The Protective Role of Mangroves and Other Coastal Forests against Tsunami Damage:Lesson Learned from Case Studies of Two Tsunamis

摘要

In this paper, some findings on tsunami damage and the protective role of mangroves and other coastal forests are presented based on case studies of two tsunamis: the Great East Japan tsunami at Sendai Airport, Sendai Bay and Yamashita of the Sendai Plain in Miyagi Prefecture, Japan; and the Indian Ocean tsunami at Banda Aceh (Sumatra, Indonesia), and at Khao Lak and Nam Kem (Southern Thailand). From our studies at Banda Aceh in Indonesia, we found that mangrove trees with ≥10 cm dbh are not damaged by tsunamis with an inundation depth of ~3 m. More than 50% of mangrove trees with ≥10 and ≥30 cm dbh would survive tsunamis with inundation depths of 4 and 6 m, respectively. Mangroves and other coastal forests do serve to reduce the impacts of tsunamis by trapping debris and mitigating flow force effects. Mangrove trees growing on sandbars at Nam Kem in Thailand were an exception as they suffered severe damage during the Indian Ocean tsunami at 3-4 m inundation depth, suggesting that soil types and possibly root anchorage are parameters to be considered in future studies. Other factors such as land subsidence, raised ground water level and liquefaction, like that which occurred in the Sendai Plain of Miyagi Prefecture in Japan during the East Japan earthquake, are noteworthy as they weaken the protective function of coastal forests against tsunami damage. The destruction of houses and other property by drift trees with projecting roots is another interesting observation. It appears that mangroves and other coastal forests do contribute to protection against tsunami damage but if they give way, they add to the devastation. The relationship between tsunami flow forces and the protective role of coastal vegetation is complex and warrants further investigation.