Indian Journal of Geo-Marine Sciences Vol. 43(7), July 2014, pp. 1140-1144

Shoreline changes and reef strengthening at Kavaratti island in Archipelago - A case study

T.N.Prakash*, L.Sheela Nair & Tiju I. Varghese National Centre for Earth Science Studies, Ministry of Earth Sciences, Government of Trivandrum 695 011 *[E-mail:[email protected]]

Received 24 August 2013; revised 22 October 2013

The Lakshadweep islands experience severe coastal erosion problem every year due to high monsoonal wave activity. Majority of the islands in the group are surrounded by a reef on the western part which acts as a first line defence for the incoming waves. It was documented from many earlier studies that the reef height in many of these islands has decreased due to both natural and human activities. In many of the islands in the group the widening of entrance channel has been carried out by cutting reef edge as early as in 1971 resulting in high waves entering the lagoonal coast causing high erosion at certain stretches of the island coast. In this paper we are presenting the Kavaratti Island as a case study through systematic collection of beach profiles and monitoring of littoral environment observations for waves and currents. Further, an attempt has also been made for a feasibility study of reconstruction of part of the reef (~400 m length) on the northern part of the island through numerical modelling studies which indicates that the reef strengthening would stabilize the beach. Adopting suitable remedial measures to allow free movement of trapped sediments could be an environment friendly soft measure that would further improve the condition of the island coastline.

[Keywords: Lakshadweep islands, Monsoon waves, Coastal erosion, Short-term and long-term, Shoreline changes, Numerical modelling] Introduction construction of part of the reef in the northern Coastal erosion is a recurring natural hazard part which was removed earlier also was carried in the Lakshadweep islands, which affects the out with the help of numerical studies (Fig. 1). stability of the coastline. Erosion is the result of natural processes as well as anthropogenic Materials and Methods activities. The natural reefs bordering the lagoon Data collection and analysis on the western part of the island is protecting the A beach monitoring survey involving beach coast from the fury of monsoon waves. Over the profiling, collection of foreshore sediment years, the reef height in many of these islands samples and longshore current measurements appears to have decreased due to natural were carried out from 20 permanent reference processes and human activities1,2. As part of the stations established by the National Centre for developmental activities over the years, the Earth Science Studies formerly CESS, Lakshadweep Administration have built harbour Trivandrum at 200 to 250 m interval in the and coastal structures including the widening of Island (Fig. 2) during 1990-19934. Totally five channel for the entry of bigger boats/vessels sets of beach profile data were collected near to the island for the transport of goods from representing pre-monsoon and early part of post- the mainland. In many of these islands the widening of the entrance channel has been carried out earlier by cutting the reef edge3. This has resulted in high waves crossing over the reef edge and hitting the lagoon coast resulting in serious coastal erosion problem. The present study was taken up in this context to identify the critical areas of erosion in Kavaratti Island that need immediate protection on the basis of both field observations and numerical Fig. 1—Reef strengthening on the northern part of the model studies. Feasibility study for re- Kavaratti Island PRAKASH et al.: SHORELINE CHANGES AND REEF STRENGTHENING AT KAVARATTI 1141 monsoon periods and the beach volume changes Results were computed to classify the beaches into Waves, Currents, Wind and Other Littoral eroding, accreting and stable types. Further in Parameters the island bi-monthly beach profiling was also Recorded wave data are limited. However, carried from the same stations for the northern the wave data recorded at Kavaratti Island5 were part of the island during 2007-2008. used to understand the wave regime of Lakshadweep Archipelago. The wave height ranges from 0.56 to 8.95 m with lowest being in

