Abstract
As a mitigation measure against a tsunami inundation, vegetation-embankment hybrid structures received attention after the 2011 Great East Japan Tsunami, and some structures have already been constructed or are under construction in Japan. The present study conducted a series of numerical simulations using a hybrid system comprised of an artificial structure (an embankment, moat) and a natural component (vegetation) that was experimentally proposed in previous studies as an effective structure for tsunami mitigation. After validating the numerical model using published data, this study investigated differences in the performance of the hybrid system by changing the tsunami period and height characteristics of the tsunami-like surge-type flow. As a result, the delay in tsunami arrival time (ΔT) was not affected by the tsunami wave period for the investigated hybrid structures. Among the investigated structures, Case Ve40ME (where Ve40, M, and E represent vegetation, moat, and embankment, respectively, in that order from seaward) showed the maximum performance of ΔT. The reductions of overflow volume (ΔQ), fluid force index (RFI), and moment index (RMI) declined during the tsunami period. The tsunami mitigation effect is closely related to the relationship between the development times of backwater rise, hydraulic jump, and the tsunami period. Case Ve40ME was effective for ΔT, ΔQ, and RMI. Case EMVe40 was especially effective for RFI. When the tsunami period is short, the water level at the shoreline starts to decrease before full development of the hydraulic jump generated in the hybrid system. Thus, overflow volume to landward decreases, and the mitigation effects increase. When the tsunami period is long, the receding phenomenon at the peak water level does not affect the maximum values, thus the mitigation effects become smaller compared with the short period. However, the superiority to other structures is maintained in Case Ve40ME and Case EMVe40 with seaward vegetation and landward vegetation, respectively.
Highlights
Disaster prevention capabilities of a single coastal embankment system were significantly exceeded in the 2011 Great East Japan Tsunami (GEJT) and resulted in the loss of human lives and properties and disaster prevention and mitigation structures like coastal forests in the Tohoku and Kanto regions of Japan [1,2,3]
Since the 2011 Great East Japan Tsunami, the effects of different configurations on tsunami mitigation have been described in experiments and numerical simulations through a combination of a coastal forest and moat [13], double embankment system [34,35], combination of embankment, moat, and forest [10,22], canal in the shoreline direction [14], and a vertically double layer of vegetation behind a sea embankment [21]
The results showed that a forest can reduce the fluid force (RFI = 71–82% using the parameter in this study) just behind of a forest compared with a no-forest condition
Summary
Disaster prevention capabilities of a single coastal embankment system were significantly exceeded in the 2011 Great East Japan Tsunami (GEJT) and resulted in the loss of human lives and properties and disaster prevention and mitigation structures like coastal forests in the Tohoku and Kanto regions of Japan [1,2,3]. Since the 2004 Indian Ocean tsunami, a coastal forest to mitigate a tsunami has received attention. It reduces the destructive fluid force, entraps debris, and provides soft landing places [4,5,6]. To target an L2 tsunami, a compound and/or hybrid mitigation system comprising both artificial (coastal embankment) and natural We experimentally investigated the effectiveness of hybrid structures comprised of an artificial structure (an embankment, moat) and a natural component (vegetation) in emergent vegetation [10,21] or submergent vegetation [22] conditions, the incident wave characteristics investigated were limited
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