River morphological evolution in earthquake-hit region: Effects of floods and pulsed sediment supply

In mountainous areas, fluvial rivers can reach mobile-bed equilibrium under alternating flood and base flow. However, a massive earthquake as well as subsequent landslides and debris flows can deliver huge amounts of sediment into river channels, driving a river far from its earlier equilibrium state and leaving it to gradually recover through a succession of floods and base flows afterwards. In this paper, we explore a river's mobile-bed equilibrium and its response to increased pulse sediment supply subsequent to a violent earthquake. In order to obtain a first but credible analysis, the simulated gravelbed river was simplified to a rectangular channel with abundant erodible sediment, and a one-dimensional numerical model was implemented for simulation. The results show a hydrograph boundary layer in the mobile-bed equilibrium under the condition of cyclic hydrograph and sediment supply. The river bed responds rapidly after the disruption of the equilibrium but recovers slowly as the disruption disappears. The adjustment of river morphology near the entrance exhibits a nearly exponentially decay rate, which justifies the applicability of a geomorphic rate law. However, the decaying rate for the local bed slope has been found to differ remarkably between the aggradation and degradation stages. This might challenge the conventional use of the rate law, and thus needs to be explored in detail.