Ocean modellers use bathymetric datasets like ETOPO5 and ETOPO2 to represent the ocean bottom topography. The former dataset is based on digitization of depth contours greater than 200 m, and the latter is based on satellite altimetry. Hence, they are not always reliable in shallow regions. An improved shelf bathymetry for the Indian Ocean region (20 • E to 112 • E and 38 • S to 32 • N) is derived by digitizing the depth contours and sounding depths less than 200 m from the hydrographic charts published by the National Hydrographic Office, India. The digitized data are then gridded and used to modify the existing ETOPO5 and ETOPO2 datasets for depths less than 200 m. In combining the digitized data with the original ETOPO dataset, we apply an appropriate blending technique near the 200 m contour to ensure smooth merging of the datasets. Using the modified ETOPO5, we demonstrate that the original ETOPO5 is indeed inaccurate in depths of less than 200 m and has features that are not actually present on the ocean bottom. Though the present version of ETOPO2 (ETOPO2v2) is a better bathymetry compared to its earlier versions, there are still differences between the ETOPO2v2 and the modified ETOPO2. We assess the improvements of these bathymetric grids with the performance of existing models of tidal circulation and tsunami propagation.

Estimates of return periods of extreme sea level events along the coast are useful for impact assessment. In this study, a vertically integrated 2D model was developed for the simulation of storm surges in the Bay of Bengal. The bathymetry for the model was derived from an improved ETOPO-5 dataset, which was prepared in our earlier work. The meteorological forcing for the model was obtained from the cyclone model of Holland using the data available for 136 low pressure systems (LPS) that occurred during 1974-2000 in the Bay of Bengal. The simulated total sea level and the surge component were obtained for each event. The simulated peak levels showed good agreement with the observations available at few stations. The annual maxima of sea levels, extracted from the simulations, were fitted with Gumbel distribution using r-largest annual maxima method to estimate the 5-year and 50-year return periods of extreme events at 26 stations along the east coast of India. The return periods estimated from simulated sea levels showed good agreement with those obtained from observations. The 5-year and 50-year return levels of total sea level along the east coast of India show a considerable increase from south to north, with the 50year return total sea levels being as high as 6.9 m and 8.7 m at stations along the north eastern coast such as Sagar Island and Chandipur, respectively. The high return levels are expected at these stations as the cyclones developed in the Bay of Bengal generally move north or north-west, producing extreme events in the northern part and moreover, these stations are characterized by high tidal ranges. However, at some regions in the southern part such as Surya Lanka and Machilipatnam, though 50-year return levels of total sea-level are not very high (2.98 m and 2.97 m, respectively) because of the relatively lower tidal ranges, high return levels of surges (0.84 m and 0.57 m, respectively) are found. In addition to the role of shallow bathymetry (-5.0 m and 6.1 m, respectively) at the two stations, the high return levels of surges are attributed to the effect of geometrical configuration at Surya Lanka and width and orientation of continental shelf (100 km) at Machilipatnam.

A vertically integrated 2D numerical model was developed for the simulation of major tidal constituents (M 2 , S 2 , N 2 , K 1 and O 1 ) in the Bay of Bengal. The bathymetry for the model domain was derived from an improved ETOPO5 dataset prepared in our earlier work. The simulated tidal elevations showed good agreement with the hourly tide gauge observations at Paradip, Visakhapatnam, and Chennai. The amplitudes and phases of M 2 , S 2 , K 1 , and O 1 at the coastal stations, obtained from harmonic analysis of simulated tides, were found to agree well with those obtained from Admiralty Tide Tables with the RMS misfit 9.2, 5.6, 2.9 and 3.1 cm, respectively. In the Bay of Bengal, semi-diurnal tides (M 2 , S 2 , and N 2 ) attain highest amplitudes (180, 80, 30 cm, respectively) in the Gulf of Martaban while amplitudes of diurnal tides (K 1 , O 1 ) reaches maximum (20, 12 cm, respectively) in the Malacca Strait. The continental shelf in the head bay and along the southern coast of Myanmar is about 200 km wide and the amplitudes of semi-diurnal tides are doubled in these regions while the diurnal tides amplify only marginally, which is consistent with Clarke and Battisti theory. In the north eastern end of the head bay and the Gulf of Martaban, the geometrical configuration of the coastline, in additionto the wide continental shelf, could contribute to the amplification of both semi-diurnal and diurnal constituents. In the Malacca Strait, the amplitudes of both semi-diurnal and diurnal tides are found to increase gradually from the northern end to the 2.5 • N and decreases towards southern boundary. The co-tidal and co-range charts of M 2 and S 2 tidal constituents also show the presence of two degenerate amphidromic points in the head bay. A virtual amphidromic point for M 2 is identified in the Malacca Strait.