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

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1 Indian Journal of Geo-Marine Sciences Vol. 43(7), July 2014, pp Shoreline changes and reef strengthening at Kavaratti island in Lakshadweep 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 India Trivandrum *[ thiru.prakash@gmail.com] 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 Coastal erosion is a recurring natural hazard in the Lakshadweep islands, which affects the stability of the coastline. Erosion is the result of natural processes as well as anthropogenic activities. The natural reefs bordering the lagoon on the western part of the island is protecting the coast from the fury of monsoon waves. Over the years, the reef height in many of these islands appears to have decreased due to natural processes and human activities 1,2. As part of the developmental activities over the years, the Lakshadweep Administration have built harbour and coastal structures including the widening of channel for the entry of bigger boats/vessels near to the island for the transport of goods from the mainland. In many of these islands the widening of the entrance channel has been carried out earlier by cutting the reef edge 3. 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 model studies. Feasibility study for reconstruction of part of the reef in the northern part which was removed earlier also was carried out with the help of numerical studies (Fig. 1). Materials and Methods Data collection and analysis A beach monitoring survey involving beach profiling, collection of foreshore sediment samples and longshore current measurements were carried out from 20 permanent reference stations established by the National Centre for Earth Science Studies formerly CESS, Trivandrum at 200 to 250 m interval in the Island (Fig. 2) during Totally five sets of beach profile data were collected representing pre-monsoon and early part of post- Fig. 1 Reef strengthening on the northern part of the Kavaratti Island

2 PRAKASH et al.: SHORELINE CHANGES AND REEF STRENGTHENING AT KAVARATTI 1141 monsoon periods and the beach volume changes were computed to classify the beaches into eroding, accreting and stable types. Further in the island bi-monthly beach profiling was also carried from the same stations for the northern part of the island during Fig. 2 Map showing the beach monitoring stations 1 to 20 in the island For calculation of long-term shoreline change in the island the cadastral maps of 1967 and High Tide Level (HTL) positions mapped 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 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 the island. Results Waves, Currents, Wind and Other Littoral Parameters Recorded wave data are limited. However, the wave data recorded at Kavaratti Island 5 were 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 fairweather 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 lagoon 6. The direction of longshore current along the eastern and lagoonal side of the island is towards north during SW monsoon period. But during the NE monsoon period, southerly long shore current is predominant. In the island winds are light to moderate during October to April. Towards late summer, winds begin to strengthen and continue to be strong in the SW monsoon. Fig. 3 Typical beach profile at CSK-7

3 1142 INDIAN J. MAR. SCI., VOL. 43, NO. 7 JULY 2014 Northerly or northeasterly winds prevail during the months of December to February. Thereafter, winds gradually become northwesterly by April and westerly by May. Southwesterly to westerly winds prevail in June and July, becoming westerly to northwesterly in August and September. In October and November, winds turn to northwest or north. The average annual wind speed in the island is around 12.6 km/hour. Long-term changes ( ) The long-term erosion/accretion in the island are shown in Fig. 6. These changes have been calculated by comparison of shoreline positions of 1967 and 1999, which gives the trend of erosion, accretion in the islands 7. In order to compare the long-term erosion trends among the different inhabited islands a semi-quantitative 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. 4 Net beach volume changes along the beach monitoring stations 1 to 20 for the period Fig. 5 Net beach volume changes for the selected station on the northern part of the island during Short-term changes The net beach volume change in the island is shown in Figs. 4&5. The short-term change in the island shows moderate erosion. A stretch of 2.8 km was affected by erosion. It was observed at station CSK-15&17 on the SW part of the island near the Chicken Neck area and also on the NE portion of the island coast between CSK- 4 to 6 and 9. Fig. 6 Long-term erosion/accretion along the monitoring stations 1 to 20 for the period 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 and near helipad shows maximum shoreline recession. The remaining length of coastline is more or less stable. Discussions Shoreline Changes The morphological changes of the beach are usually reflected in changes in the shoreline position. The results of the short term beach changes during and Dec 07 to

