[MIS01-P09] Reduction Effect of Tsunami Sediment Transport by Coastal Forest: Numerical Simulation of the 2011 Tohoku Tsunami on Sendai Plain, Japan
キーワード：tsunami simulation、sediment transport modeling、coastal forest、composite equivalent roughness coefficient
The reduction effect of the energy of tsunami waves during their passage through coastal forest has been recognized since the 1998 Papua New Guinea tsunami and the 2004 Indian Ocean tsunami (e.g., Dengler and Preuss, 2003, PAGEOPH; Nandasena et al., 2008, JSCE). Numerical simulations conducted to investigate the effects of coastal forest on tsunamis (e.g., Shuto, 1987, Coastal Engineering in Japan; Harada and Imamura, 2005, Advances in Natural and Technological Hazards Research) have showed that tsunami height, inundation distance and flow velocity are markedly reduced by the presence of coastal forest. However, the reduction effect of coastal forest on the transport of tsunami sediments has not been evaluated. In this study, we examined the contribution of coastal forest to tsunami sediment movement by using the numerical simulation of tsunami sediment transport on Sendai plain, Japan, during the 2011 Tohoku tsunami.
To account for varying roughness coefficient in areas of coastal forest, the composite equivalent roughness coefficient was used (e.g., Aburaya and Imamura, 2002, JSCE; Imai et al., 2013, JSCE). Tree occupancy and trunk width are given as 1 %, and 0.15 m, respectively, on the basis of the previous studies (e.g., Harada et al., 2000, JSCE; Imai et al., 2009, JSCE). The area of coastal forest was determined from aerial photographs and land use maps compiled before the 2011 Tohoku earthquake.
Our results revealed that the simulated mean sediment concentration, wave height, flow velocity, and inundation limit were abruptly reduced by the presence of coastal forest. The volumes of net deposition and erosion were reduced to 52-85% of those in the simulation without coastal forest; these reductions were particularly remarkable for fine sand. The maximum extent and thickness of sand layer are also reduced. This evidence of the inhibition of sediment transport by coastal forest suggests that land use conditions and the vegetation environment at the time of a tsunami should be considered in evaluations of historical tsunamis on the basis of geological evidences.
To account for varying roughness coefficient in areas of coastal forest, the composite equivalent roughness coefficient was used (e.g., Aburaya and Imamura, 2002, JSCE; Imai et al., 2013, JSCE). Tree occupancy and trunk width are given as 1 %, and 0.15 m, respectively, on the basis of the previous studies (e.g., Harada et al., 2000, JSCE; Imai et al., 2009, JSCE). The area of coastal forest was determined from aerial photographs and land use maps compiled before the 2011 Tohoku earthquake.
Our results revealed that the simulated mean sediment concentration, wave height, flow velocity, and inundation limit were abruptly reduced by the presence of coastal forest. The volumes of net deposition and erosion were reduced to 52-85% of those in the simulation without coastal forest; these reductions were particularly remarkable for fine sand. The maximum extent and thickness of sand layer are also reduced. This evidence of the inhibition of sediment transport by coastal forest suggests that land use conditions and the vegetation environment at the time of a tsunami should be considered in evaluations of historical tsunamis on the basis of geological evidences.