Assessing river dynamics from 2D hydraulic modelling and high resolution grain-size distribution

A new approach is proposed to study gravel riverbed stretches at high resolution. During a two-year period, flood events in the River Isabena (Central Pyrenees) were reproduced using a steady-state 2D hydraulic model (Hydro2de) with the aim of studying gravel riverbed dynamics. The parameters included detailed grain size at 1.5 x 1.5 m resolution, topographic data and field measurements along an 11 km stretch. Discharge data were obtained from 2 gauging stations and velocity and maximum water width were measured at 15 control cross-sections. The Aguirre-Pe formula was selected to estimate flux resistance using detailed grain size data from a previous study. Observed and modeled results show good agreement; and some geomorphological features could also be quantified. Additionally, shear stress for several discharges and related critical shear stress were compared for the whole river stretch. The initiation of sediment motion occurred at a lower discharge in the lower stretches than in the middle and upper stretches, where the mean grain size was coarser. This matched riverbed erosion measures. Results were obtained at 1.5 x 1.5 m resolution, showing a good potential as bases for subsequent fluvial applications (i. e. entrainment, channel scour).