Nguyen Ba Thuy

Tsunami mitigation by coastal vegetation considering the effect of tree breaking

Damage to vegetation by tsunami moment and reduction of potential tsunami force are discussed based on a numerical simulation. A numerical model based on two-dimensional nonlinear long-wave equations that include drag forces and turbulence-induced shear force due to the presence of vegetation was developed to estimate tree breaking. The numerical model was then applied to a coastal forest where two dominant tropical vegetation species, Pandanus odoratissimus and Casuarina equisetifolia, were considered. The threshold water depth for tree breaking increased with increasing forest width, and the analysis was consistent with the field investigation results that the critical tsunami water depth for breaking is around 80% of the tree height for P. odoratissimus. C. equisetifolia is stronger than P. odoratissimus against tsunami action, but P. odoratissimus can reduce a greater tsunami force than C. equisetifolia due to its complex of aerial root structures. Even if breakage occurs, P. odoratissimus still has high potential to reduce the tsunami force due to its dense aerial root structures. Previous numerical models that do not include the breaking phenomena may overestimate the vegetation effect for reducing tsunami force. The combination of P. odoratissimus and C. equisetifolia is recommended as a vegetation bioshield to protect coastal areas from tsunami hazards.

THE STUDY ON QUANTITATIVE ASSESSMENT OF STORM SURGE COMPONENTS BY NUMERICAL MODEL

In this study, components of storm surge caused by wind, atmospheric pressure and wave are calculated and analyzed by using integrated numerical models of tides, waves and storm surge (SuWAT - Surge, Wave and Tide). The influence of tide is also considered. In which, tide and storm surge are calculated based on two-dimensional nonlinear shallow water equations considering surge component generated by wave radiation stress that is calculated from the SWAN model, a component model in SuWAT model. The model is applied to calculate storm surge during Xangsane typhoon in Da Nang in September 2006 with a number of different computing cases. The results show that influence of tide is negligible due to small tidal amplitude, surge caused by atmospheric pressure is only significant in offshore areas where storm intensity is still strong. Meanwhile, surge components caused by wind stress and wave radiation stress dominate total water level in coastal areas.

Assessment of Storm Surge along the Coast of Central Vietnam

ABSTRACT Thuy, N.B.; Kim, S.; Chien, D.D.; Dang, V.H.; Cuong, H.D.; Wettre, C., and Hole, L.R., 2017. Assessment of storm surge along the coast of central Vietnam. In the present paper, the interaction of surge, wave, and tide along the coast of central Vietnam is assessed using a coupled model of surge, wave, and tide. A series of storm surge simulations for Typhoons Xangsane (2006), Ketsana (2009), and Nary (2013) are carried out, considering the effects of tides and waves that combines wave-dependent drag and wave-induced radiation stress to find a predominant factor in storm surge generation. The results indicate that the surge–wave interaction is crucial to the storm surge simulation in this area. In particular, the wave-dependent drag improves an accuracy of the storm surge level up to 30%. In addition, the radiation stress contributes up to 15%. However, the tide–surge interaction is negligible because there is less than 2% difference in results with and without the tide. A series of coupled surge and wave simulations for 49 historical typhoons in the period of 1951 to 2014 show that mean peak surge levels along the coast are 2.5 m. The highest peak surge level reached 4.1 m at Cuaviet in the Quangtri Province during Typhoon Harriet (1971).