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This study addresses the vivid internal flow structure variations through horizontal double-layered vegetation (HDLV) under subcritical flow conditions for an inland tsunami. The computational domain was built in ANSYS Workbench, while post-processing and simulation were performed using the computational fluid dynamics (CFD) tool FLUENT with the three-dimensional (3D) Reynolds stress model (RSM). Two alternative arrangements of HDLV were considered, namely Configuration 1 (short submergent layer (L s) + tall emergent layer (Lt)) and Configuration 2 (tall emergent layer (L t) + short submergent layer (Ls)) along with varying flow depths. Strong inflections in velocity and Reynolds stress profiles were observed at the interface near the top of Ls, Whereas, these profiles were almost constant from bed to the top of vegetations inside Lt. A shear layer zone was formed above the top of Ls, which extended to the downstream region in Configuration 2 while it was restricted by Lt in Configuration 1. The normal Reynolds stresses at the bed were significantly greater within Ls in Configuration 2 than inside Lt in Configuration 1. Hence, Configuration 1 was performed relatively better than Configuration 2 in terms of reducing velocity within the vegetation, while Configuration 2 played a key role in attenuating the increased velocities and confining the shear layer above the short submergent layer. [ABSTRACT FROM AUTHOR]

The effectiveness of coastal vegetation as a barrier to mitigate a tsunami greatly depends on the magnitude of tsunami and vegetation structure. This paper summarizes a series of laboratory experiments that investigated the upstream flow structure and energy loss due to a hydraulic jump in a steady super-critical flow. The characteristics of the jump were determined against vegetation of variable density (G ∕ d , where G = spacing of each cylinder in cross-stream direction, d = diameter of cylinder), thickness (dn, where d = diameter of cylinder, n = number of cylinders in the stream-wise direction per unit of cross-stream width), and initial Froude number (Fro, where Froude number is obtained from a model without vegetation in the flume). In super-critical flow (F r o - 1. 6 7 –1.83), a weak hydraulic jump formed on upstream side of vegetation. The height of the jump, its location, and the resulting energy loss were increased by increasing both the vegetation density and thickness. Due to reduced reflection at vegetation front, the drag force against sparse vegetation (G / d = 2. 1 3) was higher compared to intermediate (G / d = 1. 0 9) and dense (G / d = 0. 2 5) vegetation. Under these conditions, the maximum energy reduction due to a weak hydraulic jump reached 9.4% for dense vegetation while it was 8.1% and 7.8% for intermediate and sparse vegetation, respectively. [ABSTRACT FROM AUTHOR]

In order to design a vegetation structure to mitigate floods resulting from extreme events like tsunamis, vegetation density and thickness (width) are important parameters. Flow passing through vegetation faces great resistance, which results in a backwater rise on upstream (U/S) vegetation, increases the water slope inside the vegetation, and for some cases, forms a hydraulic jump downstream (D/S) of the vegetation, thus transforming a subcritical flow to supercritical [Pasha, G. A. and Tanaka, N. [2017] "Undular hydraulic jump formation and energy loss in a flow through emergent vegetation of varying thickness and density," Ocean Eng.141, 308–325.]. Like the concepts of critical velocity and critical slope, this paper introduces the concept of "critical resistance of vegetation," which is defined as "resistance offered by vegetation that transforms a subcritical flow to supercritical." An analytical approach to find the water depths U/S, inside, and D/S of vegetation is introduced and validated well by laboratory experiments. Critical resistance was determined against vegetation of variable densities (G ∕ d , where G = spacing of each cylinder in the cross-stream direction, d = diameter of the cylinder), thicknesses (dn, where d = diameter of a cylinder and n = number of cylinders in a stream-wise direction per unit of cross-stream width), and the initial Froude number (Fro). A subcritical flow (F r o = ∼ 0. 5 5 − 0. 7 5 , without vegetation) was transformed to a supercritical flow (D/S vegetation) with a range of Froude numbers of 1.6–1.9, 1.1–1.2, and 0.85–0.98 against G ∕ d ratios of 0.25, 1.09, and 2.13, respectively, thus defining G ∕ d = ∼ 1. 0 as the critical resistance. However, altering vegetation thickness did not change the results. [ABSTRACT FROM AUTHOR]

Debris produced by coastal forest destruction due to the tsunami after the Great East Japan Earthquake caused secondary damage to buildings by collision. To limit such destruction, the trapping action of a finite-length forest was examined in a flume considering the effects of 'forest density', 'debris length to forest width ratio', and 'forest width-length ratio (aspect ratio)' because the trapping height greatly affects the rate of damage to the forest itself. Higher forest density and a higher aspect ratio decrease the velocity in front of the forest. Debris having a specific gravity up to 0.80 floated after collision but oscillated vertically as forest density was increased. With debris of a higher specific gravity (0.90-1.05), increased forest density resulted in debris attachment closer to the ground, which reached a plateau beyond a forest density of 0.48 cylinders/cm2. In sparse forest, when debris was longer than the forest width; most debris fell at the foot of trees, while it was caught in the upper half of water depth in dense forest. The flow structure in front of and around a forest greatly affected the debris trapping capacity. It was deducted that the inland forest with a density of 0.48 cylinders/cm2 and an aspect ratio of 1.7 trapped most of the debris of all lengths at the foot of trees. [ABSTRACT FROM AUTHOR]

After the Indian Ocean Tsunami in 2004, several studies quantitatively investigated the effects of coastal vegetation on tsunami mitigation, but the effects of a limited forest with a small aspect ratio on tsunami mitigation were not yet elucidated. Therefore, the objective of this study was to estimate numerically the effect of the width-length ratio (aspect ratio) of a coastal forest on tsunami mitigation. Numerical simulations were performed using two-dimensional nonlinear long-wave equations that included bed resistance, drag, and turbulence-induced shear forces due to interaction with the forest. When a limited dense forest exists, the tsunami at the edge of the forest diffracts and collides behind the forest, and the fluid force becomes larger than the case without a forest. In particular, when the aspect ratio is from 1 to 4, the effect of a collision behind the forest becomes very great. However, if the aspect ratio is 4 or larger, the effect of a collision becomes smaller. [ABSTRACT FROM AUTHOR]