Author(s): Pierre Lecostey; Guillaume Gomit; Sebastien Jarny; Lionel Thomas

Linked Author(s): Pierre Lecostey, Guillaume Gomit, Sébastien Jarny, Lionel THOMAS

Keywords: Erosion; Cohesive sediment; Model sediment; Turbulent flow; PIV

Abstract: The purpose of this paper is to establish a link between rheological properties of cohesive sediment (Bingham yield stress, τb), turbulent flow (bottom shear stress, τf) and erosion (critical bed shear stress, τcr). Three paralleled techniques have been deployed to study model cohesive sediment under turbulent flow. The erosion of cohesive sediment is a complex subject since numerous sediment properties affect this process (salinity, density, concentration, pH…) [Grabowski2011]. However, rheological properties, especially yield stress, seem to be a significant factor in erosion [Perigaud1983]. Zhang [Zhang2017] recently concluded that rheological properties strongly influence the incipient motion of cohesive sediment. Furthermore, cohesive sediment is fluidized and destabilized by flow oscillations [Yang2014]. To improve emperical formula based on natural cohesive sediment studies, Pouv [Pouv2011] and Tarhini [Tarhini2016] open the door with the realization of a model cohesive sediment made in laboratory. This sediment produced with laponite and Carboxymethyl cellulose is transparent and can be studied by optical measurement methods. The rheological study reveals that the model sediment behaves as a Hershel-Bulkley fluids, similarly to real sediments [Mclinge2015] [Kervella2009]. Parameters of the Hershel-Bulkley model, in particular the yield stress, have been determined based on flow test for different concentration. Moreover, to explore oscillation effects, rheological tests have been made to determine viscoelastic modulus of the sediment (loss and storage). The study of the model sediment under turbulent flow has been performed in a square closed channel. Particle Image Velocimetry (PIV) coupled with a surface detection algorithm have been used. The used of the PIV allows the estimation of the bottom shear stress of the flow with various methods. In addition, the correlation of velocity field data from the PIV with the surface detection allows the analysis of the relationship between the flow structures and the bed sediment motion. The first tests performed in the channel have shown the apparition of ripples on the sediment surface. It seems that ripple amplitude is related to velocity flow and sediment concentration. The ripple characteristics will be analyzed to be linked to flow characteristic (turbulent structure, wall shear stress …) and rheological properties of the sediment (yield stress, viscoelastic modulus….). Ref Grabowski2011: Grabowski et al., Earth Sci. Rev., 2011 Perigaud1983: Périgaud, La Houille Blanche, 1983 Yang2014: Yang et al, J. Sediment Res., 2014 Zhang2017: Zhang et al, Water Resour. Res., 2007 Pouv2011: Pouv, PhD, 2001, Tarhini2016: Tarhini, PhD, 2016 Mclinge2015: Mclinge, Rencontres Universitaires Génie Civil, 2015 Kervella2009: Kervella, PhD, 2009

DOI: https://doi.org/10.3850/IAHR-39WC252171192022499

Year: 2022