Correlation Between Coastal Geomorphology and ... - Springer Link

J. Ocean Univ. China (Oceanic and Coastal Sea Research) DOI 10.1007/s11802-012-1794-0 ISSN 1672-5182, 2012 11 (1): 1-6 http://www.ouc.edu.cn/xbywb/ E-mail:[email protected]

Correlation Between Coastal Geomorphology and Tsunami Inundation Along the Coast of Kanyakumari, India N. Chandrasekar1), S. Saravanan1), *, M. Rajamanickam1), C. Hentry1), and G. V. Rajamanickam2) 1) Centre for GeoTechnology, Manonmaniam Sundaranar University, Tirunelveli, India 2) Department of Disaster Management, SASTRA University, Thanjavur, India (Received September 28, 2010; revised May 16, 2011; accepted November 11, 2011) © Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2012 Abstract An investigation has been carried out in the vicinity of the coastal villages of Kanyakumari District, India to decode the influence of coastal geomorphology on inundation degree and run-up level. Even though the tsunami waves approach the study area in different patterns, the consequences are found to be mainly dependent upon the coastal configuration and local geographic setting, the study area are considered to be of three types based upon the geomorphic arrangement, namely shallow coast, elevated coast and estuarine coast. The inundation and run-up level vary from coast to coast even though there is no remarkable variation in the intensity of the approaching tsunami surge. The inundation extent ranges from to 54 m to 413 m with maximum along estuarine coast and minimum along elevated coast. Estuarine coast recorded the maximum run-up level of about 6 m and the minimum of about 1 m along the elevated coast. The percentage of inundated area in the total coastal area varies between 19% to 10% along estuarine coast and elevated coast respectively. Inundation and run-up level cannot be appreciable in the inland along the elevated coast. The beaches of elevated coast are less affected whereas those of estuarine coast are highly affected. Inundation is limited in the elevated beaches along the study area. Key words

tsunami; India; coast; geomorphology; inundation

1 Introduction Tsunami is a Japanese word meaning ‘harbour waves’ used to denote a system of gravity waves formed in the sea as a result of large-scale disturbance of sea surface over a short duration of time. In the process of sea surface returning to equilibrium through a series of oscillation, waves are generated which propagate outward from the source region. It is the most devastating coastal hazard which is quite a familiar scenario along the Pacific coast (ring of fire). But tsunami is a rare event along the coast of the Indian Ocean (Kumanan, 2005). This rare phenomenon came into action on the early hours of 26th December 2004 due to an earthquake near the Sumatra Island (Indonesia) which happened due to the collision of the India plate with Burma Micro-plate. The tsunami thus generated then propagated to its maximum coverage in the Indian Ocean and conveyed immeasurable damage along the coastal regions of Indonesia, Thailand, Sri Lanka, India, part of Africa, etc. It claimed thousands of lives and inestimable properties. Tsunami Warning Systems had been established and actively functioning along the Japan coast and Pacific coast, which proved to be a * Corresponding author. E-mail: [email protected]

paramount, efficient method for monitoring and minimising the tsunami damage (Fig.1). The damage rendered by the tsunami urges the need of establishing such a system along the Indian Ocean. Researches have been carried out or in progress to decipher the science of tsunami. Dawson (1994) suggested that geomorphological processes associated with tsunami run-up and backwash are highly complex. He showed that coastal landscapes may be greatly altered not only by direct tsunami run-up orthogonal to the shoreline but also by episodes of vigorous backwash and by water flow sub-parallel to the coastline. Scheffers and Kelletat (2005) observed the relics of tsunami of the 1st November 1755 in the form of single large boulders, boulder ridges, pebbles and shells high above the modern storm level. They found that the deposition of large amount of sand by the tsunami has intensified aeolian rock sculpturing. Shuto (2001) reported that barrier spits, tombolos and sandbars were often cut by overflowing tsunamis along the coast of Japan. Channels were deepened by tsunami-induced currents and sometimes made shallow by transported sediments. He concluded that apart from erosion, tsunami could also build coastal topography. Goff et al. (2006) reported that the waves accompanying 1975 Kalapana tsunami deposited a discontinuous basalt boulder and carbonate sand veneer on the Halape-Apua Point coast of

