediment transport was 0.448 × 10⁶ m³.

 

The study shows that the average annual net sediment transport was 0.095 × 10⁶ m³ in the southerly direction and average annual gross sediment transport was 0.432 × 10⁶ m³. The net sediment transport is towards the south, since the wave activity is strong mainly during the northeast monsoon period for this coast. The annual net sediment transport computed using the estimated breaker angle gives a value of 0.098 × 10⁶ m³ and the difference between the estimated sediment transport rates based on the two methods is around 3.5%. This is due to error in observation of the surf zone width, which is based on visual observation. Also the wave–wave and the wave– current interactions in the surf zone are not considered. Hence the sediment transport rates estimated based on CERC formula using the measured wave characteristics can be taken as the representative value for this coast.

 

An earlier study shows that transport rate was around 0.1 × 10⁶ m³/month in November and December, and was low (< 0.03 × 10⁶ m³/month) in March, April and July. The predominant direction of transport is northerly from March to October and southerly from November to February. Though the annual gross transport was found to be 0.6 × 10⁶ m³/year, the annual net transport was very low showing less than 0.006 × 10⁶ m³/year (towards the north). The difference in longshore sediment transport rates between the present study and the earlier study was that in the present study the wave direction of the sea and swell waves was considered in estimation, whereas it was neglected in the earlier study. The energy at the second peak was more than 50% of that at first peak in 46% of the data collected. The coastal inclination of the study area is 359° with respect to north. A wave direction more than 89° will cause a northerly sediment transport and one less than 89° will cause a southerly transport. The average direction of sea waves was 106° and that of the swell waves was 99°. Hence a small error of the order of 0.5° in the estimate of the breaker angle may cause a change in direction of the sediment transport rate. The error in wave directions can lead even to a wrong direction for the net calculated sediment transport. The present alignment of the coastline is found to be sensitively balanced, since any slight increase in approaching wave angles may significantly increase the volume and may also alter the direction of sediment transport in this region. For a coastal inclination of 4° with respect to the north (5° more than the present alignment), the net sediment transport increases to 1.85 times the present value, and for 354° (5° less than the present alignment) the value reduces to 0.12 times the present value.

 

It is important to notice that no cyclone had occurred during both the study periods, which is otherwise common during the northeast monsoon period. It has been observed that for the occurrence of every cyclone, there was a permanent loss of beach due to erosion1. As the Palk Bay is well protected for southerly waves, no mechanism is set to transport these deposited materials towards the north.

 

The comparison of the wave data collected during 1995–96 and those collected during 1998–99 shows that the statistics of the wave parameters was almost the same during both the periods, but the events did not occur at the same time. The average value of the breaker angle for the sea waves was – 3.3° and that for the swells was 2°; the breaking sea and swell waves were mainly from the southerly direction during March–October and mainly from the northerly direction during the rest of the year. The ratio of spectral energy at the first and second spectral peaks estimated shows that the energy at the second peak is more than 50% of that at first peak in 46% of the data collected in 1995–96 and 43% collected in 1998–99. The daily longshore currents measured show that the average longshore current speed was 0.25 m/s; the longshore current was predominantly towards the north during March–October and towards the south during the rest of the year. The difference between the estimated sediment transport rates based on the two methods is around 3.5%. This is due to the error in observation of the surf zone width. The annual net sediment transport was 0.098 × 10⁶ m³ in the southerly direction and annual gross sediment transport was 0.448 × 10⁶ m³, and this contributes to the supply of sediment to the Palk Bay.” (V. Sanil Kumar, N. M. Anand and R. Gowthaman, “Variations in nearshore processes along Nagapattinam coast, India”, in CURRENT SCIENCE, VOL. 82, NO. 11, 10 JUNE 2002, p-1381 – 1389)

 

4.2.14. Studies conducted on the Wave Climate, Tides and Currents by the NEERI EIA

 

‘The winds blowing over the ocean surface has the direct effect on wave generation as it is related to wind speed, extent of fetch and wind duration. The south west monsoon influences this pattern from June to September. The average speed of the wind during south west monsoon period is about 35 km per hour frequently rising up to 45-55 km per hour. The average speed of the wind during north east monsoon (October to January) prevails around 20 km per hour. Tropical storms known as cyclones frequently occur in the Bay of Bengal during October to January.

