ADCP measurements of bedload due to hydropeaking versus natural floods

A hydropeak is a rapid increase in river discharge induced by a hydroelectric dam when optimizing energy production.  These flow fluctuations occur in many regulated rivers and can influence sediment transport and fluvial habitat. The present study investigates the relative importance of hydropeaks versus natural floods for bedload sediment transport in the Ésera River, Central Pyrenees, Spain. An acoustic Doppler current profiler (ADCP) was used to measure both stationary time series and spatial distributions of apparent bedload velocity, which is the bias induced in ADCP bottom track velocity (Doppler sonar) due to bedload transport. A Sontek RiverSurveyor M9® ADCP, coupled with a Leica GS15® Real-Time Global Navigation Satellite System (RTK-GNNS), was deployed on a tethered floating survey platform from the road bridge at the Santaliestra monitoring section, which is approximately 13 km downstream from the hydropower plant.During two measurement campaigns in 2019 and 2020, a total of 29 of the stationary ADCP apparent bedload velocity measurements distributed across the channel section were coupled with synchronous adjacent physical bedload samples collected with a Helley-Smith sampler.  Correlation of such paired samples can be used to develop a calibration relation between observed ADCP apparent bedload velocity (m/s) and bedload sediment transport rate (kg/m/s). The physical bedload samples were processed in the laboratory to obtain fractional bedload transport rates. The paired data set was insufficient to develop a strong overall calibration relation, but fractional results aligned with calibration relations developed in other rivers with similar mixed sand/gravel bed materials.A total of 13 spatial surveys of apparent bedload velocity were obtained for different flow rates, during both hydropeaking events and natural floods. Initial observations suggest natural floods result in greater sediment transport in the Santaliestra section due to the input of sediment from tributaries. Nonethless, hydropeaks were observed to partially destabilize/mobilize the bed, and thus contribute to sediment transport and morphodynamic processes.