Scale‐Dependent Evanescence of River Dunes During Discharge Extremes

S Naqshband,A J F Hoitink

doi:10.1029/2019gl085902

Abstract

AbstractDuring high river discharge extremes, the growth of dunes can reach a maximum beyond which a transition to upper stage plane bed may occur, enhancing the river's conveyance capacity and reducing flood risk. Our predictive ability of this bedform regime shift in rivers is exclusively built upon high Froude number flows dominated by asymmetric dunes with steep downstream‐facing slipfaces that are rare in natural rivers. By using light‐weight polystyrene particles as a substrate in an experimental flume setting, we present striking dune morphodynamic similarity between shallow laboratory flow conditions and deep rivers, preconditioned that both flow and sediment transport parameters are accurately scaled. Our experimental results reveal the first observation of upper stage plane bed in a shallow laboratory flume that is reached for a Froude number well below unity. This work highlights the need to rethink widely used dune scaling relationships, bedform stability diagrams, predictions of flow resistance, and flood risk.

Highlights

Dunes are periodic sediment structures that arise from the interaction between a flow field and the underlying mobile bed, in fluvial environments dominated by coarse silt, sand, or gravel
Our predictive ability of this bedform regime shift in rivers is exclusively built upon high Froude number flows dominated by asymmetric dunes with steep downstream‐facing slipfaces that are rare in natural rivers
Our analysis reveals a striking dune morphodynamic similarity between shallow laboratory flows and deep rivers, preconditioned that both Froude number and sediment transport parameters are well aligned with conditions in deep rivers

Summary

Introduction

Dunes are periodic sediment structures that arise from the interaction between a flow field and the underlying mobile bed, in fluvial environments dominated by coarse silt, sand, or gravel. Dunes in deep rivers are primarily symmetric, with significantly lower slipface angles, often less than 10° (referred to as low‐angle dunes [LADs]). They possess complex leeside morphologies with superimposed bedforms on both the stoss and the lee sides (Galeazzi et al, 2018; Hendershot et al, 2016). With increasing flow intensity and transport of bed material into suspension, HADs in flumes decay, make a regime shift, and wash out to upper stage plane bed (USPB) at a suspension number that is twice lower compared to LADs in deep rivers (Naqshband et al, 2014a; Bradley & Venditti, 2017). We present the first observation of USPB in a shallow laboratory flume that is reached for a Froude number well below unity, paving the way for advancement in our understanding of causative mechanisms governing dune morphology and leeside dynamics

Methods and Laboratory

Dune Kinematic Evolution to Upper Stage Plane Bed

Dune Morphology

Implications for Flow Resistance and Flood Risk