 

Wave Measurement

 

The observations on wave measurement show that significant wave height varied from 0.46 to 1.12 in March, 0.33 to 1.18m in April, 0.46 to 1.74 m in May, 0.71 to 1.78 m in June, 0.68 to 1.6 m in July, 0.68 to 1.49 m in August, 0.64 to 1.76 m in September, 0.54 to 1.35 m in October, 0.40 to 1.13 m in November, 0.40 to 1.12 m in December, 0.35 to 1.03 m in January and 0.35 to 1.23 m in February.

 

The maximum wave height varied from 0.67 to 1.78 m in March, 0.44 to 1.73 m in April, 0.66 to 2.81 m in May, 0.98 to 2.72 m in June, 0.91 to 2.45 m in July, 0.89 to to 2.48 m in August, 0.89 to 2.96 m in September, 0.66 to 2.94 m in October, 0.59 to 1.60 m in November, 0.48 to 1.73 in December, 0.47 to 1.68 m in January and 0.45 to 1.79 m in February.

The wave direction (with respect to north) mostly prevailed 140⁰ to 230⁰ in southwest monsoon (June to September), 85⁰ to 150⁰ during north east monsoon (October to January), and 90⁰ to 200⁰ during fair weather period (February to May). The wave direction is highly variable in January and May. The zero crossing wave period predominantly varied 3 to 8 s in December to April, 4 to10 s in May and 4 to 9 s during rest of the year.

 

Wave refraction

 

Arimunai to Vedaranyam

 

This segment of the coastline lies in Palk Bay and waves propagating from south (during south west monsoon and fair weather period) do not enter this region. Studies are indicating that even during the north east monsoon, waves are found not entering the bay and get attenuated across the shoals of middle banks and south banks between Vedaranyam and Matakal (Sri Lanka). Part of wave energy with less magnitude enters the bay through Pedro Channel and reach the coast between Puduvalasai and Gopalpatnam. Wave refraction between Arimunai and Vedaranyam during NE monsoon is shown in the following figure.

 

Wave Period

 

During SW monsoon, the wave period predominantly persisted 9 to 10 s between Vembar and Keelamundal, and 6-8 s between Uthalai and Dhanushkodi. During the NE monsoon, it predominantly persisted 5-10 s between Vembar and Keelamundal, and 5-8 s between Uthalai and Dhanushkodi east. In fair weather period, it remained 6-10 s along Vembar to Keelamundal and 9-10 s along Uthalai to Dhanushkodi. The study shows that the waves approaching the coastline consist of both seas and swells.

 

Tides and Currents

 

The tides in this region are semidiurnal. The various important tide heights with respect to chart datum near Pamban pass are as follows:

 

Mean Higher High Water Springs     =   0.70 m

Mean High Water Neaps                 =   0. 48 m

Mean Sea Level                             =   0.41 m

Mean Low Water Neaps                  =   0.32 m

Mean Low Water Springs                =   0.06 m

 

It shows that the average spring tidal range is about 0.64 m and the neap tidal range is about 0.16 m.

 

Longshore Currents

 

The longshore current speed remained weak (< 0.1 m/s) throughout the year between Keelamundal and Vedalai and along the northern coast of Rameshwaram from Arimunai to Ariyaman. Consequently, it was relatively moderate (>0.1 m/s) throughout the year between Sippikulam and Naripaiyur and along the southern coast of Rameshwaram i.e. from Uthalai to Mukkuperiyar.

