Effectiveness of double-layer rigid vegetation in reducing the velocity and fluid force of a tsunami inundation behind the vegetation

摘要

Vegetation can act as a bio-shield in disastrous events like tsunamis or storm surges. The effectiveness of a single layer of emergent vegetation against tsunamis has been revealed by extensive research, while the role of a double-layer rigid vegetation (submerged vegetation coupled with emergent vegetation) as a defense system against a tsunami is still not clearly understood. In this study, the mitigation effects of double-layer vegetation were investigated experimentally paying special attention to the reduction of velocity and fluid force within and behind the vegetation. Experiments were conducted in a flume under a quasi-steady subcritical flow condition with a high inundation depth. The double-layer vegetation produced a large water rise in front of the vegetation and entrained a large amount of air bubbles in the flow behind the vegetation. The double-layer vegetation generated a mild water gradient behind the vegetation which prevented the direct collision between the ground and flowing water. In comparison with single-layer emergent vegetation, the double-layer vegetation reduced the velocity near the ground in front, within, and behind the vegetation by maximums of 18%, 74%, and 33%, respectively. Moreover, the maximum mean flow velocity and fluid force behind the vegetation were reduced around 25% and 29%, respectively.