4 PRAKASH et al.: SHORELINE CHANGES AND REEF STRENGTHENING AT KAVARATTI 1143 Dec 08 2 indicate that majority of the stations on the eastern and lagoon coast shows stable type beach with moderate accretion except near the Chicken neck and NE part of the island between CSK-4 to 7 and 9. The long-term shoreline changes also show similar trend. A coastline length of about 4.15 km (35%) is affected by erosion and is mostly seen along the stations CSK-3 to 8 i.e. on the NE corner of the island. Along the lagoon coast, erosion is (a) observed towards south of Katchery jetty between stations CSK-13 and 14. In a nutshell, the erosion at any location along the island coast is more or less compensated by a corresponding accretion at another coastal location within the island. Erosion/Accretion and Bed Level Changes The bed level changes for the three seasons (Fig. 7a-c) were computed by including the sediment Transport Module while running the Flow Model. (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 (c) Fig. 7 Mean bed level change during (a) premonsoon and (b) monsoon (c) post-monsoon The eastern part of the island coast is subjected to negative bed level change 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 postmonsoon. 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 This growth essentially acts like a groin thereby protecting the adjoining coast towards the west. However

5 1144 INDIAN J. MAR. SCI., VOL. 43, NO. 7 JULY 2014 the presence of this growth has a negative impact on the north-eastern part as it hinders the transport of sediment to the other side. Due to this there is a shortage of sediment supply to the eastern side and this could have been the main reason for high erosion reported on the coast adjoining stations 5 to 7. The validation of the simulation results for bed level change has been done by comparing with the data obtained from various sources like plots showing short-term and long-term coastline changes, field photographs (Fig. 8) taken during the various seasons. Strengthening of Reef on the Northern part of the Island The detailed analysis of the coastal circulation pattern in the island indicated that the alongshore transport is the dominant mode compared to Fig. 9 Simulation of shoreline change with reef and without reef offshore transport except near the main entrance channel. The alongshore current in the lagoon is predominantly towards north during monsoon whereas it is towards south during the postmonsoon season. The simulated current pattern also indicates that the northern region of the island extending from stations 5 to 8 experiences high current during all the three seasons compared to other areas. The feasibility study for the reconstruction of part of the reef (~400 m length) which was removed earlier as part of the channel widening at the entrance channel was modelled. Numerical models studies were conducted to simulate the nearshore wave climate and circulation pattern with the introduction of submerged artificial reef on the northern part. Impact on shoreline changes due to the introduction of artificial reef construction was also attempted. It was observed that by providing artificial reef the condition of coastal stations 9 and 10 would improve but the portion immediate east of station 7 would have a negative effect (Fig. 9). As per the simulation results this is likely to accelerate the existing erosion in this region. However as this part of the beach is mainly composed of coarse sediments with pebble/shingle and also being protected by tetrapods and concrete blocks the impact would not be that significant and can be controlled by further strengthening or proper maintenance of the existing protection. Acknowledgements The authors are grateful to the Director, National Centre for Earth Science Studies (NCESS), Trivandrum for encouragement and providing the facilities. Our thanks are due to Director, Science and Technology, Union Territory 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) Prakash, T.N. and Sheela, L.N., Studies on Shore Protection Measures for Lakshadweep Islands. Final Report Submitted to DST, UT Lakshadweep, CESS, Trivandrum (2010) Prakash, T.N. and Sheela, L.N., Studies on Shore Protection Measures for Lakshadweep Islands. Interim Report Submitted to DST, UT Lakshadweep, CESS, Trivandrum (2008) Suchindan, G.K., Prakash, T.N. and Prithviraj, M., Studies on coastal erosion, sediment movement and bathymetry in selected islands of the UT of Lakshadweep ( ) - Kavaratti, Agatti, Amini and Bangaram (Phase-I): Department of Science & Technology, Union Territory of Lakshadweep (1993) Baba, M., Shahul Hameed, T.S., Kurian, N.P. and Subhash chandran K S, Wave power of Lakshadweep Islands. Final report submitted to Department of Ocean Development, Government of India, CESS (1992) Chandramohan, P., Anand, N.M. and Nayak, B.U., Shoreline dynamics of the Lakshadweep Islands. Indian J. Mar. Sci., 22(1993) Thomas, K.V. and Prakash, T.N., HTL, LTL and No Development Zone for the islands of Lakshadweep. Project Report Submitted to DST, UT Lakshadweep, CESS, Trivandrum (2002)1-36.