The hydrodynamic paradox of submerged roots: numerical investigation of multi-scale flow and local scour mechanisms

Despite the widespread implementation of vegetative bioengineering, riverbank failure remains prevalent. Uncovering the hydrodynamic mechanisms governing vegetated banks is therefore imperative. To elucidate these interactions under submerged conditions, this study establishes a 3D coupled water-root-soil interface numerical model integrating in-situ observations and parameterized L-systems. Realistic interfacial dynamics are captured by simulating roots that are partially embedded in the soil and fully submerged in the flow. Utilizing the SST k−ω model, a dual-scale analysis of the velocity field, turbulent kinetic energy (TKE), and wall shear stress (WSS) is performed across three cases: root-free control, single-row in-line, and double-row staggered arrays. Results indicated: (1) The double-row staggered case induced pronounced cascading wake interference, yielding a 51.88% macroscopic velocity reduction and a 61.39% velocity deficit within the near-bank boundary layer, altering vertical velocity profiles into asymmetric S-shaped mixing layers. (2) The root array drove bidirectional TKE evolution. While the double-row staggered case macroscopically suppressed near-wall turbulence by 66.70% compared to the root-free control, pronounced turbulent kinetic energy concentrations remained highly localized within the root interstices. (3) The WSS spatial distribution revealed a defining multi-scale hydrodynamic paradox. Macroscopically, the blocking effect of the double-row staggered case established a broad wake sheltering zone, reducing the high-risk detachment area from 28.41% to 17.74%. Paradoxically, driven by microscopic pore-flow constriction and downwash secondary vortices, localized stress exacerbation emerged at the root-bank interface, evidenced by a pronounced secondary probability density function (PDF) peak at 1.87 Pa. Overall, while bank-protection efficacy followed the hierarchy of double-row staggered > single-row in-line > root-free control, this study uncovers the multi-scale hydrodynamic paradox of submerged roots, where macroscopic bank protection is intricately intertwined with localized scour exacerbation. These findings provide a theoretical foundation for optimizing ecological revetments in complex riverine environments.