Observations of canonical flow resistance in fast-flowing sand bed rivers

Abstract

Field data are here re-interpreted using the Vortex–Drag equation for evaluating the alluvial resistance in a sand bed river. The Vortex–Drag equation introduces the Rossiter resonance concept into alluvial hydraulics. It appears that this equation is more consistent than the Manning approach for the lower alluvial regime since a ripple configuration effectively induces a lower flow resistance coefficient than a duned configuration. A systematic decrease in the control factor down to a canonical value during the transition from the lower to the upper alluvial regime is found for well-contrasted freshet events. It results that during high stream power events with maximum river bed reshaping, the flow pattern is reproduced by simply imposing this canonical value. This is most welcome in alluvial hydraulic routing, because peak-discharge events are generally these which are the most difficult to characterize in a straightforward way, both experimentally and numerically.

Keywords:

• Alluvial regime
• drag reduction
• in-phase waves
• mobile bed forms
• movable-bed resistance
• optimum bed form configuration
• separated and reattaching flow
• vortex drag model

Acknowledgements

The authors thank the YRCC for fruitful collaboration and access to the data. The research took place when the third author was benefiting from a research stay grant provided by the Brussels Regional District (RiB 2006-14). The study contributes to the AquaTerra Project “Integrated modelling of the river-sediment-soil-groundwater system” funded by the European Sixth Framework Programme, research priority 1.1.6.3 Global change and ecosystems (European Commission, Contract No. 505428-GOCE). It is part of Flux3 “Input/output mass balances in river basin: dissolved and solid matter load”, a sub-component of the AquaTerra Integrated Project.