Effect of Sediment Supply on Morphodynamics of Free Alternate Bars: Insights from Hydrograph Boundary Layer

Huang Dai,Toshiki Iwasaki,Yasuyuki Shimizu

doi:10.3390/w13233437

Abstract

Sediment supply plays an essential role in river morphology. However, the specific impact of sediment supply on river morphology is not apparent. According to the hydrograph boundary layer (HBL) concept, upstream riverbed changes caused by the imbalance between sediment supply and the capacity can propagate only a limited length and have a negligible effect on the riverbed beyond such a short length. We performed a two-dimensional morphodynamic calculation to test the concept of HBL, which was proposed under a one-dimensional simulation, meaning that the concept of HBL is still valid for plane changes in river morphology. We employed an unsteady flow with equilibrium or constant sediment supply in a straight, modeled gravel-bedded channel with an unerodible bank to simulate alternate bar morphodynamics. The results show that regardless of the sediment supply condition, the alternate bar features formed downstream of the HBL are considerably similar. This suggests that sediment disturbance at the upstream end has a negligible effect on the mobile-bed dynamic processes, including alternate bar formation and development downstream of the HBL.

Highlights

Catastrophic flood events result in intense sediment transport and subsequent changes in river morphology, causing water and sediment-related disasters
According to Wong and Parker [18], the bed fluctuation of the latter run propagates downstream but has an influential role within the limited river reach from the upstream end, that is, hydrograph boundary layer (HBL); there should be no difference in the morphodynamic features downstream of the HBL in both conditions. This is valid for 1D cases; in this study, we investigated whether this concept is still valid for the 2D morphodynamic phenomena developed beyond HBL, such as the formation and development of alternate bars that take place far downstream from a sediment source into the river
We investigated the effect of sediment supply on the morphodynamics of free alternate bars in gravel-bed rivers in terms of the HBL concept proposed by Wong and Parker, using a

Summary

Introduction

Catastrophic flood events result in intense sediment transport and subsequent changes in river morphology, causing water and sediment-related disasters. The river morphologies and their dynamics are influenced by numerous physical and environmental factors such as water discharge [7], bed slope [8], grain size [9], anthropogenic stresses [10], and sediment supply [11]. Among these factors, sediment supply plays a vital role because the imbalance between sediment supply from the upstream of river and sediment transport capacity causes bed aggradation or degradation in the downstream reach and controlling sediment transport features and overall river morphologies. The downstream alternate bar disappeared because the block of the dam considerably reduced the sediment supply

Objectives

Methods

Results

Discussion

Conclusion