 

The spit between Dhanushkodi and Ariyaman in Gulf of Mannar experienced relatively stronger currents during fair weather period (March to may) and remained weak during southwest monsoon and northeast monsoon periods (June to February). It indicates that the stronger currents prevailing in the adjacent coasts during SW/NE monsoons becoming weaker between Dhanushkodi and Arimunai. This phenomenon of sudden weakening of littoral currents causes the littoral drift to deposit and form series of sand shoals near Arimunai. Such prolonged deposition of littoral drift over many years can be attributed to formation of numerous islands and shallow shoals across the strait between Arimunai and Talaimannar (SriLanka) called Adam’s Bridge.

 

The Uthalai coast facing Gulf of Mannar experienced stronger longshore currents (0.2 – 0.5 m/s) throughout the year, followed by a segment of the coast between Vembar and Naripayur (0.2 – 0.4 m/s) with exposure to relatively high wave energy environment.

The prevalence of weak longshore currents between Keelamundal and Vedalai is causing deposition of littoral drift on either side, as evidenced by the occurrence of many offshore islands and submerged shoals.

 

Although the Pamban Pass, connecting Palk Bay and Gulf of mannar break the continuity of longshore current between the mainland and Rameshwaram Island, the magnitude of the current on either side of Pamban Pass is found to be very weak. This reduces the volume of littoral sediments approaching Pamban Pass which in turn reduces the quantity of sediment passing through Pamban pass from Gulf of mannar to palk Bay.

 

The longshore current direction prevailed northerly during SW monsoon and fair weather period, and southerly during NE monsoon between Sippikulam and Uthalai. The entire coast of Rameshwaram facing Gulf of Mannar, experienced the current in the westerly direction throughout the year, except in June, july. This phenomenon of northerly currents along the mainland and westerly current along Rameshwaram creates a zone, wherein, most of the littoral drift will get deposited. Only a fractional proportion is expected to move from this region by tide induced currents towards the Adam’s Bridge and in turn the quantity of sediment entering Palk Bay from Gulf of Mannar. These sediments deposited at shoals is supplied back to the littoral system for the mainland, when the longshore currents move towards south during the ensuing monsoon.

 

Although the longshore current was extremely weak along the sand spit facing Palk Bay, it tends to be easterly during SW monsoon/fair weather period and westerly during NE monsoon. Similarly, at Ariyaman, the longshore current direction was southerly during SW monsoon / fair weather period and northerly during NE monsoon, indicating just opposite to the phenomenon observed in Gulf of Mannar. Such processes once again indicate the accumulation of littoral drift on either side of Rameshwaram Island during SW monsoon and removal during NE monsoon making this region as a sediment storage reservoir.

 

Ocean Current Studies

 

Continuous measurements on tidal current speed and direction were carried out for three seasons at 4 locations viz., 1) stn. C1 – off Arimunai-Adam’s Bridge, 2) stn. C2 – off Uthalai (Gulf of mannar), 3) stn. C3 – Pamban Pass, and 4) stn. C4 – of Tharuvai (Palk Bay). The measured currents were resolved into parallel and perpendicular components with respect to the coastline.

 

SW monsoon (June to September)

 

Near Arimunai (stn.C1) the average current speed occurred around 0.2 m/s with the maximum and minimum speed of 0.3 m/s and 0.05 m/s respectively both at surface and bottom. The variation of current direction had not followed the tidal phase. It showed consistent northwesterly flow over one tidal cycle and changed to southeasterly flow for the subsequent tidal cycle. It indicates that current shifted its flow direction for alternate tidal cycles rather than flood and ebb tidal phases. The shore parallel component of currents indicates that for larger tidal range, the flow was in westerly direction and for small range in easterly direction. The shore perpendicular component of currents indicates that the flow consistently existed from Gulf of Mannar into Palk Bay. The northwesterly and south westerly currents over different tidal cycles were found to bw equally predominant.

