Characterization of bedload discharge in bores and very unsteady flows in an ephemeral channel

Abstract

Bedload flux under steady flow conditions is calculated by a multitude of available methods and equations. Yet, very little is known about the effect of very unsteady flows, such as flash floods and specifically bores, on bedload flux. The unpredictable nature of the floods together with many logistic difficulties and safety issues in monitoring explain this gap in knowledge. Global climate change may increase flood event occurrence, making their understanding even more crucial. The methodology of our study is based on automatically monitoring bedload flux (Reid slot samplers). Automation allowed high frequency monitoring of hydraulic parameters and bedload flux. Added novelty includes pipe and plate microphones for capturing acoustic signals of bedload sediments and 3-D velocimetry for characterizing turbulence. Alerting sensors and cellular data transmission enabled onsite presence upon bore arrival. Calibration between the acoustic indirect sensors and the direct slot samplers allows determination of bedload flux at a frequency of 1 Hz. Analyses of flood events indicate an increase in turbulent kinetic energy, instantaneous vertical velocities, shear stress and bedload flux during the rising limb within the first two minutes after bore arrival. This has implications for the likely destabilization of the channel bed and for bedload transport after passage of the bore and during subsequent, less unsteady flows.