A parameter study on bank shear stresses in curved open channel flow by means of large-eddy simulation
Meandering rivers and streams are a common planform in the world’s populated areas. Furthermore the recent increased focus on renaturalization projects has lead policy makers to also consider the partial remeandering of previously trained rivers. Economic factors such as navigation, man-made infrastructure and valuable farm land set the boundary conditions for such rivers. Understanding the behavior of the near bank flow in schematized open channel bends could help in understanding the behavior of meandering rivers and predict locations of potential damage, as well as aid in the development of design criteria of stable river banks. The interaction of the hydrodynamics with the bed and banks causes meandering river to change their course over time. Hickin and Nanson (1975) showed that the yearly migration rate in meandering rivers depends on the ratio of the river width to the radius of curvature B/R of a river bend. They showed that there is a peak migration rate at (B/R)max between 0.5 and 0.33. For milder curvature B/R < (B/R)max, but also for sharper curvature B/R > (B/R)max the yearly migration rate is lower. The explanation for this behavior is lacking. Curved open channel flows exhibit complex flow structures (such as the outer bank cell) near the outer bank. To model these flow structures requires flow solvers with advanced turbulence modeling capabilities. Large-eddy simulation is able to capture the complex flow structures occurring in curved open channel flows. Using a well-validated large-eddy simulation code, a large set of axi-symmetric simulations (infinite length bend) were performed. The simulations are based on a wide range of mildly and sharply curved bends (represented by the parameter space B/R and Cf-1H/B). The results indicate that for B/R > 0.1 the magnitude of the bank shear stress decreases with increasing inverse aspect ratio H/B. The magnitude of the bank shear stress was found to increase strongly for small increasing B/R. For large increasing B/R the magnitude of the bank shear stress no longer increases, but even slightly decreases. The bed shear stress magnitude, however, still increases for large increasing B/R, which suggests that sharply curved channels tend to deepen rather than migrate laterally. Furthermore, considering the bank shear stress to depend quadratically on the sum of the velocity excess and the bulk velocity multiplied by a bank friction factor Cf,bank, a correction factor outer bank is derived. The correction factor outer bank, which depends on B/R and H/B, represents the increase of bank 1918 friction factor compared to a straight channel flow due to the complex curved outer bank hydrodynamics.