Differences in effectiveness of a hybrid tsunami defense system comprising an embankment, moat, and forest in submerged, emergent, or combined conditions. (15th July 2020)

• Record Type: Journal Article
• Authors: Kimiwada, Yuya; Tanaka, Norio; Zaha, Takehito
• Is Part Of: Ocean engineering. Volume 208(2020)
• Publication Date: 2020-07-15
• Subjects: Energy reduction -- Hydraulic jump -- Vegetation resistance -- Fluid force -- Overflow volume; Ocean engineering -- Periodicals; Ocean engineering; Periodicals; 620.4162
• DOI: 10.1016/j.oceaneng.2020.107457
• Languages: English
• ISSNs: 0029-8018

Abstract

Abstract: A hybrid tsunami defense system comprising vegetation (V), an embankment (E), and a moat (M) while changing the order of the components and the inundation depth of vegetation was investigated in a flume using a surge-type flow produced by quickly lifting a gate. Compared with the hybrid defense system in which vegetation is emergent, the tsunami mitigation effects of seaward vegetation became smaller than the other types because the reflections were weakened by the reduction of vegetation resistance in the submerged condition. Based on the decrease in overflow volume, fluid force, and moment by fluid force, the optimal structures of seaward and landward submerged vegetation types were the same order of 'V, M, and E (VME type)' and 'E, M, and V (EMV type)' from seaward, respectively, as those in the emergent condition. Differences in tsunami mitigation effects due to vegetation renewal were demonstrated in VME and EMV types by changing the horizontal vegetation structure. Although the mitigation effects of horizontally combined types were in between emergent and submerged conditions, the water surface gradient behind the forest, which is related to erosion and driftwood production, showed a clear difference in the order of submerged and emergent vegetation. Highlights: Reduction of wave energy by combination of embankment, moat & vegetation was noted. Effectiveness of hybrid systems was examined using a surge-type unsteady flow. Reduction of fluid force and moment by fluid force were investigated. Differences of reflection, backwater rise & hydraulic jump were the key phenomena. The optimum layout was discussed based on the available land of the existing forest.