Analytical solution of the distributive thrust forces due to storm surges in coastal area

Abstract

Most of the countries in the tropical area of the world are threatened by storm surge generated by cyclonic wind. Storm surges are generated by cyclonic winds and the atmospheric pressure drop associated with a cyclone. The major contribution comes from the winds that exert a stress on water surface which is proportional to the square of wind velocity. In addition to human lives, storm surges cause severe damages to the coastal infrastructure due to tremendous force exerted by the surge wave on these structures. To assess the damage on coastal infrastructure, an essential parameter is the resultant force exerted on these structures. To evaluate these damages, there is hardly any quantitative method available to compute this force. In this research we have developed an analytical model, named as Dynamic Force Model (DFM), by using Variational Iteration Method to compute the distributive thrust force which is generated by the cyclonic wind and moving surge. As governing equations, we have used the Saint-Venant equations which are basically 1D shallow water equations derived from the Navier-Stokes equations. During solution, steady and uniform state of flow represented by Manning‘s equation is used as initial condition. The model is verified by applying the model in a hypothetical channel on which wind is blowing. The computed water velocityby analytical solution is compared with a finite difference solution and reasonable agreement is found. Wind drag coefficient is used as the calibration parameter. It is found that wind drag coefficient increases with the increase of wind speed. Flow field of DFM is validated by comparing the surge velocity computed by DFM with the surge velocity computed by a numerical model Delft3D. The coastal zone of Bangladesh is selected as the study area during model validation. After verification, calibration and validation – the DFM is applied in the coastal zone of Bangladesh. The model computes distributive thrust force in the entire coastal zone for the following events (1) Cyclone SIDR (2) 1991 cyclone (3) Hypothetical SIDR-like cyclone (4) Impact of Sunadarban (5) Impacts of coastal afforestation (6) Impacts of coastal embankments. The results show that the model is capable to compute spatial variation of thrust forces due to cyclones of different intensities and landfall locations. The model is also capable to compute the distributive thrust force due buffering effect of Sundarban mangrove forest and artificial afforestation along the coastal belt. The model can differentiate the thrust force due to the momentum of cyclone wind only and combined impact of cyclone wind & surge wave.