Effects of Air Bubbles on Storm Surge

Abstract

The development of real time operational surge prediction system in the Bay of Bengal incorporating air bubble effects is still limited and is needed immediate attention. In this respects the study has been focused the wave energy dissipation, sea level rise due to air entrainment during storm surge and its various consequences. Furthermore, the thesis has also been investigated the two most severe cyclones of 1991 and AILA, 2009 that crossed the coast of Bangladesh. The energy dissipation by entrained air bubbles under breaking waves is studied theoretically both in spilling and plunging breakers. Results show that there is a correlation between the sudden reductions of wave height and entrained air bubbles into water. The study also focuses that in plunging breaking waves, the wave energy is dissipated on a very short way, whereas, in a spilling breaker this way is of the order of some wave lengths. Mathematical formulations of depth-averaged hydrodynamic equations have also been developed in terms of air bubbles. The results show that the water level increases with the increasing of air bubbles. Moreover, we have also found that wave energy increases with the increasing of void fraction. A nonlinear storm surge model in terms of air bubble effect has been developed for the Bay of Bengal along Bangladesh coast The model is included the analysis area of the head Bay of Bengal. These equations have been solved by finite difference method where the coasts and islands are included in the stair step method. The dynamic effect of pressure gradient outside the storm surge region in the model is found to be negligible in comparison with the dominant effect of wind. Using the model storm surges are simulated for the severe cyclones April, 1991 and "AILA" 2009. The simulation is made at some coastal and island locations of Bangladesh and simulated water levels are found higher in presence of air bubbles. That is, it observes that approximately 1.5%-3.5% water level increases with the increasing of void fraction from 10%-70% associated with AILA, 2009, whereas it increases up to 2%-4.5% for void fraction 10%-70% in the cyclone April, 1991. The results are in good agreement with the reported post storm survey information.