Pure Appl. Geophys. Ó 2019 Springer Nature Switzerland AG https://doi.org/10.1007/s00024-019-02379-x Pure and Applied Geophysics Effect of Wave Radiation Stress in Storm Surge-Induced Inundation: A Case Study for the East Coast of India 1 2 1 1 3 P. L. N. MURTY, A. D. RAO, K. SIVA SRINIVAS, E. PATTABHI RAMA RAO, and PRASAD K. BHASKARAN Abstract—Tropical cyclone-induced coastal inundation is a ecosystem and thickly populated coastal cities. Low- potential hazard for the east coast of India. In the present study, two lying coastal areas along the ECI are highly suscep- case studies are presented to examine the significance and impor- tance of wave radiation stress in storm surge modeling during two tible and vulnerable to the impact of storm surges due extreme weather events associated with the Phailin and Hudhud to landfalling tropical cyclones that originate in the cyclones. Model computations were performed using the advanced Bay of Bengal basin. The national weather agency circulation (ADCIRC) model and the coupled ADCIRC ? SWAN (Simulating Waves Nearshore) model for these two events. Mete- has categorized both Phailin and Hudhud cyclones as orological and astronomical forcing were used to simulate the very severe cyclonic storms (VSCS) that made hydrodynamic fields using the ADCIRC model run in a stand-alone landfall along the ECI near Gopalpur, Odisha and mode, whereas the coupled ADCIRC ? SWAN model also incorporated the wave radiation stress attributed from wave Visakhapatnam, Andhra Pradesh, respectively. These breaking effects. Cyclonic wind fields were generated using the two VSCS resulted in a trail of destruction during revised Holland model. Results clearly indicate an increase in the landfall, thereby causing widespread damage to peak surge of almost 20–30% by incorporating the wave radiation infrastructure, property and a few instances of loss of stress and resulting inundation scenario in the coupled model simulation. The validation exercise showed that the coupled human life in the coastal regions of Odisha and ADCIRC ? SWAN model performed better than the ADCIRC northeastern Andhra Pradesh. Coastal flooding or model in stand-alone mode. Key findings from the study indicate inundation is a catastrophe having significant impact the importance of wave-induced setup due to radiation stress gra- dients and also the role of the coupled model in accurately on any coastal zone, causing loss of life and property simulating storm surge and associated coastal inundation, espe- and widespread damage to coastal infrastructure. cially along flat-bottom topography. Timely warnings of storm surge and inundation Keywords: Wave radiation stress, coupled model, storm forecasts during cyclones can potentially reduce the surge, inundation extent, ADCIRC ? SWAN, unstructured mesh. loss of life by issuing bulletins and alerts to stake- holders. Numerical modeling of coastal inundation during cyclonic events is a vital element in issuing warnings for emergency planning and evacuation 1. Introduction measures. While several studies had focused on storm surge modeling along the ECI (Murty et al. 1986; The east coast of India (ECI) is characterized by a Dube et al. 1997, 2000; Rao et al. 1994), very few highly productive estuarine environment, mangrove studies have focused on modeling inundation sce- narios during storm surge events. Rao et al. (2012) investigated coastal inundation scenarios along the ECI attributed due to tropical cyclones. Bhaskaran 1 Earth System Science Organisation (ESSO), Indian et al. (2014) performed a detailed analysis on the National Centre for Ocean Information Services (INCOIS), Hyderabad 500 090, India. E-mail: [email protected]; potential of coastal inundation at several coastal [email protected] locations off Tamil Nadu. A study by Srinivasa et al. 2 Centre for Atmospheric Sciences, Indian Institute of (2015) investigated the possible worst-case scenario Technology (IIT), Hauz Khas, New Delhi 110 016, India. by considering the combination of highest high water 3 Department of Ocean Engineering and Naval Architecture, Indian Institute of Technology Kharagpur, Kharagpur 721 302, levels with storm surge that generates huge West Bengal, India. P. L. N. Murty et al. Pure Appl. Geophys. devastation during cyclones. Their study highlights roughness that in turn affects the currents in coastal the importance of the tidal phase and its relevance in circulation models (Smith et al. 1992). Although controlling the extent of inland inundation during short- and long-period waves are separated in a wave tropical cyclone landfall. spectrum, they interact. Breaking waves in shallow In a shallow-water environment, storm surge and water generate radiation stress gradients that in turn wind waves can interact with each other by transfer- drive wave setup and currents affecting the vertical ring energy from one system to the other. It is well momentum mixing, bottom friction and the resultant known that wind waves can ultimately