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.

Experimental Investigation of Flood Energy Dissipation by Single and Hybrid Defense System

“…where ∆h 1 = backwater rise at the upstream of dike, h o = initial water depth without any model, To minimize the sidewall effects on the flow structure the ratio between the flume width and the cylinder diameter should be greater than five [4,45]. In this study, this ratio was kept greater than 5 to minimize the sidewall effect.…”

“…In various European countries, whenever there is flood in main rivers, the surrounding small rivers are also flooded at the same time creating heavy damage to properties and human lives [3]. In 2011, the Great East Tsunami in Japan arrived with a return period of 1000 years [4] which destroyed embankments, coastal forests, tsunami gates, and thousands of human lives [5][6][7]. Similarly, the other south Asian countries were also affected by heavy floods in the recent past.…”

“…In addition to the above-mentioned solutions, the hybrid or compound defense system were also studied by different researchers. Various studies consist of the energy dissipation through a compound defense system of vegetation [28], combined effects of sea embankment and coastal forest [4,29], a moat with/without a forest [30] and a double embankment system [31]. Furthermore, Zaha (2019) conducted a series of experiments to investigate a hybrid defense system consisting of a forest, a moat (depression) and an embankment under unsteady flow conditions, considering the dike as unbreakable [32].…”

“…However, the energy dissipation phenomena of a hybrid defense system and assessment of the effect of various shapes of its components still need further investigation [4,32]. Due to the hybrid defense system, the flow structure becomes more complex because of the formation of a hydraulic jump.…”

“…Due to the hybrid defense system, the flow structure becomes more complex because of the formation of a hydraulic jump. The jump characteristics and its location should be identified to improve the efficiency of the hybrid defense system against the destructive forces [4]. In the present paper, the hydraulic jump developed in SDS and HDS was classified on the basis of its location and the Froude number.…”