River Flow 2020 Estimation of the transverse mixing coefficients in shallow gravel-bedded rivers

Abstract : Transverse mixing coefficients are key parameters for predicting pollutant dispersion in rivers. While most studies deal with river reaches considered homogeneous, the literature review indi-cates that shallow, gravel-bedded rivers with a longitudinally-varied bed morphology are still poorly documented. The motivation of the present work is that the transverse mixing coeffi-cient in rivers with varied hydraulic parameters could be estimated with a succession of coeffi-cients of more homogeneous sub-reaches. This work applies a 1D-diffusion model to estimate these local transverse mixing coefficients from several local injections. Slug injections of Rho-damine WT are performed at various locations of the Durance River, France. Cross-profiles of tracer concentration at selected cross-sections delineating as homogeneous as possible sub-reaches are measured using fluorometers located near the bed. In addition, hydraulic data are derived from ADCP measurements. Best-fitting solutions of the 1D-diffusion model against the measurements suggest substantial variations of the transverse mixing coefficient between con-secutive sub-reaches.