Simulation of Water Levels and Extent of Coastal Inundation Due to a Cyclonic Storm Along the East Coast of India

Nat Hazards DOI 10.1007/s11069-012-0193-6 ORIGINAL PAPER Simulation of water levels and extent of coastal inundation due to a cyclonic storm along the east coast of India A. D. Rao • P. L. N. Murty • Indu Jain • R. S. Kankara • S. K. Dube • T. S. Murty Received: 28 September 2011 / Accepted: 6 April 2012 Ó Springer Science+Business Media B.V. 2012 Abstract The devastation due to storm surge flooding caused by extreme wind waves generated by the cyclones is a severe apprehension along the coastal regions of India. In order to coexist with nature’s destructive forces in any vulnerable coastal areas, numerical ocean models are considered today as an essential tool to predict the sea level rise and associated inland extent of flooding that could be generated by a cyclonic storm crossing any coastal stretch. For this purpose, the advanced 2D depth-integrated (ADCIRC-2DDI) circulation model based on finite-element formulation is configured for the simulation of surges and water levels along the east coast of India. The model is integrated using wind stress forcing, representative of 1989, 1996, and 2000 cyclones, which crossed different parts of the east coast of India. Using the long-term inventory of cyclone database, synthesized tracks are deduced for vulnerable coastal districts of Tamil Nadu. Return periods are also computed for the intensity and frequency of cyclones for each coastal district. Considering the importance of Kalpakkam region, extreme water levels are computed based on a 50-year return period data, for the generation of storm surges, induced water levels, and extent of inland inundation. Based on experimental evidence, it is advocated that this region could be inundated/affected by a storm with a threshold pressure drop of 66 hpa. Also it is noticed that the horizontal extent of inland inundation ranges between 1 and 1.5 km associated with the peak surge. Another severe cyclonic storm in Tamil Nadu (November 2000 cyclone), which made landfall approximately 20 km south of Cuddalore, has been chosen to simulate surges and water levels. Two severe cyclonic storms that hit Andhra coast during 1989 and 1996, which made landfall near Kavali and Kakinada, respectively, are also considered and computed run-up heights and associated water levels. The simulations exhibit a good agreement with available observations from the different A. D. Rao (&) Á P. L. N. Murty Á I. Jain Á S. K. Dube Centre for Atmospheric Sciences, Indian Institute of Technology, New Delhi 110 016, India e-mail: [email protected] R. S. Kankara ICMAM-Project Directorate, NIOT Campus, Pallikaranai, Chennai 600 100, India T. S. Murty Department of Civil Engineering, University of Ottawa, Ottawa, ON, Canada 123 Nat Hazards sources on storm surges and associated inundation caused by these respective storms. It is believed that this study would help the coastal authorities to develop a short- and long-term disaster management, mitigation plan, and emergency response in the event of storm surge flooding. Keywords ADCIRC model Á Storm surges Á Return period Á Inland inundation Á Water levels 1 Introduction Natural disasters such as tropical cyclones heavily impact the Indian coasts. The destruction due to flooding caused by extreme wind waves generated by the cyclones is a serious concern along the coastal regions of India. The rise of sea level would be more if the cyclone crosses the coast during high-tide conditions. Therefore, it is important to know the maximum total water elevation that could possibly occur at a particular coastal location due to the combined effect of surge, tide, and wind waves. As the effect of storm surges is confined to few 100 km along the coast around the landfall location of the cyclone, the prediction will hopefully be improved when high- resolution location-specific models are used. Since modeling of storm surges in complex terrain requires true representation of coastal geometry as well as detailed onshore topography and bathymetry, finite-element models may be more useful for surge prediction and associated inland inundation. When used operationally, the numerical models are useful to provide early warning to low-lying areas and guide evacuation and rescue operations. In India, the models based on finite-difference methods have been developed and used extensively to simulate/predict storm surges for the last two decades. Johns et al. (1982) employed a finite-difference method, which involves a continuously moving lateral boundary. In this model, the coastline representation is based on a conformal mapping procedure that cannot be used in case of sharp curvatures in the boundary. A detailed review of the problem concerning storm surges in the Bay of Bengal is provided by Rao et al. (1994) and Dube et al. (1985). Rao et al. (2009) describes a comprehensive com- parison of surge simulations using finite-difference and finite-element models. The study concludes that resolving the atmospheric forcing of the cyclone is as important as providing the detailed coastline geometry