At Uthalai (stn.2) in GOM, the average current prevailed around 0.1 m/s with the maximum and minimum of 0.2 m/s and 0.05 m/s respectively. Similar to Stn.C1, the bottom current was seen responding to tides flowing east over one tidal cycle and west during the subsequent tidal cycle. The direction of flow was predominant in southeasterly direction for larger tidal range and north westerly direction for small tidal range. The shore parallel component of currents indicates that the flow shifted in southeast and north west both at surface and bottom. The shore perpendicular component of currents indicates that the flow shifts towards north east and south west both at surface and remains consistently northeast at bottom.

The current speed near Pamban Pass (stn.C3) was found to be strong showing an average of 0.5 m/s, with the maximum of 1 m/s and minimum of 0.1 m/s. Current direction remained consistently northeast flowing from GOM into Palk Bay. Variation of current speed shows that the magnitude of current speed was more during flood and less during evv tide indicating the influence of tides over the seasonal unidirectional flow. The shore parallel component of currents indicates that the flow is into Palk Bay with high speed during flood tide and low speed during ebb tide. The shore perpendicular component of currents indicates that the flow is across the Pamban with high speed during flood tide and low speed during ebb tide.

At Tharuvai (stn.C4), the average current speed of 0.2 m/s with the maximum of 0.3 m/s and minimum of 0.1 m/s were observed both at surface and at bottom. The flow was unidirectional towards south east but the current speed varied with tidal phase. Current speed was high during flood tide and low during ebb tide indicating the strong influence of seasonal circulation current towards northeast during south west monsoon.  The shore parallel component of currents indicates that the flow was towards southeast at surface and bottom. The shore perpendicular component of currents indicates the flow was towards northeast both at surface and at bottom.

The measurement shows that during SW monsoon when tidal range is large, the opposite direction of flow prevails between Adam’s Bridge (stn.C1) and Uthalai (stn.2) would cause the water mass to flow from the GOM to Palk Bay. This flow would transport sediments into Palk Bay from GOM. On the other hand, when the range is small, the divergence of flow occurring near the Adam’s Bridge and Uthalai would initiate a flow from Palk Bay into GOM through Adam’s Bridge.

 

 

 

 

Northeast Monsoon (October to January)

 

Near Arimunai (stn.C1), current was generally weak showing an average of 0.1 m/s, with the maximum of 0.2 m/s and minimum of 0.05 m/s. The flow direction remained unidirectional towards west both at surface and bottom. The current speed increased during flood tide and reduced during ebb tide. The shore parallel component of currents indicates that the flow was consistently towards northwest at surface and bottom. The shore perpendicular component of currents indicates the flow prevailed northeast at surface and south west at bottom.

The variation of currents at Uthalai (stn.2), showed an average current speed of 0.08 m/s, with the maximum of 0.15 m/s and a minimum of 0.04 m/s. The bottom flow was nearly unidirectional towards southeast. The shore parallel component of currents indicates that the flow was oscillating in southeast and northwest at surface and remaining consistently southeast at bottom. The shore perpendicular component of currents indicates that the flow was towards northeast both at surface and bottom. 

The currents at Pamban pass (stn.C3) prevailed strong with the average of 0.1 m/s, maximum of 1.4 m/s and minimum of 0.5 m/s. Currents remained consistently unidirectional around 225⁰. The change in tidal phase caused the variation in current speed showing stronger currents during ebb tide and reduction in current speed during flood tide. It indicates that the flood tide propagates fro GOM to Palk Bay and vice versa. The shore parallel component indicates that the flow was consistently from Palk Bay into GOM both during ebb and flow tides. The shore perpendicular component of currents indicates the flow was across the Pamban from Rameshwaram to Mandapam.

