Development for the design of colonnade with the trapping effects of tsunami flotsam

A coastal forest is useful as a countermeasure for tsunami disaster, and it is verified in the 2011 Tohoku-oki earthquake tsunami that a coastal forest is effective in dissipating the energy of tsunami and in obstructing and trapping drifting objects. In a previous research, Shuto (1985) attempted to evaluate tsunami mitigation effects of a coastal forest from the field investigation. The other previous research (e.g., Harada & Imamura, 2003; Imai & Matsutomi, 2008) have shown positive effects of trees in flow energy dissipation and potential risk of damage to the trees. On the other hand, current knowledge on the trapping effects of tsunami flotsam is only based on case studies. There is no detailed investigations held on the trapping effects of trees along a coastline or city streets, nor general awareness of the potential trapping capacity of the trees. It is predicted that quantitative evaluation of the trapping capability will lead us to find a suppressing method for the indirect damage by drifting objects of tsunami. To increase the reliability of general flood mitigation efforts of the trees, it is also important to provide the physical limitations of the trees, such as inclining and lodging. In this research, field survey and hydraulic experiments were held to clarify the trapping mechanism of drifting and floating objects caused by tsunami and the capability of the lined trees. From the field survey, it was confirmed that drifting objects flowing in a group can be caught by the trees even if their length is shorter than the distance between the trees. From the hydraulic experiments, relations between the physical factors and the capturing function of the tsunami flotsams were shown. The associated physical factors made it possible to propose a simple evaluation formula of the trapping mechanism. In addition, we show the limitations of lodging of the trees with tsunami flotsams being caught. Using this evaluation method, we confirmed the consistency and the specifications of the trees and flotsams of the 2011 Tohoku-oki earthquake tsunami in Tagajo City, Miyagi. And we reproduced the conditions of the trees in Tagajo which did not lodge even with tsunami flotsams being caught, and the value of the capture rate. The reproducibility of an actual phenomenon by using our evaluation model is indicated in this research.