Effect of bedload sheets beyond hydrograph boundary layer on alternate bar morphology

Abstract

Sediment supply has several essential roles in downstream river morphology and grain size; however, quantifying the relationship between sediment supply effect and river morphodynamics remains difficult. This is because of our limited understanding of the spatiotemporal scale of the nonequilibrium effect caused by the imbalance between sediment supply and transport capacity on downstream river dynamics. Here, we investigate the morphology of alternate bar with mixture-sized sediment under cycled hydrographs and constant sediment supply. This upstream water and sediment supply condition results in repeated aggradation/degradation and fining/coarsening near the upstream end. Under well-sorted sediment condition, such variation of bed and grain size takes place only limited upstream reach so-called hydrograph boundary layer (HBL). On the other hand, poor-sorted sediment breaks down the concept of HBL, causing a grain-sorting wave such as bedload sheet that migrates beyond HBL. This sorting wave represents a long-distance effect of upstream sediment supply on the downstream river morphology. The amplitude of this sorting wave is generally small, but may affect downstream bar dynamics since previous experimental and numerical studies have indicated that alternate bars with mixture-sized sediment are more unstable to some disturbance than those with uniform-sized sediment. To understand this, we perform the numerical calculation by using a two-dimensional morphodynamic model, iRIC-Nays2DH, to clarify how bedload sheet migrating beyond HBL as a disturbance affects the downstream alternate bar morphology. The computational condition is determined based on the Otofuke River, Japan, a typical gravel-bed river. The river has wide grain size distribution and well-developed alternate bars, so this will be a suitable case to understand the effect of bedload sheet on the alternate bar dynamics. The numerical result shows bed elevation variation during a single hydrograph close to the upstream end corresponding to the HBL, and small-amplitude grain sorting wave migrates downstream beyond HBL. The alternate bars developed slightly downstream of HBL reach so that the upstream alternate bar is continuously affected by the migration of bedload sheet. The bedload sheets cause irregular alternate bars upstream to a certain extent, but this has a limited role in more downstream alternate bar dynamics. This will be because of the much larger sediment storage of bars than small-amplitude bedload sheet. This result may suggest that the bedload sheet can indeed migrate downstream of HBL, but, has a small effect on the downstream alternate bar dynamics.