Study Of Beach Profile Changes Under Tsunami Waves

Tsunami waves may move a significant amount of coastal sediment and cause significant changes of beach profiles in the affected coastal regions.The 2D laboratory experiments were performed to investigate beach profile changes and sorting of sand grains under the tsunami waves on the fine beach. The incident wave was N-wave. The initial 1/10-1/20 combined beach slope mainly composed of two or three sediment grain sizes was exposed to the N-waves in three different water depths. The incident wave also used regular waves and irregular waves for comparative study. The wave breaking, uprush, back-wash, hydraulic jump, bed profiles, initial and final distributions of sediment grain size were measured.The experimental results show that the wave motion and sediment transport were not really affected by the bed profile changes that occurred after running waves on the bed profile. The strong down rush resulted in erosion on the foreshore and deposition seaward of wave run down. The results agreed qualitatively with other experimental results. These was no changes of beach profiles under N-wave action when the water depth enough. This result agreed qualitatively with coastal retreat field observed after tsunamis. Those were all suggested that the majority of the deposit sources were from the beach rather than the deep ocean floor.The influence of sorting of sand grains and beach slope were considered in this experimental study. The characteristics of beach profile changes and sorting of sand grains under the tsunami waves were obtained. The sands sampling was performed in ten different feature locations after 6 waves. It was shown that the mean grain size increased in the deposition region. The results agreed qualitatively with the post-tsunami field observations. The nonuniform sediment transport and coarsening of the bed were laboratorial investigated for the first time.Also our experimental final profile was similar to some regular wave experimental results which were classified into bar type. However, the N-wave experimental grain sorting was opposite to regular wave results. Two results were in accord with basic rules that finer grains are winnowed from the bed in the most energetic areas by a turbulent process and are carried away to less energetic areas, resulting in a coarsening of the bed in the more energetic areas ((?)eliko(?)lu, 2006).Based on experimental investigations, the comprehensive two dimension plane numerical model was developed with the capability of modeling tsunami wave, including the effects of sediment transport, morphological changes and changes of sediment grain size distribution. The hydrodynamic model adopts on highly nonlinear and weekly dispersive Boussinesq equations and couples the sediment transport and morphological evolution models. The Weighted Essentially Non-Oscillatory Schemes (WENO) was implemented for spatial discretization of morphological bed level equations. The wave breaking model, bottom friction, source function method (Wei, 1999) and moving boundary algorithm were all implemented in the numerical model.The combined model was validated using available laboratory experiments. The coupling numerical model could well resolve the solitary wave propagate, break, runup and rundown on the beach, also could simulate scouring process and sorting of sand grains. The Engelund (1972), Ackers (1973) and Van Rijn (1984) total load empirical formulas were also investigated.The changes of beach profile and sorting of sand grains by tsunami-like waves were investigated by this numerical model. The characteristics of beach profile changes and sorting of sand grains by tsunami-like waves were further obtained.