This study investigates the role of emerged vegetation in enhancing the performance of submerged spur dikes for better flow control and bank protection in river systems. The research utilizes a numerical model based on Computational Fluid Dynamics (CFD), validated with experimental data. After validation, the study explores various configurations of vegetated spur dike, adjusting the submergence heights of the impermeable spur dike to achieve porosity ranging from fully impermeable to highly porous. Porosity levels of 24%, 48%, and 72% were chosen based on the spur dike height and the effectiveness of staggered vegetation arrangements in maximizing drag and reducing velocity. This approach aims to determine their impact on flow structure, velocity, and turbulence characteristics. The results reveal that impermeable dikes create strong recirculation zones downstream, increasing the potential for bank erosion. Introducing vegetation on the dike, particularly at moderate porosities (24% and 48%), effectively disrupts this behavior by reducing downstream velocity and mitigating mass and momentum exchange concentration between the spur dike field and the mainstream. However, the highest porosity case (72%) offered reduced flow resistance, limiting its protective effectiveness. Analysis of velocity and stress distributions showed that vegetation porosity significantly impacts normal and shear stresses, influencing flow stability at critical points around the spur dike. The findings signify the potential of integrating vegetation into spur dike designs to achieve a balance between effective flow conveyance and erosion control even in the spur dike submergence condition. This approach can outperform conventional emerged impermeable spur dikes, as supported by previous studies that demonstrate the effectiveness of porous and vegetated structures in reducing flow resistance, minimizing stagnation zones, and improving sediment deposition compared to impermeable designs.

Water overflowing from a levee generates scour holes on the toe, which progresses towards the backward crest of the levee and results in nappe flow generation. The direct collision of nappe flow on the downstream area causes levee failure. It is important to introduce a novel countermeasure against scouring caused by nappe flow. Hence, the present study utilized a new technique to reduce scouring due to nappe flow by introducing a combination of pooled water and geogrids. Herein, laboratory experiments were conducted with the three cases for rigid bed (R), named as NR, G1R, G2R (N, G1 and G2 represent no geogrid, geogrid 1 and geogrid 2, respectively), and moveable bed (M), named as NM (nothing moveable), G1M (geogrid 1 moveable), G2M (geogrid 2 moveable), to elucidate the effect of dimensionless pooled water depth (D_P*), overtopping depth (D_C*) and the aperture size of geogrids (d*) on flow structure and scouring. The results showed that the scour depth was reduced by around 17–31% during the NM cases, 57–78% during the G1M cases and 100% during the G2M cases by increasing the D_P* from 0.3 to 0.45. Hence, the combination of geogrids with pooled water (G1M, G2M) performed a vital role in suppressing the scouring, but the results of G2M were more advantageous in terms of scouring countermeasures.

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.