The Indian Ocean Tsunami: The Global Response to a Natural Disaster

4: Tsunami Inundations and Their Impact in the Kaveri River Delta, Tamil Nadu, India

99 4 Tsunami Inundations and Their Impact in the Kaveri River Delta,Tamil Nadu,India S. Rani Senthamarai and J. Francis Lawrence The Indian Ocean Tsunami inundated large areas along the southeast coast of India. This chapter discusses the mapping of tsunami inundations and the impact of seawater intrusions on the groundwater of the area. Inundation distance, run-up level, and post-tsunami water quality of groundwater were measured and assessed. The methodology for these measurements was developed by the Department of Science and Technology, government of India, to record and document the run-up, inundation, and infiltration along the tsunami-affected areas on the Tamil Nadu coast. The methodology is schematically described in figure 4.1 using the geospatial technologies of remote sensing, global positioning system (GPS), and geographic information system (GIS). The study is restricted to two areas as case studies: (1) the coastal stretch running westward from Kodiyakarai (Vedharanyam) and (2) the area southward from Karaikal of Puducherry (Pondicherry) state. These stretches are located in the delta districts of Thanjavur, Nagapattinam, and Tiruvalur of Tamil Nadu and the Karaikal region of the Union territory of Puducherry. Tsunami Inundation When tsunami waves approach the shore, their speed declines as they begin to “feel” the bottom, and their height drastically increases. As the waves strike shore,they may inundate low-lying coastal areas,resulting in mass destruction and loss of life. When the waves of a tsunami approach the land, their appearance and behavior depends on the topography of the seafloor, the shape of the shoreline,and the nature of the land topography.When a tsunami wave encoun- 100 Senthamarai and Lawrence ters shallow waters surrounding the shoreline, its height can increase from 1 m (3.3 feet) or less to over 20 m (66 feet). Wave heights may increase when concentrated on headlands or when traveling into bays having wide entrances that become progressively very narrow.Geomorphic features of the near shore and coastal land of an area may alter the inundation pattern of tsunami waves. For instance, the presence of an offshore coral reef or shoal in the near shore and sand dunes or coastal forestry on the land may dissipate the energy of a tsunami, decreasing the impact on the shoreline (Subramaniam 2006). During the tsunami, the maximum vertical height to which the water is observed with reference to sea level (spring tide or mean sea level) is referred to as run-up. The maximum horizontal distance that is reached by a tsunami is referred to as inundation. The run-up and horizontal inundation during a tsunami may be highly variable in a local area depending on the underwater topography, orientation to the oncoming wave, tide level, and the magnitude of the tsunami. The most common method for determining tsunami wave height is measuring the run-up, the highest vertical point reached by the wave. Run-up heights Collection of Ground Control points (GCP) Geo referencing: • IRS LISS III Imagery- Post tsunami • Cadastral Maps of coastal Villages Identification of maximum extent of inundation by: •Deposition of washed materials •Degraded grass/vegetation •Seawater level mark on the building walls •Local enquiry Maximum extent of inundation mapping: • Using ARCPAD Differential Global Positioning System • "Polyline" Features - interval of 2 meters Elevation Profile to determine the run-up level • Measurement of land elevation by Real Time Kinematic Global Positioning System • Different sites along the coast Ground water Samples: Bore Wells, Hand Pumps, Dug Wells and Dug cum Bore wells Geochemical parameters: • TDS, major cations and anions harness, Corrosivity Ratio and CI / HCO3 + CO3 Geophysics Measurements • Resistivity Preparation of Thematic Layers • Cadastral land Parcel • Elevation of Contours • Geomorphology • Land cover/land use • Extent of Inundation • Ground water Quality Map Fig. 4.1. Methodology for tsunami water inundation mapping and groundwater quality. Tsunami Inundations and Their Impact in the Kaveri River Delta 101 are measured by looking at the distance and extent of salt-killed vegetation and the debris left once the wave has receded. Measurements of run-up level are useful in determining the extent of an area’s vulnerability to tsunami waves and delineating areas vulnerable to coastal hazards to manage the coastal ecology and environment. Vulnerability maps are prepared, using spatial data for the area on land use, geomorphology, land elevation, bathymetry, extent of inundation , and socioeconomics. Mathematical models may be applied to define vulnerability from multiple and complex parameters, and remote sensing and...