Abstract:
Storm surges, in combination with tidal and wave dynamics, can lead to severe levels of catastrophic flooding, critical damage to human infrastructure, and habitat disturbance in coastal zones during tropical cyclones. Due to its low-lying topography and exposure to the Bay of Bengal, the Bangladesh coastline experiences devastating storm surge impacts. This highlights the importance of accurate total water level modeling for disaster preparedness and resilience planning. However, traditional storm surge and wave modeling techniques often rely on linear summation methods, which assume that tides, surges, and waves independently contribute to the total water levels. However, these approaches typically overpredict surge elevations due to ignoring nonlinear interactions between these hydrodynamic elements. A two-dimensional nonlinear shallow water model assesses the nonlinear interactions among storm surges, tides, and waves along the coast of Bangladesh during Cyclone 1991, Cyclone Sidr (2007), and Cyclone Amphan (2020).
For different cyclonic events, a hydrodynamic model was established to simulate the interactions of storm surges, tides, and waves. The model had been validated against measured data from met stations, including a mean absolute error of 0.467m for Cyclone Sidr (2007), root mean squared error of 0.632m for Cyclone Sidr (2007), and correlation coefficient of 0.707 at Hiron point (good agreement between modeled and observed data). The study covered three significant cyclones that hit each of the five regions along the Bangladesh coastline: Cyclone 1991, which hit Chittagong on April 29, 1991; Cyclone Sidr, which struck the Kuakata-Patharghata coast on November 15, 2007; and Cyclone Amphan, which struck West Bengal on May 20, 2020. Simulations were run with a six-hour lead to peak tidal phases and a six-hour lag to peak tidal phases to investigate the effects of tidal phase on surge propagation.
The calculated surge heights using the nonlinear model were found to be lower than those predicted using linear superposition methods. Linear summation generated 15m surge heights at Cox’s Bazar, however the nonlinear model estimated a greater range between 9-10m, a 5m overestimation. On Anwara, linear sums gave 14m, nonlinear models 11m — an overestimate of 3m. Sandwip, Teknaf and Nijhum Dwip also logged similar overestimations of between 20 and 30 percent. The analysis further demonstrated how flood tides enhanced wave heights, while ebb tides suppressed them due to tidal current impacts. Waves heights were different on both eastern as well as western coast of Bangladesh owing to bathymetry and geometry of coastline. In certain areas, surge-wave interactions elevated water levels as much as 1.8m due to wave breaking and energy dissipation close to shore. In summary, it was observed that for both surge and waves, the non-linear interaction is higher at locations away from the landfall locations. This is due to the strong wind force of the cyclone dominating the surge height and wave height at the landfall locations which is absent far from the landfall locations. It was notable that for most cases, the maximum nonlinear interaction took place at Sandwip (3.3m in case of tide-surge and 3.5m in case of tide-surge and wave) for cyclone 1991, where the tidal range is very high. The proximity of the observation location from the coastline also influences this non-linear interaction between surge-tide and waves.
Surge propagation was found to largely depend on the phase of the tide. At low tide, surge propagation was slower due to less water depth and bottom friction, while surge propagation was faster at high tide, enhancing inundation potential in coastal lowlands. Surge clustering effects closer to the right-hand side of the cyclonic tracks, especially for Cyclone Sidr and Cyclone Amphan, resulted in peak surge coinciding with high tide in parts of the region. The nonlinear surge-tide interaction model has been an improvement over conventional superposition methods that have led the model to predict peak surge times more accurately by 15–20 percent and improved coastal flood risk assessments.
The study highlights the role of nonlinear hydrodynamic modeling in lifting storm surge predictions. This has implications for how flood protection planning is carried out, as the over-reliance on linear models can lead to overstated coastal flooding risks. The findings are a vital contribution to coastal zone management, specifically in terms of improving early warning systems and optimizing the construction of coastal defenses. Incorporating the nonlinear surge tide wave dynamics into the advice given to policymakers and disaster management can allow for better mitigation of floods, and promote long-term resilience of coastal communities.