Numerical modelling of mixed-sediment consolidation

Abstract

Sediment transport modelling in estuarine environments, characterised by cohesive and non-cohesive sediment mixtures, has to consider a time variation of erodibility due to consolidation. Generally, validated by settling column experiments, mud consolidation is now fairly well simulated; however, numerical models still have difficulty to simulate accurately the sedimentation and consolidation of mixed sediments for a wide range of initial conditions. This is partly due to the difficulty to formulate the contribution of sand in the hindered settling regime when segregation does not clearly occur. Based on extensive settling experiments with mud-sand mixtures, the objective of this study was to improve the numerical modelling of mixed-sediment consolidation by focusing on segregation processes. We used constitutive relationships following the fractal theory associated with a new segregation formulation based on the relative mud concentration. Using specific sets of parameters calibrated for each testwith different initial sediment concentration and sand content-the model achieved excellent prediction skills for simulating sediment height evolutions and concentration vertical profiles. It highlighted the model capacity to simulate properly the segregation occurrence for mud-sand mixtures characterised by a wide range of initial conditions. Nevertheless, calibration parameters varied significantly, as the fractal number ranged from 2.64 to 2.77. This study investigated the relevance of using a common set of parameters, which is generally required for 3D sediment transport modelling. Simulations were less accurate but remained satisfactory in an operational approach. Finally, a specific formulation for natural estuarine environments was proposed, simulating correctly the sedimentation-consolidation processes of mud-sand mixtures through 3D sediment transport modelling.

Modelling Fine Sediment Dynamics: Towards a Common Erosion Law for Fine Sand, Mud and Mixtures

“…According to the model concept, the consolidation state of the bottom sediment is linked to the relative mud concentration, i.e., the concentration of mud in the space between the sand grains (C rel mud ). Given that the surficial sediment in our study area is weakly consolidated (erosion actually occurs in surficial layers only, typically a few mm or cm thick) and mainly composed of fine sand, consolidation was disregarded and C rel mud was set at a constant value of 550 kg·m −3 (representative of pre-consolidated sediment according to Grasso et al [41]). In addition, bed load was not taken into account in the present application, assuming that surficial sediment in our study area is mainly composed of mixtures of mud and fine sand.…”

Monitoring and characterisation of sand-mud sedimentation processes

“…pure muds). However, it has been noted as being rarely applied to mixed sediments (Grasso et al 2015) and, indeed, Van and Pham Van Bang (2013) described the method as being irrelevant for mixed sediments (e.g. sandy-mud, muddy-sand).…”

“…Toorman 1996Toorman , 1999Merckelbach and Kranenburg 2004b;Le Hir et al 2011) but with varying success in the accurate simulation of consolidation for sediment mixtures with medium to high sand contents (i.e. >10-20 %) (Grasso et al 2015). However, the consolidation model proposed by Le Hir et al (2011) was shown to provide good predictive capabilities for mud consolidation, as well as the capacity to simulate sand-mud segregation within the bed.…”

“…Several modelling techniques (of varying complexities) have been proposed to simulate these bed consolidation processes. The most comprehensive and widely used approach, at least for primary consolidation (Grasso et al 2015), is known as the Gibson model (Gibson et al 1967), which computes the time evolution of vertical concentration profiles over the unified hindered settling and initial consolidation phases of bed evolution (Toorman 1996). The Gibson theory was developed for, and has been applied successfully to simulate, consolidation processes in monodisperse cohesive sediments (i.e.…”

“…sand) sediments within estuaries and coastal regions relies on physical understanding of the complex interplay between transportation processes such as flocculation, settling, deposition, erosion and consolidation (e.g. Grasso et al 2015). In this respect, net sedimentation [i.e.…”