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Chandrasekar et al. / J. Ocean Univ. China (Oceanic and Coastal Sea Research) 2012 11 (1): 1-6

the island of Hawaii. These deposits run up to 320 m inland. They pointed out that such sudden sedimentation may result in the adjustment of the coastal geomorphology. Kench et al. (2007) observed the effects of the 26th December 2004 tsunami based on pre- and post-tsunami topographic and planform surveys of 13 uninhabited islands in South Maalhosmadulu atoll, central Maldives. Their survey shows that there was no extreme island erosion or significant change in vegetated island area. Instead, the tsunami accentuated predictable seasonal oscillations in shoreline change promoting localised retreat of exposed island scarps and deposition of cuspate spits to leeward and Vertical Island building through overwash deposition of sand and coral clasts. Clague et al. (2005) examined sediment cores to assess the record of tsunami inundations in the Fraser River delta, Canada. Their investigation revealed that even though there is no evidence of tsunami deposits in the Fraser River delta, the results suggest that tsunamis, if they have indeed occurred, have not had a noticeable impact on the geology of the delta. Chandrasekar et al. (2006) reported that the inundation extent mainly depends upon the nature of coastal geo-

morphology. Kurian et al. (2006) investigated the inundation characteristics and geomorphological changes associated with the December 2004 tsunami along the Kerala Coast, India. They detected that the river inlets had encouraged inundation. The devastation was very severe there because of the coincidence of the tsunami with the high tide. Raval (2005) reported that the severe destruction along the coast of Nagapattinam was primarily due to its typical geographic setting which favoured much inundation. Narayan et al. (2005a) insisted that the elevated landmass has played a key role in checking the inundation thereby, minimising the damage. Narayan et al. (2005b) found out that the inundation degree was strongly scattered in direct relationship to the morphology of the seashore and run-up level. Though many studies have been done on the inundation, run-up, tsunami sedimentation and tsunami-induced geomorphic changes, few studies have interpreted the role of coastal geomorphology in determining the inundation extent, run-up level. This paper aims to bring out the relationship between the geomorphic setting and the inundation and run-up level recorded along the study area.

Fig.1 Shows the damages created by tsunami waves along the low lying areas.

2 Regional Setting The study area is located along the southern coast of Tamilnadu state, India (Fig.2). The area experiences medium to high wave energy condition which results in workable deposits of placer minerals in most of the beaches. Along the shoreline between Colachel and Kanyakumari, there are a number of rocky cliffs projecting to the Arabian Sea and forming headlands. In between the headlands, wide beaches are noticed with high concentration of black sands. Between Kanyakumari and Rajakkamangalam, linear calcareous terraces covered by aeolian sands with an enrichment of heavy minerals are observed. The sandy beaches are built by the development of foredune belt as well as high sand dunes. The sand dunes are fine in constituents and well rounded with broken molluscan shells. Numerous intertidal mudflats and laccustrine ponds are

present along the study area. They do not receive any river flow but it could be a swale system. Dunes are mainly unconsolidated sand formed by aeolian activity. These coastal dunes are found in the inland sands blown to the back of the beach by tsunami wave. The coastal dunes are noticed in the areas of Tamarikulam, Thengampudur, Madhysoodhanapuram, Rajakkamangalam, and Colachel. A large parabolic dune along with a dune complex is observed in Chothavilai beach, Tamarikulam. It has a length of 5 km and the width ranges from 2 to 5 km. The dune rises up to 2 to 3 m near Tamarikulam and Rajakkamangalam. Migration of dune is also noticed between these regions. Mudflat area is seen in Tamarikulam, Madhysoodhanapuram and Rajakkamangalam. These mudflats contain silt, clay and water. They are always associated with sheltered environments like estuaries and embayments. These regions act as facilitators for tsunami wave. The most remarkable landforms like mangrove and salt marshes are also ob-


served in the Manakudi estuary, Tamarikulam. This estuary varies in length and width scored by tidal currents (Fig.3). Beach ridges are also seen in the study area with intervening sandy plains occurring parallel or sub-parallel to the shore formed by periodic wave impounding actions (Short et al., 1989). They are followed in the backshore

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by sandy plains and are discontinuous (Chandrasekar et al., 2005). These landforms act as barrier for the tsunami waves. The study area experienced the interference of reflected waves from Sri Lanka and Maldives Islands with receding waves which was responsible for intense damage reported along the study area (Narayan et al., 2005a, b).

Fig.2 Sketch map showing location of the study area.

