Mountain River Morphodynamics Subject To Varying Water And Sediment Supply

Water and sediment supply from the river basin are active factors influencing the river morphology.For alluvial mountain rivers,their water and sediment supply have strong spatio-temporal nonuniformity as a result of the watershed conditions as well as human activities.Moreover,their sediment supply has a wide range of grain sizes thus leading to evident sorting effects.However,due to the complex interaction among water,sediment,and river bed,the morphodynamic processes of mountain rivers subject to varying water and sediment supply are still not well understood.In this paper,we study both the dynamic equilibrium and the transient process of alluvial mountain rivers subject to varying water and sediment supply,using the method of numerical simulation and mathematical analysis.A 1D morphodynamic model for alluvial mountain rivers is formulated and validated with flume experiments.Then we implement this model to study the river evolution under cycled water and sediment supply from the inlet as well as repeated sediment pulses from the sidewalls.We find that after long-term evolution river can reach a dynamic equilibrium,under which hydrograph boundary layers can be observed both near the upstream end of the channel and around the sediment pulse region.River bed adjusts persistently with the unsteady water and sediment supply within the hydrograph boundary layer,but keeps constant beyond the hydrograph boundary layer.The imbalance between sediment supply and sediment transport capacity is necessary in order to form a hydrograph boundary layer.The equilibrium morphological parameters outside the hydrograph boundary layer can be affected by the unsteadiness of water supply,but are not affected by the unsteadiness of sediment supply.Mathematical analysis of the morphodynamic model shows that the governing equation is a nonlinear diffusion equation for uniform sediment,but becomes nonlinear advection-diffusion equations for sediment mixtures.The more poorly-sorted the sediment,the more advective the governing equations will be.According to the numerical simulation,the advective character of governing equations can generate sorting waves under dynamic equilibrium,thus leading to the breakdown of hydrograph boundary layer.Under such circumstances,bed fluctuations within the hydrograph boundary layer are still quite strong,but outside the hydrograph boundary layer persistent low-amplitude bedload sheets exist along the entire channel reach.More specifically,the breakdown of hydrograph boundary layer occurs only when the sediment is very poorly-sorted.The morphodynamic model is implemented to study the transient process of mountain rivers under the distrubances of an earthquake.We find that the increase of gravel(grain size larger than 2 mm)can lead to bed aggradation;but the increase of sand(grain size smaller than 2 mm)can lead to bed degradation.The reason for bed degradation is the magic sand effect,i.e.,the existence of fine sediment can significantly increase the mobility of bed materials.Since the transient process of bed elevation is dominated by diffusion process,the first-order solution of its governing equation has a form that decays exponentially with time.Therefore,the exponential delayed response model is applicable for bed elevation.The transient process of surface grain size distribution,however,is dominated by both advection and diffusion process,which can lead to a transient process characterized by multi time scale and nonmonotonicity.Therefore,the exponential delayed reponse model is not applicable for surface grain size distribution.