130 / 2025-07-16 14:09:11

Contributions of hyporheic flow to forces on riverbed sediments

Sediment deposition in mountain rivers is influenced by complex interactions between flow structures and bedforms, particularly in the presence of large boulders. While previous studies have linked Froude number (Fr) to distinct sediment deposition patterns, the underlying mechanics—especially the role of fluctuating hydrodynamic forces and hyporheic flow—remain poorly understood. This study quantifies the drag and lift forces on bed sediments around boulders at varying Fr and explores their relationship to turbulence and hyporheic exchange.
Large-eddy simulations (LES) were conducted in a channel containing boulders overlying a seven-layered bed of spherical sediments. Three cases with Fr = 0.15, 0.45, and 0.89 were examined. Drag (Fx) and lift (Fz) forces on top-layer bed spheres were computed using an immersed boundary method. Pre-multiplied spectra and cross-correlation analyses were applied to identify dominant flow structures contributing to force fluctuations, with a particular focus on boulder-induced surface turbulence and subsurface (hyporheic) flow.
Time-averaged drag and lift forces near boulders can reach 6 and 4 times the reach-averaged shear force, respectively. Their fluctuations are even more pronounced, with standard deviations up to 4.4 times the mean forces and instantaneous peaks nearly an order of magnitude higher than the average shear force. Three dominant spectral peaks—associated with boulder wake oscillations (λ≈ 2.1D), hyporheic flow (λ> 4.5D), and near-bed vortices (λ≈ 1.6D)—govern the force variability. At high Fr, hyporheic flow accounts for up to 50% of local lift force fluctuations, while wake-induced turbulence dominates at intermediate Fr. Cross-correlation analyses reveal that hyporheic flow significantly influences subsurface forces in both low and high Fr regimes. A new sediment deposition criterion based on instantaneous forces provides superior predictive agreement with experimental observations by Papanicolaou et al. (2018), compared to conventional metrics relying on time-averaged shear stress.
This study demonstrates that instantaneous drag and lift forces, strongly modulated by Fr-dependent flow structures and hyporheic exchange, critically control sediment stability and deposition near boulders. The traditional reliance on time-averaged shear stress fails to capture these dynamics, especially in mountain rivers with pronounced subsurface exchange. The proposed instantaneous-force-based criterion offers a robust alternative for predicting bedload transport and depositional patterns under varying hydraulic conditions.