TY - JOUR

T1 - The reduction effects of mangrove forest on a tsunami based on field surveys at Pakarang Cape, Thailand and numerical analysis

AU - Yanagisawa, Hideaki

AU - Koshimura, Shunichi

AU - Goto, Kazuhisa

AU - Miyagi, Toyohiko

AU - Imamura, Fumihiko

AU - Ruangrassamee, Anat

AU - Tanavud, Charlchai

N1 - Funding Information:
We especially thank H. Matsumoto, S. Tsukawaki, H. Sato, K. Hayashi, Y. Fukuda, P. Kuntasap, and S.A. Chavanich for the support they provided during our field survey. This study was financially supported by the Sasakawa Scientific Research Grant from The Japan Science Society and in part by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 18201033 and 19681019).

PY - 2009/1/1

Y1 - 2009/1/1

N2 - Using an integrated approach including satellite imagery analysis, field measurements, and numerical modeling, we investigated the damage to mangroves caused by the 2004 Indian Ocean tsunami at Pakarang Cape in Pang Nga Province, Thailand. Comparing pre- and post-tsunami satellite imagery of the study area, we found that approximately 70% of the mangrove forest was destroyed by the tsunami. Based on field observations, we found that the survival rate of mangroves increased with increasing stem diameter. Specifically, we found that 72% of Rhizophora trees with a 25-30 cm stem diameter survived the tsunami impact, whereas only 19% with a 15-20 cm stem diameter survived. We simulated the 2004 Indian Ocean tsunami using the nonlinear shallow-water wave theory to reproduce the tsunami inundation flow and investigated the bending moment acting on the mangrove trees. Results of the numerical model showed that the tsunami inundated areas along the mangrove creeks, and its current velocity reached 5.0 m s-1. Based on the field measurements and numerical results, we proposed a fragility function for mangroves, which is the relationship between the probability of damage and the bending stress caused by the maximum bending moment. We refined the numerical model to include the damage probability of mangrove forests using the obtained fragility function to investigate the tsunami reduction effect of mangrove forest. Under simple numerical conditions related to the mangrove forest, ground level, and incident wave, the model showed that a mangrove forest of Rhizophora sp. with a density of 0.2 trees m-2 and a stem diameter of 15 cm in a 400 m wide area can reduce the tsunami inundation depth by 30% when the incident wave is assumed to have a 3.0 m inundation depth and a wave period of 30 min at the shoreline. However, 50% of the mangrove forest is destroyed by a 4.5 m tsunami inundation depth, and most of the mangrove forest is destroyed by a tsunami inundation depth greater than 6 m. The reduction effect of tsunami inundation depth decreased when the tsunami inundation depth exceeded 3 m, and was mostly lost when the tsunami inundation depth exceeded 6 m.

AB - Using an integrated approach including satellite imagery analysis, field measurements, and numerical modeling, we investigated the damage to mangroves caused by the 2004 Indian Ocean tsunami at Pakarang Cape in Pang Nga Province, Thailand. Comparing pre- and post-tsunami satellite imagery of the study area, we found that approximately 70% of the mangrove forest was destroyed by the tsunami. Based on field observations, we found that the survival rate of mangroves increased with increasing stem diameter. Specifically, we found that 72% of Rhizophora trees with a 25-30 cm stem diameter survived the tsunami impact, whereas only 19% with a 15-20 cm stem diameter survived. We simulated the 2004 Indian Ocean tsunami using the nonlinear shallow-water wave theory to reproduce the tsunami inundation flow and investigated the bending moment acting on the mangrove trees. Results of the numerical model showed that the tsunami inundated areas along the mangrove creeks, and its current velocity reached 5.0 m s-1. Based on the field measurements and numerical results, we proposed a fragility function for mangroves, which is the relationship between the probability of damage and the bending stress caused by the maximum bending moment. We refined the numerical model to include the damage probability of mangrove forests using the obtained fragility function to investigate the tsunami reduction effect of mangrove forest. Under simple numerical conditions related to the mangrove forest, ground level, and incident wave, the model showed that a mangrove forest of Rhizophora sp. with a density of 0.2 trees m-2 and a stem diameter of 15 cm in a 400 m wide area can reduce the tsunami inundation depth by 30% when the incident wave is assumed to have a 3.0 m inundation depth and a wave period of 30 min at the shoreline. However, 50% of the mangrove forest is destroyed by a 4.5 m tsunami inundation depth, and most of the mangrove forest is destroyed by a tsunami inundation depth greater than 6 m. The reduction effect of tsunami inundation depth decreased when the tsunami inundation depth exceeded 3 m, and was mostly lost when the tsunami inundation depth exceeded 6 m.

KW - Thailand

KW - damage

KW - mangroves

KW - mitigation

KW - numerical model

KW - tsunamis

UR - http://www.scopus.com/inward/record.url?scp=57649195456&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=57649195456&partnerID=8YFLogxK

U2 - 10.1016/j.ecss.2008.10.001

DO - 10.1016/j.ecss.2008.10.001

M3 - Article

AN - SCOPUS:57649195456

SN - 0272-7714

VL - 81

SP - 27

EP - 37

JO - Estuarine, Coastal and Shelf Science

JF - Estuarine, Coastal and Shelf Science

IS - 1

ER -