An 11 hours survey was performed on the 17th of September 2014 in the Rhine Region Of Freshwater Influence (Rhine-ROFI) about 10 km downstream of the mouth of the Rotterdam Waterway during calm weather conditions. Suspended Particle Matter (SPM) measurements were performed during a full tidal cycle, near the seabed, at neap tide, and samples were taken at 0.6 meter above bed for on-board analysis. The measurements were performed with (a) LISST 100X, a submersible particle size analyzer, (b) LISST-HOLO, a submersible digital holographic camera, (c) a home-made underwater camera and (d) an on-board LabSFLOC2 video microscopy equipment that used in-situ collected samples. The first aim of the present study was to compare the results obtained from the different monitoring techniques and to characterize the different types of suspended particles found in-situ. It was found that that the highly anisotropic particles present in the water column lead to multiple peaks in the Particle Size Distributions (PSD) found using the LISST 100X. Using the LISST-HOLO, underwater camera and LabSFLOC2 camera these particles could properly be imaged and meaningful PSD’s were obtained using these techniques in the size range > 20 μm. LabSFLOC2, LISST-HOLO and the underwater camera moreover provide information on the size and aspect ratio of particles. On the other hand, LISST 100X can be used to detect the fine fraction (<20 μm), a size range that is not accessible for the other techniques. From the analysis of the data on the survey day, three classes of particles were identified, based on composition rather than size (the sizes given here are purely indicative): (1) mineral (inorganic) sediment particles in the range size 5 - 20 μm, (2) organic/inorganic aggregates in the size range 20 - 200 μm and (3) organic particles in the size range > 200 μm. A large range of settling velocities (0.1 - 10 mm/s) and aspect ratios between 1 and 10 were recorded by video microscopy (LabSFLOC2). This spreading in settling velocities and aspect ratio was due to the different properties (shape, effective density and size) of the particles in the water column. The second aim of the study was to reproduce the flocs found in-situ in the lab and investigate the kinetics of flocculation between inorganic and living organic matter. Laboratory experiments were conducted with grab samples obtained from Port of Rotterdam harbour and living microalgae (Skeletonema costatum). The results of these experiments showed a shift in effective density upon addition of living algae to the sediment, which confirmed the flocculation ability between sediment and microalgae. The flocculation occured on a timescale of minutes and lead to flocs having a large spread in density for a given size, due to the heteregeous inorganic/organic composition of the flocs. This spread in density was at the origin of the large range of settling velocities for a given floc size observed in-situ, which leads to conclude that organic matter should be an important input parameter in sediment transport models.

This paper presents a new model, using existing consolidation theory, suitable for long-term morphodynamic simulations; we refer to the dynamic equilibrium consolidation (DECON) model. This model is applicable for muddy systems at small suspended particulate matter (SPM) concentrations, where the sedimentation rates are smaller than the consolidation rates and small fractions of sand can be accounted for. Thus, the model assumes quasi-equilibrium of the consolidating bed. It is derived from the full consolidation (Gibson) equation and is implemented in a mixed Lagrangian-Eulerian bed model guaranteeing stable and non-negative solutions, while numeric diffusion remains small. The erosion and deposition of sand and mud is accounted for, whereas internal mixing (e.g., bioturbation) is modeled through diffusion. The parameter settings for the new consolidation model (the hydraulic conductivity, consolidation coefficient, and strength) can be obtained from consolidation experiments in the laboratory. The model reproduces one-dimensional consolidation experiments and the qualitative behavior of erosion and deposition in a tidal flume. The DECON model was also applied to more natural conditions, simulating fine sediment dynamics on a schematized mud flat and in a schematized tidal basin under tide and wave forcing. The computational results of the mudflat simulations compared well with the simulations with the full Gibson equation. For the tidal basin simulations, DECON predicted the expected landward tidal transport of fine sediment during tide-dominated conditions, while the tidal basin withstood erosion during the more energetic wave-dominated periods. Computational times for the morphodynamic simulations of the tidal basin example without waves increased by a factor of 5 when consolidation was included. For the simulations with waves, this increase in computational times was only a factor of 2, as simulations with waves are always expensive. Applying a complete consolidation model would be prohibitive. The DECON model therefore serves as a useful tool to simulate fine-sediment dynamics in complex wave- and tide-dominated conditions, as well as the effects of seasonal variations.

The Ems and Loire evolved into hyper-turbid estuaries in the course of the 20th century. Winterwerp et al. (2013 a, b) argued that narrowing and deepening of these rivers induced a regime shift towards these hyper-turbid conditions. This regime shift was presumed to be driven by a positive feed-back between tidal amplification, tidal asymmetry and sediment-induced drag reduction. As also the Sea Scheldt was/is subject to narrowing and deepening, Winterwerp et al. reasoned that also this river may be at risk to become hyper-turbid.

This article is part of the Topical Collection on the 11th International Conference on Cohesive Sediment Transport