for surge development. Inland intrusion of water associated with storm surges is the main cause for extensive damage due to extreme events along the Indian coastline. Storm surges have inundated large stretches of coastal regions, sometimes penetrating up to 10–15 km inland, particularly, when the cyclone passes through a river deltaic region (SMRC 1998). To minimize the damage, prediction of inland inundation is as important as the prediction of surge heights. Generally, in storm surge prediction models, the rigid lateral boundaries are taken as vertical sidewalls, through which there is no flux of water. In a realistic sense, water level associated with the surge will continuously move onshore. Hence, the assumption of idealized vertical walls will lead to spurious error in simulations of the surge development. In addition, simplified horizontal inland extrapolation of the predicted surge height from the fixed sidewall that estimates the distance of inland penetration do not take into account the underlying dynamics of the intrusion processes. The importance of a numerical model comes to light when providing an early warning to a low-lying area, thereby guiding evacuation and rescue operations. Earlier, Jelesnianski and Chen (1979) developed a numerical model that allows flow of water inland from the model coastline. This has been accomplished by prescribing the advancing 123 Nat Hazards waterfront to move discontinuously from one grid point to another according to a preset criterion. However, to obtain acceptable accuracy, this procedure requires extremely fine grid spacing, which may not be otherwise necessary. In order to properly describe the physics of storm surges, a numerical model must resolve coastal features that can affect storm surge generation and propagation. This means the model domain must necessarily incorporate complex coastal geometries (bathymetry and topography), large gradients in bathymetry along the continental shelf that permits reasonable boundary conditions (i.e., tidal elevations). In this context, a finite-element- based model is assumed to be the best choice, as it allows flexibility to represent a larger spatial domain while permitting higher grid resolutions near the landward boundary. Blain et al. (1994) showed in a grid convergence study that near-coastal resolution is the most critical factor in determining the accuracy of storm surge computations. Although the coastal stretch along the east coast of India is vulnerable to storm surges and associated coastal inundation, no attempt seems to have made in detail on modeling the extent of inland inundation. In the present work, we use the advanced circulation (AD- CIRC) model that integrates domain-specific tools, standard grid, and portal tools to provide an integrated environment for forecasting and information dissemination. This framework allows storm surge computations to be run in a distributed grid environment. Here, the ADCIRC model (described in details by Luettich et al. 1992) is configured for maritime states of Andhra and Tamil Nadu along the east coast of India for computation of extreme surges and associated water level. Numerical experiments are carried out to compute the storm surges using the wind stress forcing representative of 1989 Kavali cyclone, November 1996 Andhra cyclone, and 2000 Cuddalore cyclone. The selection of these cyclones in the present study is based on the availability of observations as well as importance of the geographical locations of the landfall of the storms. The model computes water levels from the more accurate onshore topography associated with extreme surges generated by the cyclonic wind field. Winds in the model are calculated by using a dynamic storm model of Jelesnianski and Taylor (1973). Water levels along the open boundary are obtained from FES95.2 database. This database was developed using a global tidal model and has been found to perform very well in deep waters. The peak water level and associated extent of horizontal inundation simulated at the time of cyclone landfall are found to be in good agreement with observations of India Meteorological Department (IMD). It is to be noted that the vertical run-off height can be used as benchmark for the design level criteria of facilities for the particular region. This study would enable the coastal authorities for appropriate planning for disaster reduction in the event of any severe cyclone crosses a particular stretch of the coast. 2 Data sources The cyclone tracks along with its intensity were collected from various sources viz; IMD reports, SMRC (1998), Unisys Hurricane Database (2006), and several research publications. Using this database, composite synthesized track is derived from observed tracks as well as the theoretical ones based on most favorable direction of the cyclone for Kalpakkam. The intensity of a cyclonic disturbance is measured by the strength of the associated winds. The feasibility study is carried out using parameters of the cyclone based on 50-year return period as an input to the model using the synthesized tracks.

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