Authors: M.V.Ramanamurthy, S.Sundaramoorthy, Y.Pari*, V.Ranga Rao, P.Mishra, M.Bhat, Tune Usha, R. Venkatesan +, and B.R.SubramanianIntegrated Coastal and Marine Area Management (ICMAM) Project Directorate, Department of Ocean Development, Chennai ,

*Institute for Ocean Management, Anna University, Chennai + National Institute of Ocean Technology, Chennai

 # Contents of this report already under publication in Current Science, India and more detailed reports on impact of tsunami available in http://dod.nic.in/ http://www.icmam.gov.in/

Summary

The December 26, 2004 earthquake with a magnitude of M 9.0 off Sumatra, Indonesia, occurred at 6.29 am (IST) generated giant tsunami. Within few minutes after the earthquake, tsunami waves hit the Andaman and Nicobar group of islands. The Nicobar group experienced loss of human life. Due to the geometry of the Andaman islands, except Little Andaman, the impact on human life was less but extensive damage was done to the structures like jetties and boats. The tsunami hit the coast of Tamilnadu at 9 am. Nagapattinam coast suffered maximum human casualty of 6065 due to its geographic location and low coastal land elevation and high population density near the coast.

Data on seawater run-up levels recorded during tsunamis can help determining vulnerability lines for human safety. Field measurements have indicated the run-up levels varying from 1.5 to 5 m in Andaman Islands and 3 to 7 m in Nicobar Islands. In the case of Tamilnadu, it was between 2.8 and 3.9 m. Locations having gentle land slope values showed penetration of seawater to long distances compared to locations with fewer slopes. Penetration of seawater during the tsunami to long distances in Car Nicobar, Little Andaman and Nagapattinam revealed the vulnerability of low lying areas having gentle slope beaches/coastal land slopes to storm surges, tsunamis etc. Land subsidence has aggravated the problem of seawater inundation in A & N islands and the low lying areas adjoining the creeks face permanent problem of seawater inundation during high tides. Adoption of elevation based setback lines or vulnerability lines is recommended for human settlement planning along the coastal areas.

Introduction

The Andaman and Nicobar islands located in the subduction zone of Burma Plate is classified as Seismic zone 5 indicating high level of risk due to earthquake. A tsunami was generated on 26 th December, 2004 by an earthquake of magnitude M 9.0 occurred near Sumatra Island of Indonesia with the epicentre located on the shallow depths of seabed(Source : USGS). Tsunami waves hit the Andaman and Nicobar group of islands within few minutes. The tsunami waves hit Indonesia, Andaman and Nicobar islands, parts of East Coast of India, Thailand, Sri Lanka, Maldives and West and East Africa. The details of wave height and velocity remain unknown as there were no measurement devices placed at these locations except the National Institute of Ocean Technology’s (NIOT) Acoustic Tide Gauge (ATG) and Survey of India tide gauge located at Port Blair to measure the sea level. Since the NIOT tide gauge is located inside a bay (Chattam island) and set only for normal tidal variations, it could record the sea water level only upto 3.5 m, which is about 1.5 m more than normal level (Figure 1 ).The sharp rise in sea water level was recorded between 7.01 and 7.25 am and again at 7.45 am on 26 th December 2004.

Methods and Analysis

Runup measurements at different sites along the A & N and Tamil Nadu coasts were made using Realtime Kinematic Global Positioning System (RTKGPS) during the months of Janunary and February, 2005. Transect run from seawater upto maximum inundation point was identified from 1) deposition of washed materials and/or 2) degraded grass/vegetation or 3) seawater level mark on external walls of buildings. These transects were drawn at each location representing characteristics of open coast with built up and transect adjacent to inlet/ backwater. All the levels taken along the transects connected to GTS bench mark wherever it was available and in other places the sea levels reported in Tide table for the nearest location were taken as references. All run up measurement levels were corrected to tide and reduced to Mean Sea Level. The water point was measured in backwater/ bay/ inlet to minimise error in measuring the Sea Level. The highest point of inundation was also mapped along the identified signature using ARCPAD Differential Global Positioning System, where the inundation points were tracked using “Polyline” feature, at an interval of 2 m. Names of locations, where run-up level measurements were made have been specified in Table 1.

Interpretation of Results

In general, the extent of vertical run-up of seawater during tsunamis depends on earthquake parameters, geographical location, direction, velocity and frequency of tsunami waves, nearshore bathymetry, beach profile and land topography. Due to these parametric variations in Andaman and Nicobar Islands (A & N) and Tamilnadu coasts, the run-up levels and landward penetration characteristics of seawater were location specific and varied within a location and even in an island (Figure 2 & Table 1). In the case of A & N, in the North and South Andaman group of islands the run up levels varied from 1.5 m to 4.5 m and the distance penetration from the coast ranged from 100 to 250m (Table 1).

2.9,3.5,3.7,3.9,4.5

Figure 2 - Run-up levels (in meters) at selected locations in Andaman (A) & Nicobar (B) Islands and Tamilnadu Coasts (C) (Names of locations are available in Table 1 against their run-up values)

The little Andaman recorded a run up of 5 m with the distance of penetration 1200m. In the two Nicobar islands, the run up levels varied from 3 to 7 m with distance of penetration ranging from 50 to 1000m with higher run up levels and longer penetration noted in Car Nicobar (Table 1).

