Energy Reduction of a Tsunami Current through a Hybrid Defense System Comprising a Sea Embankment Followed by a Coastal Forest

Abstract

The 2011 Great East Japan tsunami revealed the limit of using natural or artificial infrastructures as a single tsunami countermeasure. In recent tsunami mitigation strategy, interest in a hybrid defense system (combination of natural and artificial infrastructures) rather than a single defense structure is growing, and a pilot project has already started in Japan. Clarification of flow structures within the hybrid defense system is necessary for designing an improved mitigation system. In addition, when a hydraulic jump is expected, its position should be restricted to a protected area for the resilience of the hybrid defense system. This study performed flume tests to elucidate the mitigation effect of a hybrid defense system comprising an embankment model (EM), followed by different types of single-layer emergent forest models (SLM) or vertical double-layer forest models (DLM). Different types of hydraulic jumps were observed within the defense system, jump position and their characteristics dominated the energy reduction downstream of SLM or DLM. Experimental results showed that this hybrid defense system reduced the flow energy to 30% and 40% of maximum for SLM and DLM, respectively, compared to only the single EM. Moreover, the position of the hydraulic jump was near the EM in the combination of EM and DLMs.