Morphological response of gravel bed rivers near a knickpoint: Effect of bars on dynamic equilibrium river profile

Abstract

AbstractGeomorphological evolution is one of the main factors that increases flood damage in small or medium rivers located in upstream river reaches. These types of flood damage have been increasing at knickpoints where the riverbed slope and river width change abruptly and are likely to cause non‐equilibrium conditions for sediment transport during floods. Therefore, it is important to understand the non‐equilibrium morphological response at the knickpoint and the resulting new dynamic equilibrium state under given external forces. The effects of two‐dimensional (2D) morphological features on the dynamic equilibrium riverbed profile, however, have not been specifically studied because the methods currently in use for calculating equilibrium profiles are based on zero‐ or one‐dimensional (0D or 1D) modeling. Here, we perform numerical calculations using the 2D morphodynamic model iRIC‐Nays2DH to clarify the dynamic equilibrium profile and the process of reaching a dynamic equilibrium state. We also use an existing 1D model to show the 2D effect in the dynamic equilibrium state. To understand this, we set up three channels: slope transition point, width transition point, and both the slope and width transition point. 1D results show a constant slope profile in channels with constant width and upward‐convex profiles in channels with width expansion at the equilibrium state, owing to the adjustment of the difference in sediment transport volume in the two reaches with different widths by changing the slopes. In contrast, the 2D results show that the alternate bars create a small autogenic knickpoint even in the straight channel and significantly dampen sediment deposition at the width expansion point, as seen in the 1D model result. This was because the bars' shape increased the volume of sediment transport because the shape of the bars concentrated flow. These results suggest that 2D morphological features, such as fluvial bars, play a significant role in the equilibrium riverbed profile.