affect the water level elevations. According to the conservation coastal ocean circulation by enhancing the wind stress of wave energy flux, the wave height and steepness (Mastenbroek et al. 1993). Surface waves constitute have a tendency to increase as they propagate into sea surface roughness, and for a strong wind regime shallow water environments. The shallow-water wave such as in tropical cyclones ([ 30 m/s), they tend to process is dominated by depth-induced breaking, decrease the surface drag (Hwang 2018) when other wherein momentum flux is transferred to the water important processes start to play an important role column, raising the water levels adjacent to the coast (e.g. sea spray). The study by Hwang (2018) also (Holthuijsen 2007). The dynamic characteristics of indicates that both surface roughness and whitecaps water level elevation and wave-induced setup and due to wave breaking are driven by surface wind set-down along coastal areas, especially during an stress, and their quantification using the wind speed approaching storm, have cumulative effects that input depends on the drag coefficient formulation. A depend on the mutual interaction between waves, previous study by Rajesh et al. (2009) attempted to currents, surge and the astronomical tides. In fact, develop a parameterization of sea surface drag under tidal propagation is also modified in the presence of a varying sea states dependent on wave age. Their study surge, leading to tide-surge interaction (Horsburgh (Rajesh et al. 2009) combined the Toba-3/2 power law and Wilson 2007), and the waves are modified by the and Froude number scaling to develop a new drag presence of currents generated by the tide and surge. formulation found to perform well under conditions of They contribute to resultant water level and mean both young and mature waves, verified against time circulation through wave-induced setup and currents, series meteorological and oceanographic observations attributed to radiation stresses created by breaking in the Indian Ocean region. Usually, these physical waves in shallow water (Longuet-Higgins and Ste- processes can influence one another in numerous wart 1962). The dynamic interaction between wave– ways: wind stress that is altered by including the wave current–surge–tide components and their combined effect (Donelan et al. 1993); wave radiation stress energy balance is very strong during tropical cyclone considered to be an additional forcing term in storm activity. Hence, the nonlinear interaction mechanism surge models (Xie et al. 2001; Mellor 2003; Xie et al. and the influence of waves on coastal currents and 2004). The concept of radiation stress was first storm surge has been an important area of research introduced by Longuet-Higgins and Stewart (1962)to area in recent years. The key question lies in how analyze the mechanism of wave setup and set-down in these nonlinear processes can be properly accounted a nearshore environment. for and coupled in numerical models. In shallow waters, storm surges, tides and wind Application of coupled wave hydrodynamic waves are the major components in an ocean fore- models for the global ocean is a recent development casting system that determines the sea-state that has been observed in a few studies such as the environment of that region. It is known that these MAST III PROMISE Project (Ozer et al. 2000); the processes are present in many cases and their inter- modeling of energetic events such as hurricane actions are very significant in context to individual Katrina and Rita in 2005, and Gustav and Ike in 2008 dynamic processes (Longuet-Higgins and Stewart in the Atlantic Ocean (Dietrich 2010); modeling of 1960; Prandle and Wolf 1978; Janssen 1992; Bao the Mediterranean Sea with focus on the Italian coast et al. 2000; Welsh et al. 2000). Wind waves play a (Ferrarin et al. 2013); and a coupled modeling system significant role in determining the sea surface for typhoon Maemi in Korean seas (Bung et al. 2013). Effect of Wave Radiation Stress in Storm Surge-Induced Inundation Arora and Bhaskaran (2012) reported on the impor- surface gravity waves in time and geographic space. tance of bottom friction under combined wave-tide It computes the wave direction (h) and relative action and its relevance to bottom boundary layer frequency (r) based on the action balance equation characteristics (Arora et al. 2010). The role of wave- (Booij et al. 1999), as given below: induced setup during an extreme event for the Kal- o o o o o S N þ c N þ c N þ c N þ c N ¼ ð1Þ pakkam coast (Nayak et al. 2012) and associated ot ox x oy y or r oh h r coastal vulnerability was studied by Nayak and Bhaskaran (2013). A few studies have emphasized where N represents the wave action density; S is the the importance of coupling storm surges, waves and source term (combination of source and sinks) in tides to enhance the accuracy of storm surge forecasts terms of energy density; ch is the propagation in coastal areas (Roland et al.