Modelling the effects of cyclonic storm surge and wave action on selected coastal embankments

Abstract

This study work has been conducted to assess the simultaneous impact of cyclone induced storm surge and wave action due to sea level rise and increase in cyclonic wind speed on selected coastal embankments in Bangladesh namely Polder 61, 62 and 64/1a in Chittagong. There are two options considered in this study, one is baseline and another is climate change which have been simulated to observe the combined cyclonic storm surge and wave effects. Nine severe cyclones have been selected in this study which directly hit the Chittagong coastal area. Cyclone SIDR has been selected as a synthetic track along the Chittagong coastal area in this study. Then there are ten severe cyclones which has been considered in this study from 1960 to 2007 to simulate the baseline and the climate change scenario's. These cyclones made their landfall at different tidal phases i.e. either at low tide or at high tide. The selected cyclones have been simulated at original tidal phase and also in the opposite tidal conditions, which means if a cyclone made landfall on low tide then both low tide and high tidal conditions of each cyclones were simulated. Total twenty simulations, ten for original phase and ten for opposite phase have been carried out at baseline and climate change condition respectively. Maximum surge level for each scenario has been extracted at different locations of the selected polders at their sea facing side. These surge levels have been analyzed to determine the storm surge level for different return periods at the selected locations. Ten spectral cyclonic wave models have been simulated to observe the wave effects along the coastline of the study area. Significant wave height has been calculated from the spectral wave model results for the same locations. Simulated significant wave height is used to calculate the wave run-up at baseline and climate change conditions. Finally climate resilient coastal embankment crest level has been designed considering the 25- 50-100 year return period surge level, significant wave at climate change condition, land subsidence value and slope roughness. The crest level of the coastal embankments varies at location. The climate resilient embankment crest level for 25 year return period of Polder 61 varies from 6.60 to 8.15 mPWD, Polder 62 varies from to 6.30 to 8.80mPWD and Polder 64/1a varies from 7.80 to 8.95 mPWD. Comparing the design crest between 25 year return period and 50 year return period, it is seen that the crest level is increased by 21-25% in Polder 61, by 22-28% in Polder 62 and by 23-31% in Polder 64/1a. From the 100 year return period analysis, it is seen that the crest level has increased by 36-44% in Polder 61, by 40-48% in Polder 62 and by 42-52% in Polder 64/1a from the 25 year return period data set.