This study predicts how the Free Surface Level (FSL) variations around finite length vegetation affect flow structure by using a numerical simulation. The volume of fluid (VOF) technique with the Reynolds stress model (RSM) was used for the simulation. Multizone Hexahedral meshing was adopted to accurately track the free surface level with minimum numerical diffusion at the water–air interface. After the validation, finite length emergent vegetation patches were selected based on the aspect ratio (AR = vegetation width-length ratio) under constant subcritical flow conditions for an inland tsunami flow. The results showed that the generation of large vortices was predominated in wider vegetation patches (AR > 1) due to the increase and decrease in the FSL at the front and back of the vegetation compared to longer vegetation patches (AR ≤ 1), as this offered more resistance against the approaching flow. The wider vegetation patches (AR > 1) are favorable in terms of generating a large area of low velocity compared to the longer vegetation patch (AR < 1) directly downstream of the vegetation patch. On the other hand, it has a negative impact on the adjacent downstream gap region, where a 14.3–34.9% increase in velocity was observed. The longer vegetation patches (AR < 1) generate optimal conditions within the vegetation region due to great velocity reduction. Moreover, in all the AR vegetation cases, the water turbulent intensity was maximum in the vegetation region compared to the adjacent gap region and air turbulent intensity above the FSL, suggesting strong air entrainment over this region. The results of this study are important in constructing vegetation layouts based on the AR of the vegetation for tsunami mitigation.

The aim of this article is to numerically explore the effects of a horizontal double layer of trees (HDLT) across the whole width of the channel on the flow structures under a steady flow rate and subcritical conditions. The numerical domain was established in ANSYS Workbench, and post-processing (i.e., meshing + boundary conditions) along with simulation was carried out by utilizing the computational fluid dynamics tool FLUENT. The three-dimensional (3D) Reynolds stress model and Reynolds-averaged Navier–Stokes equations were used to analyze the flow properties. The numerical model was first validated and then used for simulation purposes. Two varying configurations of HDLT were selected, represented as Arrangement 1 (tall emerged trees (T_t) + short submerged trees (S_t)) and Arrangement 2 (short submerged trees (S_t) + tall emerged trees (T_t)), along with different flow heights. The model accurately captured the simulated results, as evidenced by the vertical distributions of the velocity profiles and Reynolds stresses at specific locations. The strong inflection in velocity and Reynolds stress profiles was observed at the interface of S_t, contributing to turbulence and giving rise to vertical transportation of momentum between flow layers. While these profiles were almost constant from the beds to the tops of trees at those locations lying in taller trees (T_t), there was an approximate 31–65% increase in streamwise velocities at locations 1–6 in cases 1–2, along with a 54–77% increase at locations 7–10 in cases 3–4, in the unvegetated zone (Z > 0.035 m) compared to the vegetated zone (Z < 0.035 m). The magnitude of turbulence kinetic energy and the eddy dissipation rate were significantly larger inside the short submerged and tall emerged trees as compared to the unvegetated region, i.e., upstream and downstream regions. Similarly, the production of turbulence kinetic energy was approximately 50% and 70% greater inside the tree region (Z < 0.035 m) as compared to above the shorter trees during cases 1–2 and 3–4, respectively.