Fig.3 Showing the estuary along the study area.

3 Materials and Methods The study has been initiated with the base map preparation along with that of cadastral map of each coastal village on 1:8000 scale on which the satellite imageries were interpolated. Intense field work has been carried out

to survey the extent of inundation and run-up level from the high tide line in each coastal village with possible regular transects. Survey was done using hand-held GPS, levelling and surveying equipment with regular interval in each transect following standard procedures. The field data were superimposed on the satellite images (LISS 4 and PAN merged data) and interpreted using GIS tech-

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niques (ArcGIS-9.1). To find out if there is any close relationship between the geomorphology of the coast and the inundation/run-up level, the study area was studied by dividing it into three types of coast based upon their overall geomorphic setting, namely, shallow coast, elevated coast and estuarine coast (Table 1). The extent of

inundation and the total coastal areas inundated and runup level in each type of coast were interpreted using standard statistical package. The inundation distance with respect to the total coastal area was deduced and the percentage of inundation in different types of coast pictured.

Table 1 Categorisation of study area based upon their geomorphic arrangement Category

Description

Beaches falling under the category

Shallow coast

Relatively equivalent (or not well above) to the MSL

Thengampudur, Madhysoodhanapuram, Dharmapuram, Rajakkamangalam, Manavalakurichi and Lakshmipuram

Elevated coast

Relatively well above the MSL

Azhakappapuram, Kanyakumari, Agastheeswaram and Kadiapatnam

Estuarine coast

Neighbouring a rivermouth/waterbody

Tamarikulam and Colachel

4 Results and Discussion The extent of inundation due to tsunami is shown in Table 1. Maximum inundation of 413 m occurrs in the estuarine coast neighbouring a river mouth/estuary. Such geomorphic feature facilitates the uprushing Tsunami waves to inundate much of the coastland, which obviously had damaged adjoining the host and the beaches. So, it is deduced that estuary and low-lying areas are the most negative features for a coast which makes it most vulnerable to such hazards (Chandrasekar et al., 2006). Similar condition has been observed by Kurian et al. (2006) along the Kerala coast where the devastation was more remarkable along the coast adjoining river inlet due to the high inundation experienced by it. The severe inundation in the river inlet/mouth not only could affect the adjoining coast but also alter the pattern of the river inlet/mouth itself by depositing the tsunami-borne sediments there or by eroding the sediments from the estuary/waterbody or by redistribution of sediments (Grauert et al., 2001; Goff et al., 2006), which is also attested by similar events along the Japan coast (Shuto, 2001). Next to estuarine coast, shallow coast had, obviously, experienced more inundation of 183 m. As it is situated at an elevation almost equivalent to the mean sea level (MSL), the invading tsunami surge would not possibly encounter with sturdy obstacle and hence could effortlessly inundate much of the coastland (Chandrasekar and Immanuel, 2005; Narayan et al., 2005a, b). Mohan (2005) observed similar scenario along the northern coast of Tamilnadu. The shallow coast though seems to be more vulnerable to tsunami, there are also various other factors which could stabilise and protect it. Mohan (2005) reported that elevated coastal dunes and beach ridges near the coastline could act as barriers, which in turn would minimise the rate of inundation. Kench et al. (2007) found that the vegetated islands of Maldives were less affected by the 26th December 2004 tsunami. This has been proved by the present study, as the inundation along the shallow coast is not much higher when compared to the estuarine coast and most of the shallow coast along the Kanyakumari district is encircled by coconut plantation and well-developed dunes. The beaches along the elevated coast were found to be less