February and highest in August. The wave activity is less intense during November to March. During the SW monsoon period higher wave activity was observed and significant wave height was around 5 m. About two-third of the distribution of wave period was in the range of 6-8 s during SW monsoon and 5-9 s during fair- weather period. The dominant wave directions are from 200° to 210° and 260° to 270°. Due to the presence of deep and wide reef flats on the open seaside, the wave induced longshare currents along the eastern coast are relatively stronger (0.2 to 0.3 m/s) compared to the lagoon side of the island. On the lagoon side the longshare current is very feeble or no movement is observed. The variations of the longshore current velocity and direction with change in tide levels were measured at Kavaratti lagoon6. The direction of longshore current along the eastern and lagoonal side of the island is Fig. 2—Map showing the beach monitoring stations 1 to 20 in the island towards north during SW monsoon period. But during the NE monsoon period, southerly long shore current is predominant. In the island winds For calculation of long-term shoreline change are light to moderate during October to April. in the island the cadastral maps of 1967 and Towards late summer, winds begin to strengthen High Tide Level (HTL) positions mapped and continue to be strong in the SW monsoon. during 1999 were used and trends of erosion/accretion over a period of 30 years were calculated. Further the long-term beach changes in the island were also estimated by comparing the profiles of 1990 and December 2007. Along with the bi-monthly beach profiling work, littoral environmental observations were also made. The nearshore wave heights and circulation pattern during three principal seasons were simulated using the Spectral Wave and Hydrodynamic Modules of MIKE 21 modelling system respectively. This was calibrated with the available long-term wave and current data in Fig. 3—Typical beach profile at CSK-7 the island.

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Northerly or northeasterly winds prevail Long-term changes (1967-1999) during the months of December to February. The long-term erosion/accretion in the island Thereafter, winds gradually become are shown in Fig. 6. These changes have been northwesterly by April and westerly by May. calculated by comparison of shoreline positions Southwesterly to westerly winds prevail in June of 1967 and 1999, which gives the trend of and July, becoming westerly to northwesterly in erosion, accretion in the islands7. In order to August and September. In October and compare the long-term erosion trends among the November, winds turn to northwest or north. different inhabited islands a semi-quantitative The average annual wind speed in the island is around 12.6 km/hour.

Shoreline Changes Both short-and long-term shoreline changes in the island are worked out for the island. Short-term shoreline changes have been calculated using beach profile data. A typical beach profile of the island is given in Fig. 3. Long-term shoreline changes are also presented for the island.

Fig. 6—Long-term erosion/accretion along the monitoring stations 1 to 20 for the period 1967 – Fig. 4—Net beach volume changes along the beach 1999 monitoring stations 1 to 20 for the period 1990-1993 approach utilising the length of coastline affected by erosion has been used.

Both erosion and accretion are evident in the island. About 30-40% of the shoreline is affected by erosion. On the whole, the erosion at a shoreline stretch in an island is more or less compensated by accretion at another place in the same island. The island coast along northeast Fig. 5—Net beach volume changes for the selected and near helipad shows maximum shoreline station on the northern part of the island during 2007- recession. The remaining length of coastline is 08 more or less stable. Short-term changes The net beach volume change in the island is Discussions shown in Figs. 4&5. The short-term change in Shoreline Changes the island shows moderate erosion. A stretch of The morphological changes of the beach are 2.8 km was affected by erosion. It was observed usually reflected in changes in the shoreline at station CSK-15&17 on the SW part of the position. The results of the short term beach island near the Chicken Neck area and also on changes during 1990-1993 and Dec’07 to the NE portion of the island coast between CSK- 4 to 6 and 9.

PRAKASH et al.: SHORELINE CHANGES AND REEF STRENGTHENING AT KAVARATTI 1143

Dec’082 indicate that majority of the stations observed towards south of Katchery jetty on the eastern and lagoon coast shows stable between stations CSK-13 and 14. In a nutshell, type beach with moderate accretion except near the erosion at any location along the island coast the Chicken neck and NE part of the island is more or less compensated by a corresponding between CSK-4 to 7 and 9. The long-term accretion at another coastal location within the shoreline changes also show similar trend. A island. coastline length of about 4.15 km (35%) is Erosion/Accretion and Bed Level Changes affected by erosion and is mostly seen along the The bed level changes for the three seasons stations CSK-3 to 8 i.e. on the NE corner of the (Fig. 7a-c) were computed by including the island. Along the lagoon coast, erosion is sediment Transport Module while running the Flow Model.

