Definition of an erosion law for cohesive sediments from Gironde estuary using rheology

The accumulation of cohesive sediments is one of the most prominent issues in many tidal estuaries, as it has major implications on estuarine morphodynamics and on water quality and dredging strategies to support harbor activities. The work presented here focuses on the Gironde estuary, and more specifically on the Cadillac site located 40 km upstream of Bordeaux, where sediments were taken from the banks at low tide.

The first step was to characterise the physical properties of these materials by means of a granulometric analysis and rheological tests. Granulometric analysis (Mastersizer 3000, Malvern) is used to ensure the homogeneity of the sediments taken during the various campaigns. These reworked materials are then studied from a rheological point of view (HR2, TA Instruments) in order to identify in particular their yield stress for setting in motion. To do this, flow tests were carried out in a coaxial disc geometry in order to define the corresponding rheogram. The Hershel-Bulkley behaviour law is then applied to the descent curve to identify the yield stress. Once the protocol has been established, the effect of concentration is measured by diluting the sediments taken and the concentration is measured a posteriori by weighing the samples before and after drying. In this way, the law governing the evolution of yield stress as a function of concentration can be established.

Erosion tests were then carried out in a laboratory channel. The sediments were placed in the space left free between two ramps placed at the bottom of the channel. Initially, the threshold stress for setting the various samples in motion was established by gradually increasing the flow rate in the channel for a given height of water. The flow rate at which movement starts to occur on the sediment surface is noted. This flow rate is then translated into parietal stress on the bottom using PIV calibration on a rigid surface. It is assumed that the friction stress on the rigid bottom is equal to the minimum stress required to set the sediment in motion. These tests are repeated for different sediment concentrations. In a second phase, flows greater than the minimum flow are applied for 45 minutes and the quantity of sediment eroded is obtained by a double weighing method and by optical measurement using a 3D camera. In this way, the erosion rate, defined by the change in mass over time on a given reference surface, can be established and its evolution as a function of flow parameters and rheological properties can be determined. The final aim is to be able to propose an erosion law corresponding to the sediments of the Gironde estuary, which could then be used in numerical modelling.

This work was supported by the French National Agency for Research in the context of EMPHASE project (ANR-19-FQSM-0003).

This work pertains to the French Government program “Investissements d’Avenir” EUR INTREE, reference ANR-18-EURE-0010.