Preliminary conclusions drawn by Bilham et.al. (2005) on the slip pattern of 26 Dec 2004 indicate that due to high rate of slip in the southern 650 km of the 1300 km North -South rupture zone of 2004 Andaman-Sumatra earthquake, the principal tsunami was generated in the Sumatra area. Time lag between earthquake and land subsidence in Port Blair (S.Andaman) on 26 Dec 2004 which is estimated to be 30-38 min has been interpreted as that the rate of slip was slow in the Andaman region resulting generation of no tsunami in this zone. Therefore, the wide variation in the run up levels between Andaman and Nicobar islands was primarily due to the remoteness of North, Middle and South Andaman islands relative to Nicobar group to the tsunami source zone and also due to nature of land topography in the run-up level measurement locations.

In order to correlate the run up levels with respect to land topography, the coastal land slope values were calculated based on RTKGPS data and these values are given in Table 2. The data indicates penetration of seawater to a short distance in Andaman islands (except Little Andaman) compared to the Nicobar group, which is due to the presence of elevated areas within short distance from the coasts in the North, Middle and South Andaman. The Little Andaman and the Car Nicobar islands that had relatively gentle slopes along the coast compared to the South Andaman island, experienced farthest penetration of seawater (Table 2) apart from the fact that they are located close to Tsunami wave source. The slope value of 1 in 32 for Chidyatopu in South Andaman compared to slope values of 1 in 325 for Little Andaman and 1 in 167 for Car Nicobar support this interpretation (Table 2). This clearly indicates the vulnerability of low lying areas with gentle beaches/land slopes to inundation of seawater during storms, tsunamis etc. The low lying areas adjoining the creeks which facilitate travel of tsunami waves far inland, are too vulnerable as indicated by the landward penetration of seawater upto 2 km from the creek in Sippighat area of Port Blair, Andaman (Table 1).

Table 1. Run-up level of sea water during tsunami at selected locations in Andaman & Nicobar Islands and along Tamilnadu coasts

Table 2. Coastal land slope values of various locations in Andaman and Nicobar Islands and Tamilnadu

Another major reason for inundation of seawater in South Andaman and Nicobar islands is land subsidence.. Location specific observations made by ICMAM PD in association with the Andaman and Lakshadweep Harbour Works indicate 0.8m around Port Blair and 1.3 m in Great Nicobar. Such a land subsidence is evident from the high tide water entering into the paddy fields of Sippighat area that registered penetration of seawater upto 2 km during tsunami. Inundation of inland low lying areas during high tide has become a cause for concern to local population as their houses are marooned in seawater. The concern is likely to get aggravated during the monsoon months when the rain water antagonizes movement of high tide water. The net effect would depend on the velocity of rain water flowing from low lying areas through sluice gates to the adjoining bay. If the tidal force dominates, it may be possible that the rain water may accumulate in all low lying areas and both the freshwater and sea water would increase the height of water level and likely to spread to the neighbouring elevated areas too. However, atleast a full year observation is required to confirm this apprehension.

The run up levels along the Tamilnadu coast, showed almost a similar trend as noted in A & N islands. The worst affected Nagapattinam showed longer penetration of seawater (750m) upto an elevation of 3.9 m due to the gentle slope of coastal land combined with the effect of tsunami wave diffraction caused by northern tip of Sri Lanka Presence of creeks like Vedaranyam canal in Nagapattinam facilitated the seawater inundation upto 2.2 km inland (Figure 3). The Chennai areas showed less landward penetration of seawater (45 to 200m) due to prevalence of wider elevated beach (2.8m), which have acted as barriers. The slope value of 1 in 227 for Nagapattinam compared to 1 in 39 at Chennai showing gentleness in Nagapattinam further supports this interpretation (Table 2). The presence of offshore shoals along the Ennore coast near Chennai (4 – 7 km from coast) subdued the effect of tsunami waves, saving the village of Kalanji where the seawater had penetrated only upto 45 m (Table 1).

The run up level of 7m in Car Nicobar and 3.9m in Nagapattinam and the prevalence of these elevations at distances of 1.1 km and 750m respectively, where moderate to large settlements occur close to the coast, indicate the need to consider elevation based setback line in human settlement planning along the coastal areas of the country (Figures 3 & 4). In Nagapattinam, there are dense households from the coast upto 1.5 Km. Tsunami has devastated the Nagapattinam area with a death toll of 6065 people and damaging as many as 40,000 houses (Source : TN Govt. website dt. 14 Feb.2005). The loss of human life in Car Nicobar was 1138 (Source: A&N Administration Notice dated 23.01.2005). In fact, it is well known that Nagapattinam is one of the low lying coastal areas of the country and has experienced the fury of storm surges several times as it is known to be one of the possible landfall points for cyclones. The present run-up levels and inundation distances can be used as guidance to determine the safe locations for resettlement of affected population for the present.

Figure 3 – Satellite image of Nagapatinam showing inundation of seawater in settlement areas during tsunami

The observations of protection offered to human settlement by wide and elevated beaches and shoals along the coastal stretches clearly indicate the need to protect them from the threat of erosion.

Acknowledgement

The authors express their sincere thanks to Dr. Harsh K. Gupta, Secretary, Department of Ocean Development for guidance and encouragement.

Reference:

Bilham, R, E.R.Engdhal, N.Feldl and S.P.Satyabala. Partial and Complete Rupture of the Indo-Andaman plate boundary 1847-2004. Seism.Res.Lett. (2005). Draft dt. 4 March 2005 published in the internet (http://cires.colorado.edu/~bilham/IndonesiAndaman2004_files/AndamanSRL4Mar.htm)

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