(a) (a)

(b)

(b)

Fig. 8—Field photographs of the northern part of the island at station CSK-8 (a) Erosion towards west (b) Accretion towards East

The eastern part of the island coast is subjected to negative bed level change (c) compared to that of western part indicating higher erosion in this region. The beach in front of coastal station 8 is invariably an accreting beach even though there is a slight reduction in the accretion during pre-monsoon and post- monsoon. Maximum accretion is seen during post-monsoon season and the minimum during monsoon. The accretion tendency is clearly due to the impact of the recent spit like growth that is seen adjacent to Station 8 formed during the passage of cyclone in 20043. This growth essentially acts like a groin thereby protecting Fig. 7—Mean bed level change during (a) pre- monsoon and (b) monsoon (c) post-monsoon the adjoining coast towards the west. However

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the presence of this growth has a negative also attempted. It was observed that by impact on the north-eastern part as it hinders the providing artificial reef the condition of coastal transport of sediment to the other side. Due to stations 9 and 10 would improve but the portion this there is a shortage of sediment supply to the immediate east of station 7 would have a eastern side and this could have been the main negative effect (Fig. 9). As per the simulation reason for high erosion reported on the coast results this is likely to accelerate the existing adjoining stations 5 to 7. The validation of the erosion in this region. However as this part of simulation results for bed level change has been the beach is mainly composed of coarse done by comparing with the data obtained from sediments with pebble/shingle and also being various sources like – plots showing short-term protected by tetrapods and concrete blocks the and long-term coastline changes, field impact would not be that significant and can be photographs (Fig. 8) taken during the various controlled by further strengthening or proper seasons. maintenance of the existing protection. Strengthening of Reef on the Northern part of the Island Acknowledgements The detailed analysis of the coastal The authors are grateful to the Director, circulation pattern in the island indicated that National Centre for Earth Science Studies the alongshore transport is the dominant mode (NCESS), Trivandrum for encouragement and compared to providing the facilities. Our thanks are due to Director, Science and Technology, of Lakshadweep for sponsoring the project.

References 1 Pillai, C.S.G. and Thomas, P.A., Environmental deterioration in Lakshadweep and need for conservation of living resources, Geol. Surv. India Spec. Publ., 56(2001)171-174. 2 Prakash, T.N. and Sheela, L.N., Studies on Shore Protection Measures for Lakshadweep Islands. Final Report Submitted to DST, UT Lakshadweep, CESS, Fig. 9—Simulation of shoreline change with reef and Trivandrum (2010)1-60. without reef 3 Prakash, T.N. and Sheela, L.N., Studies on Shore Protection Measures for Lakshadweep Islands. offshore transport except near the main entrance Interim Report Submitted to DST, UT Lakshadweep, channel. The alongshore current in the lagoon CESS, Trivandrum (2008)1-30. is predominantly towards north during monsoon 4 Suchindan, G.K., Prakash, T.N. and Prithviraj, M., Studies on coastal erosion, sediment movement and whereas it is towards south during the post- bathymetry in selected islands of the UT of monsoon season. The simulated current pattern Lakshadweep (1990-1993) - Kavaratti, Agatti, Amini also indicates that the northern region of the and Bangaram (Phase-I): Department of Science & island extending from stations 5 to 8 Technology, Union Territory of Lakshadweep experiences high current during all the three (1993)1-116. 5 Baba, M., Shahul Hameed, T.S., Kurian, N.P. and seasons compared to other areas. Subhash chandran K S, Wave power of Lakshadweep The feasibility study for the reconstruction of Islands. Final report submitted to Department of part of the reef (~400 m length) which was Ocean Development, , CESS removed earlier as part of the channel widening (1992)1-63. 6 Chandramohan, P., Anand, N.M. and Nayak, B.U., at the entrance channel was modelled. Shoreline dynamics of the Lakshadweep Islands. Numerical models studies were conducted to Indian J. Mar. Sci., 22(1993)198-202. simulate the nearshore wave climate and 7 Thomas, K.V. and Prakash, T.N., HTL, LTL and No circulation pattern with the introduction of Development Zone for the islands of Lakshadweep. submerged artificial reef on the northern part. Project Report Submitted to DST, UT Lakshadweep, CESS, Trivandrum (2002)1-36. Impact on shoreline changes due to the introduction of artificial reef construction was