Effectiveness and Limitations of Coastal Forest in Large Tsunami: Conditions of Japanese Pine Trees on Coastal Sand Dunes in Tsunami Caused by Great East Japan Earthquake

Abstract

Coastal vegetation is widely recognized to reduce tsunami damage, but coastal forests in large areas of the Tohoku and Kanto districts of Japan were destroyed by the tsunami after the Great East Japan Earthquake on 11 March 2011. A field survey in the affected area elucidated the critical breaking conditions of Japanese coastal pine trees and identified the regions of large-tree debris production. Most broken trees remained in the vegetated region, but scoured regions behind the sea wall or on the down-slope regions of sand dunes produced large pieces of tree debris. In contrast, the pine trees in the inland zone were found to trap much debris. This trapping function should be utilized more in future designs. A numerical model that includes the breaking phenomena of trees estimates the capacity of a coastal forest to reduce the washout region of houses by around 150 m. This is not a negligible component of the mitigation measures when a large tsunami occurs and overflows the sea embankment.

Flume Experiments on Flow Analysis and Energy Reduction through a Compound Tsunami Mitigation System with a Seaward Embankment and Landward Vegetation over a Mound

“…To model the vegetation as a replica, the Japanese pine tree found on the Sendai Plain [2] (having an average height of 15 m and diameter at breast height of 0.4 m) was taken as a reference. Hence, each tree was modeled with a circular wooden cylinder having a diameter (d) of 0.4 cm and was placed in a staggered arrangement on a wooden plate.…”

“…The 2011 Great East Japan Tsunami (GEJT) significantly exceeded the disaster prevention capability of the present single coastal embankment system and resulted in loss of human lives, and extensive damage to the buildings and coastal forests in the Tohoku and Kanto regions of Japan [1,2]. After this disastrous event, the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) of Japan divided tsunamis into two groups, with level 1 (level of disaster prevention) having a recurrence interval less than hundred years, and level 2 (level of disaster mitigation) having a recurrence interval several hundred to a thousand years [3].…”

“…Due to its wide range of functions, such as reducing the destructive fluid force, entrapment of debris, and providing soft landing places, [17], coastal vegetation is being considered a natural component of mitigation techniques that is not only economically feasible, but also environmentally friendly [18,19]. Many experimental and numerical studies have shown the effectiveness of a coastal forest (including mangrove forest) in tsunami mitigations [2,7,8,15,20]. Despite that, constructing only a coastal forest along the coast would not be sufficient in case of a large tsunami event, because the direct impact of the approaching tsunami flows causes many of the trees to be washed out and produces numerous debris [21].…”

Rehabilitacion de centros urbanos afectados por el tsunami 2010 en la Comuna de Pelluhue, Chile

“…En un periodo de 5 años y tras producirse dos nuevos tsunami en la costa chilena en 2014 y 2015 Catalán et al, 2015;Contreras-López et al, 2016), aún no existen obras de mitigación de tsunami en ninguno de los centros urbanos de la comuna. Esta desatención ignora el nivel protector ofrecido por las dunas (e.g., Mascarenhas & Jayakumar, 2008) ubicadas en las ZAV y la posibilidad de crear bosques de protección de tsunami (e.g., Irtem et al, 2009;Tanaka, 2012). Las obras de mitigación propuestas en esta línea por el PRES no tuvieron protagonismo en la cartera de proyectos, por lo que actualmente no existe en la comuna una estrategia a escala urbana de protección ante tsunamis.…”

Scoured depth and length of pools and ditches generated by overtopping flow from embankments during the 2011 Great East Japan Tsunami

“…One of the mechanisms of the embankment failure observed was scouring of the substrate supporting the structures. The presence or breaching of a sea embankment greatly affected the damage to coastal forests and wooden buildings in the Sendai area (Tanaka, 2012;Tanaka et al, 2012Tanaka et al, , 2014. Bricker et al (2012) and Tokida and Tanimoto (2012) conducted post-tsunami surveys and investigated the scoured depth around coastal structures.…”