The current was found to be weak off Tharuvai at Palk Bay (stn.4) showing the average speed of 0.1 m/s, maximum of 0.13 m/s and minimum of 0.04 m/s. Similar to stn.C3, the current flow was unidirectional towards 250⁰, but the speed was high during ebb tide and low during flood tide. The shore parallel component of currents indicates that the flow was towards northeast both at surface and bottom. The shore perpendicular component of currents indicates that the flow was towards southwest both at surface and at bottom.

The observation during northeast monsoon indicates thet the current flow was more influenced by seasonal flow than by tides. Stronger currents were observed during ebb tides flowing from Palk Bay into GOM through Pamban pass. The currents were generally weak in GOM and Palk Bay (stns.C2 and C4). Significant flow from Palk Bay was observed through Adam’s Bridge also. Such current pattern during NE monsoon can transport and exchange the sediments from Palk Bay into GOM.

 

Fair weather period (February to May)

 

The current at Arimunai (stn.1) was generally weak showing average of 0.1 m/s, with the maximum of 0.2 m/s and minimum of 0.05 m/s. The current flow was found to be unidirectional towards northwest both at surface and bottom. The shore parallel component of currents indicates that the flow was towards northwest both at surface and at bottom. The shore perpendicular component of currents indicates the flow was changed in direction in north east and south west both at surface and bottom.

At GOM (stn.C2), the current was weak with average of 0.1 m/s, maximum of 0.2 m/s and minimum of 0.04 m/s. The flow remained unidirectional consistently towards 305⁰, but the current speed varied randomly between 0.04 and 0.12 m/s. The shore parallel component of currents indicates that the flow was towards northwest both at surface and bottom. The shore perpendicular component of currents indicates the flow changed the direction from northeast to southwest both at surface and bottom.

The flow through the Pamban Pass (stn.C3) was quite distinct, showing the average speed of 0.3 m/s, maximum of 0.6 m/s and minimum of 0.04 m/s. Current flow was noticed towards 45⁰, i.e., into Palk Bay during flood tide and towards 225⁰, i.e., into GOM during ebb tide. The shore parallel component of currents indicates that the flow was into Palk Bay during flood tide and into GOm during ebb tide. The shore perpendicular component of currents indicates the flow was changing its direction across the Pamban Pass between Mandapam and Rameshwaram.

During fair weather period, the change in current was observed over the tidal phases at Pamban Pass. The study shows that the current flows mostly parallel to the coast. The general circulation of current in northwesterly direction dominates the tide induced current. This would help the sediments to move by tide induced currents from GOM to Palk Bay prevailing through Pamban pass and to some extent through Adam’s Bridge. (NEERI EIA, p- 2.2 – 2.11).

 

4.2.15. Studies on Sedimentary Transport by the NEERI EIA

General Information

Rameshwaram Island, the geological formation of coral atoll with huge sand cover between India and Sri Lanka plays a vital role on the processes of exchange of littoral drift between east coast and west coast. It separates the sea in the north by Palk Bay and south by Gulf of Mannar. The wave sheltering effect due to Sri Lanka Island, the large siltation in Palk Bay, the presence of numerous offshore islands in GOM, the growing sand spit along Dhanushkodi and the shallow reef (Adam’s Bridge) between Arimunai and Talaimannar (Sri Lanka) largely modify the sediment movement. It is strongly evident that the coastal processes taking place around the Rameshwaram Island and the exchange of the littoral drift between GOM and Palk Bay significantly determine the supply of sediments to the rest of the east coast and in turn the stability of the region.

 

 

Longshore sediment transport

During SW monsoon, the longshore sediment transport was considerable (>10X10³ m³/month) along the spit facing GOM and negligible on Palk Bay side. Very close to the tip i.e., near Arimunai, the longshore transport direction dominated in easterly direction indicating the movement from GOM to Palk Bay through Adam’s Bridge.

During NE monsoon, the values of longshore transport rate was relatively low along the spit facing GOM and negligible in Palk Bay. It is noticed that the long shore sediment transport rate was considerable (>10X10³ m³/month) in January between Uthalai and Mukkuperiyar. The sediment transport direction was consistently towards west in GOM and east in Palk Bay.

