Estimating the resistance of Japanese black pine stands on coastal sand dunes to tsunami damage

Mechanical properties of Japanese black pine (Pinus thunbergii Parl.) planted on coastal sand dunes: resistance to uprooting and stem breakage by tsunamis

Kazuki Nanko(a), Satoru Suzuki(a), Hironori Noguchi(b), Yoji Ishida(c), Delphis F. Levia(d), Akira Ogura(e), Hiroaki Hagino(b), Hiroshi Matsumoto(c), Hiromi Takimoto(f), Tomoki Sakamoto(a)

(a) Department of Disaster Prevention, Meteorology and Hydrology, FFPRI, Tsukuba, Ibaraki, Japan.

(b) Tohoku Research Center, FFPRI, Morioka, Iwate, Japan.

(c) Ishikawa Agricultural and Forestry Research Center, Hakusan, Ishikawa, Japan.

(d) Department of Geography University of Delaware Newark, USA.

(e) Kenou General Agriculture Forestry Office Ishikawa Prefecture, Kanazawa, Ishikawa, Japan.

(f) Minamikaga General Agriculture Forestry Office Ishikawa Prefecture, Komatsu, Ishikawa, Japan.

Japanese black pine (Pinus thunbergii Parl.) is the predominant tree species in Japan’s coastal forests. It is crucial to quantify the resistance of P. thunbergii to tsunami damage for estimating the mitigation effect of tsunamis on coastal forests. Tree damage can be broadly categorized as either uprooting or stem breakage. Currently, research regarding the resistance of large-diameter P. thunbergii trees to uprooting or stem breakage is limited.

On-site pulling tests and indoor bending tests were conducted using P. thunbergii logs with a diameter at breast height of ≥20 cm, and an equation was devised to calculate the resistance of P. thunbergii to uprooting and stem breakage. Tsunami inundation of standing P. thunbergii trees was simulated, and the critical tsunami water depth at which a P. thunbergii specimen would fail was calculated. The results showed that P. thunbergii trees planted on coastal sand dunes were more prone to uprooting than to stem breakage and that the resistance of P. thunbergii to tsunamis increased with increasing height, base diameter, and root ball depth. P. thunbergii specimens with a height of 10 m could withstand a tsunami water depth of 3–4 m.

Recently, coastal forests in Japan are required to perform different functions depending on the region in which they grow including windbreak and sand stabilization, and techniques must be developed to manage such forests to meet these requirements. The results of the present study may find application in the development of forestry management techniques to increase the resistance of coastal P. thunbergii forests to tsunami damage.
 

Photos: A P. thunbergii log (length 3.6 m, median diameter 14 cm) is subjected to a bending test. Left, before the test; right, after the test. The Young’s modulus (elasticity) of the log is calculated from the amount of force applied and amount of deformation displayed.