A settling velocity parameterization for sand/mud mixture in a 1D flow during a flushing event

Settling velocity is one of the key parameters to understand the deposit phenomena and to succeed in its representation in numerical models. To evaluate this parameter in-situ, a field campaign was carried out during a flushing event of the upper Arc River (Northern French Alps) in June 2011. Since most of the measurement methods are unsuitable in the complete concentration range of this flushing event (0.2 to 20 g/l), a simple method with a settling column was used. Water samples were regularly collected in the river from bridges and directly poured into settling columns. About ten measurements were done for each of the three experimental sites along the river system, to estimate the temporal and spatial dynamics of the settling velocity during the flushing event. Grain size characteristics of the suspension were determined using a laser grain sizer. Regarding these results and the measured concentration of each sample, a detailed description of the different settling velocity regimes is proposed. In the same time, concentration data measured during the same event give information on the suspended sediment concentration (SSC) propagations. Measurements of SSC were done at eleven sampling sites along a 100 km reach downstream from the last dam. Water level was also measured at several locations as well as water discharge. These measurements allow us to calibrate a 1D hydraulic model, used to estimate the suspended sediment mass balance between each measurement site. The deposition fluxes given by this method are compared to the measured values obtained in the settling column.