inundated (54 m) as they are positioned well above the sea level and in many cases guarded by features such as presence of sea cliff. Narayan et al. (2005a) found that the inundation and hence, the damage was very small on or behind elevated landmass of coastal region along the Nagapattinam and Kanyakumari Districts of Tamilnadu. Kanyakumari and Kadiapatnam coast are known for their sea cliff shaped as headland whereas Azhakappapuram and Agastheeswaram coast are shielded by rock outcrops which made the coast fortified. Further, the steep gradient of the elevated coast would not support the uprushing sea water to inundate much of the coastland, making it invulnerable to such hazards (Chandrasekar et al., 2005). Table 2 and Figs.4 and 5 show the areas inundated along the study area. Out of the total shallow coast area of 80.82 km2, 11.14 km2 were inundated whereas 5.79 km2 of land were inundated out of the total elevated coastal area of 49.42 km2. The total coastal area and the inundated area along the estuarine coast are 18.86 km2 and 4.37 km2, respectively (Table 2). It is very evident from Fig.4 that large area has been inundated along the shallow coast whereas there was not much greater variation between the areas inundated along the elevated and estuarine coast. Fig.5 clearly shows that the study area mostly comprises shallow coast followed by elevated and estuarine coast. Similarly, the inundated area was also found to be very large along the shallow coast. Raval (2005) observed that among the facts which resulted in the severe destruction of Nagapattinam District, Tamilnadu, one is that most of the beaches along the coast were shallow and open. So, similar such features along the present study area were responsible for the high inundation which set the Kanyakumari District to be one of the severely affected coasts on the Indian subcontinent (Fig.6). Mohan (2005) and Reddy et al. (2005) reported similar results along the coast of Northern Tamilnadu and Andhra Pradesh, India respectively. Table 2 Total coastal area and inundated area Location Shallow coast Elevated coast Estuarine coast

Total area (km2)

Inundated area (km2)

80.82 49.42 18.86

11.14 5.79 4.34


Fig.4 Areas inundated along the study area.

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Fig.5 Inundation extent with respect to the total coastal area.

Fig.6 Shows the inundation along the open coast.

The run-up level recorded along the study area is given in Table 3. Perceptibly the run-up level reaches the maximum of about 5 to 6 m along the estuarine coast whereas a low run-up level of about 1 to 2 m has been recorded along the elevated coast. Moderate level of run-up has been reported along the shallow coast. Kurian et al. (2006) investigated the run-up level measured along the Kerala Coast and found a varied range from 1.5 to 5 m. Narayan et al. (2005b) reported the range of run-up level along the major coasts of Tamilnadu which varies between 1 to 10 m. The tsunami run-up readings are strongly scattered in direct relationship to the measured inundation extent (Narayan et al., 2005b). Estuarine coast endures high inundation and run-up level whereas these two devastating parameters have been found to be low along the elevated coast. The percentage of areas inundated in the total coastal area along the study area has been calculated for different types of coast and is given in Table 3 and Fig.7. The proportion of inundated area to the unaffected area is higher (19%) along the estuarine coast whereas it is lower (10%)

along the elevated coast. Clague et al. (2005) investigated the sediment cores of the Fraser river delta, Canada to assess the record of tsunami inundation there and concluded that there was no evidence of tsunami deposits. However, their results suggest that tsunamis, if they have indeed occurred, have not had a noticeable impact on the geology of the delta. But the estuaries along the study area should have definitely contained ample amount of tsunami-borne sediments as the inundation here was very high (Shuto, 2001; Grauert et al., 2001; Goff et al., 2006). Nearly, 12 % of the areas were inundated along the shallow coast. Though the total coastal areas inundated by the tsunami surge is very high along the shallow coast (11.14 km2), the percentage of inundation is only reasonably high along the estuarine coast, where 4.34 km2 of the total coastal area was inundated. Though ample literature is available on the various aspects of tsunami inundation, geomorphic changes induced by tsunami and run-up level, no authentic report is available indicating the percentage of coastal area inundated by the tsunami surge, especially along the study area.

Table 3 Inundation extents and run-up levels along the study area Location

Inundation extent (m)

Run-up level (m)

Inundated area (%)

Unaffected area (%)

Shallow coast Elevated coast Estuarine coast

183 54 413

3–4 1–2 5–6

12 10 19

88 90 81

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Fig.7 Percentage of inundated (pale colour) and unaffected area in different types of coast.

5 Conclusion The present study reveals the various links between the coastal geomorphology and the tsunami inundation/runup level. The extent of inundation was high along the coast adjoining rivermouth/estuary. Shallow/open coast were found to endure high inundation whereas inundation and run-up level could not dominate along the elevated coast. Run-up level increases with the increase in inundation degree. From the study, it is comprehended that the vulnerability of a coast to such hazards depends mainly upon its geomorphic setting. Further detailed studies are to be done to develop inundation models which rely on finer data on the inner shelf bathymetry, hydrodynamic characteristics and coastal topography.

Acknowledgements The authors wish to thank the National Resource Data Management System (NRDMS) Division of the Department of Science and Technology (DST), Government of India for supplying the necessary equipment and financial assistance to accomplish this study.

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