In fair weather period, the longshore sediment transport was low along the spit facing GOM and Palk Bay. The transport direction was observed to be westerly near the tip facing GOM. It shows that in February, April and May the sediment drifts from Palk Bay to GOM and the net quantity is found to be 8000 m³, 6000 m³, 20000 m³ respectively. Consequently, in March, June, July, August and September, it drifts from GOM towards Palk Bay and the respective quantities are 8000 m³, 35000 m³, 10000 m³, 4000 m³ and 1000 m³ respectively. There was no significant movement of sediment observed during October to January. It means that during SW monsoon, sediments move from GOM to Palk Bay and during fair weather period from Palk Bay to GOM. No noticeable exchange due to wave induced longshore transport takes place in NE monsoon. It is noticed that over a period of one year, a net volume of 27000 m3 sediments as a wave induced longshore transport move from GOM to Palk Bay around Adam’s Bridge.

The sediment transport rate is practically negligible throughout the year, particularly between Valinokkam and Kondugal in GOM, and between Arimunai and Ariyaman in Palk Bay. The geomorphological formation of inner part of GOM and the presence of many offshore islands are the main reasons for wave attenuation and reduction in sediment transport.

The coastal segment between Tuticorin and Valinokkam experienced relatively higher sediment transport rate during NE monsoon, but remains calm during the rest of the year. However, the small stretch between Vembar and Naripaiyur experienced relatively higher sediment transport rate also during SW monsoon. The only coastline between Uthalai and Arimunai experienced relatively higher sediment transport rate both during SW monsoon and fair weather period, with relatively low sediment transport during NE monsoon.

The direction of sediment transport during SW monsoon remained easterly between Tuticorin and Arimunai except near Kondugal and Dhanushkodi, where it was in the opposite direction i.e., towards west. Due to the reversal of sediment transport direction near Kondugal, the easterly transport gets deposited in the vicinity of Pamban Pass, Kurusadai Tivu and Shingle Tivu. Once again easterly transport along Vedalai terminates near Dhanushkodi which would cause the formation of shoals in the vicinity of Arimunai. Such formation of submerged shoals was observed south of Arimunai during the study period. The prevalence of easterly transport at Arimunai might cause part of the sediments deposited as shoals to migrate towards Adam’s Bridge and enter into Palk Bay. This process of sediment migration was noticed close to Adam’s Bridge. Hence a small proportion of littoral drift deposited during SW monsoon close to Pamban Pass and Arimunai has the tendency to enter Palk Bay.

During the NE monsoon, the sediment transport rate was very low moving in southerly direction between Tuticorin and Valinokkam and it was negligible between Valinokkam and Mandapam. Between Kondugal and Arimunai, the transport was relatively low in westerly direction. It implies that there will be a deposition of littoral drift in the vicinity of Pamban Pass. Due to the low littoral drift taking place during NE monsoon, the quantity of sediments entering GOM from Palk Bay will be much lower than the quantity moving from GOM to Palk Bay during SW monsoon.

During fair weather period, the sediment transport rate along the entire study region except between Uthalai and Arimunai remains negligible. The sediment transport between Uthalai and Arimunai exists relatively low in westerly direction for which the source of sediment is expected from Palk Bay through Adam’s Bridge.

Due to low sediment transport prevailing in the study region, which comprises of about 10 percent compared to the rest of Indian east coast, the volume of sediment exchange is expected to be low, During SW monsoon, the sizeable portion of littoral drift from west coast passing around Kanyakumari is seen deposited before reaching Tuticorin. This deposited sediment is supplied back for the westerly transport during NE monsoon. Such deposition is evidenced from the occurrence of large beach deposits and elevated dunes along Tiruchendur – Manapad region. Similarly, the southerly transport along the east coast during NE monsoon gets deposited between Vedaranyam and Manmelkudi in Palk Bay, which is supplied back to the littoral drift cycle during SW monsoon.

