There are many methods and equations for
estimating bedload flux in steady flow conditions. Yet, very little is known
about the effect of very unsteady flows, such as flash floods, 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. This research focuses on two durations of flash floods where the
flow is most rapidly changing: a) flash flood bore arriving on dry river bed
and b) flash flood bore arriving on a column of moving water. During those
times hydraulics parameters and bedload transport are monitored and there
relation is examined and characterized.
The methodology of our study is based on the
demonstrated ability of the Eshtemoa gauging station to automatically monitor
the variation of bedload flux depending on flow and bed characteristics, along
with innovative equipment including hydrophones and geophones for capturing
acoustic signals of bedload sediments (1 Hz), video cameras for continuous monitoring
of water surface velocity (by the LSPIV method to determine its structure and
velocity) and 3-D velocimetry for characterizing turbulence (40 Hz). Additional
to these, a well-planned deployment was carried out, including alerting sensors
and cellular transmission, enabling to be onsite when bores arrive.
During the winters of 2015-2019, eleven flow
events were monitored out of which three flow events were sufficiently large to
transport significant amounts of bedload. An innovative mass aggregation based
calibration between the acoustic indirect sensor and the direct slot sampler
allow determination of bedload flux at a frequency of 1 Hz. The results
indicate an increase of the turbulent nature (increase of the turbulent kinetic
energy and the instantaneous vertical velocities), shear stress and bedload
flux during the rising limb in the first two minutes of bore arrival.