Effect of backward-facing step heights in vegetation-step model on reducing the velocity of a tsunami inundation and increasing the energy dissipation efficiency

Abstract

A coastal forest combined with a backward-facing step is an efficient facility to reduce tsunami damage to residential areas behind sea embankments. This study establishes a generalized model, and experimentally explores the water level changes upstream of the vegetation-step mitigation model as well as its energy dissipation effect under different initial Froude numbers, step heights, and vegetation conditions. The results show that the relative backwater rise increases with the growth of vegetation density, patch length and initial Froude number, representing a slowing down of the tsunami inundation. As for energy dissipation, it is mainly caused by the additional resistance of the vegetation and the hydraulic jump. And the vegetation condition not only affects the energy dissipation due to stem-scale turbulence within the patch, but also changes the hydraulic jump process of water falling from the step in cooperation with the step height. As a result, the energy dissipation efficiency always increases with the growth of vegetation density, vegetation patch length and step height. With the criterion that the energy dissipation efficiency and its growth rate can hardly change with vegetation parameters, this study innovatively defines the threshold slope and gives the principle of judging the most cost-effective vegetation conditions at different step heights. These results are expected to provide an important reference for the design of composite tsunami mitigation facilities.