Thus the study indicates that there is a break in the chain of littoral drift at Tuticorin on the south and Vedaranyam is relatively low and there exists limited quantity of exchange through Pamban Pass and Adam’s Bridge.

It signifies that theregion around Adam’s Bridge forms as significant sink for the littoral drift. The prolonged accumulation may lead to the emergence of new islands. In case of occurrence of cyclones in GOM, such prolonged deposition of sediments move north and enter in Palk Bay through Pamban Pass and Adam’s Bridge. Once the sediments enter Palk Bay, the environment favours immeadiate deposition. Hence the occurrence of cyclones in GOM and the associated high northerly waves might exchange more sediments from the southern part of Peninsular India to the northern part of the east coast. Similarly any cyclones moving in Palk Bay, would generate large southerly waves and transport sizeable amount of deposited sediments into GOM. In the event of absence of cyclones, the deposition will increase causing the enlargement of sand spit across Adam’s Bridge, but the order of sediment exchange will be limited. (NEERI EIA, p- 2.12 – 2.22)

 

4.2.16 Spit Configuration

The numerical modeling study for the region around Rameshwaram indicates that due to tidal currents, in SW monsoon, the sediment transport is 6000 m³ and 30000 m³ through Pamban Pass and Arimunai respectively moving from GOM to Palk Bay. The same phenomenon continued in fair weather period indicating 3000 m³ and 16500 m³ through Pamban Pass and Arimunai respectively moving from GOM to Palk Bay.

On the other hand, during NE monsoon, about 1500 m³ and 21000 m³ of sediments are being transported through Pamban Pass and Arimunai respectively from Palk Bay to GOM. It shows that in an annual cycle, a net exchange of 6000 m³ of sediment is found to move from Palk Bay to GOM through Pamban Pass and 25,500 m³ of sediment moves from GOM to Palk Bay through Arimunai. The modeling study indicated that the volume of sediment exchange due to tidal current (25,500 m³/year) is very close to the volume being transported through littoral drift in breaker zone (24000 m³/year).

The annual gross longshore sediment transport rate along the study region remained less than 0.1X10⁶ m³/year, which shows only 10 percent of the rest of the Indian east coast.

In February, April, May the wave induced littoral drift is taking place from Palk Bay to GOM and the net quantity is found to be 8000 m³, 6000 m³, 2000 m³ respectively. Consequently, in March, June, July, August and September, it drifts from GOM to Palk Bay and the quantity is 8000 m³, 35000 m³, 10000 m³, 4000 m³ and 1000 m³ respectively. There was no significant movement of sediment between October and January. Over a period of one yea, the net volume of 24000 m³/year of sediments moves from GOM to Palk Bay. Adam’s Bridge forms as noticeable sink for the littoral drift. The prolonged accumulation leads to the emergence of new islands.

 

Season	Pamban Pass (m3)	Adam’s Bridge (m3)
SW monsoon	-6000	-30000
Fair Weather	-3000	-16500
NE monsoon	+15000	+21000
Net	+6000 m3/year	-25500 m3/year

(-)   =  Towards Palk Bay

(+)  =  Towards GOM

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             

The modeling study indicates that over an annual cycle, the net volume of sediment exchange due to tidal current is 6000 m³, from Palk Bay to GOM through Pamban Pass and 25500 m³ from GOM to Palk Bay through Arimunai.

The satellite imageries show that the spit gets deflected towards Palk Bay during SW monsoon indicating erosion of GOM side and deposition on Palk Bay side. During NE monsoon, the spit gets deposited on GOM side and eroded in Palk Bay side, but the overall length increased by 150 m towards Adam’s Bridge.

The sand spit extended 455 m in seven years indicating an average growth of 65 m in a year; the width increased 200 m at 1 km distance from the tip.  (NEERI EIA, p-2.22 – 2.33)