Wind-Induced Resuspension and Transport of Contaminated Sediment from the Rove Canal into the Etang De Berre, France

The present study concerns the erosion and transport of severely contaminated sediments in a Canal. It begins in the context of an engineering project aimed to re-introduce a forced convection at the entrance of this Canal by pumping marine water. The local wind is often strong enough to overpass the resuspension threshold; thus, there is a serious risk of downstream contamination of a Mediterranean lagoon. So, the goal is to evaluate this risk as a function of the pumping rate; this contamination is transported by the fine suspended particles. Different scenarios are investigated to determine the downstream transport of suspensions in terms of runoff. These scenarios (of 24 h) contains a succession of 3 periods: constant wind speed, wind slowdown and calm, for two opposite wind directions. Special attention is devoted to the modeling of complex mechanisms of erosion and resuspension during wind periods, deposition during windless periods and sediment consolidation. The main results concern the total flux of the suspended particles through the exit of the Canal at the confluence with the lagoon. It is shown that even for moderate runoff (<6 m3/s) this total flux is large enough, not only during the wind period, but also after several hours of calm.

Grey Box Modelling of Decanter Centrifuges by Coupling a Numerical Process Model with a Neural Network

Continuously operating decanter centrifuges are often applied for solid-liquid separation in the chemical and mining industries. Simulation tools can assist in the configuration and optimisation of separation processes by, e.g., controlling the quality characteristics of the product. Increasing computation power has led to a renewed interest in hybrid models (subsequently named grey box model), which combine parametric and non-paramteric models. In this article, a grey box model for the simulation of the mechanical dewatering of a finely dispersed product in decanter centrifuges is discussed. Here, the grey box model consists of a mechanistic model (as white box model) presented in a previous research article and a neural network (as black box model). Experimentally determined data is used to train the neural network in the area of application. The mechanistic approach considers the settling behaviour, the sediment consolidation, and the sediment transport. In conclusion, the settings of the neural network and the results of the grey box model and white box model are compared and discussed. Now, the overall grey box model is able to increase the accuracy of the simulation and physical effects that are not modelled yet are integrated by training of a neural network using experimental data.

Consolidation of gas hydrate-bearing sediments with hydrate dissociation

Quantifying sediment deformation induced by depressurization of gas hydrate reservoirs and hydrate dissociation is crucial for the safe and economic production of natural gas from hydrates, and for understanding hydrate-related natural geological risks. This study uses our recently developed fully-coupled Thermo-Hydro-Mechanical formulation for gas hydrate-bearing geological systems implemented in the 3D Code_Bright simulator. First, the model formulation is briefly presented. Then, the model is applied to reproduce published experimental consolidation tests performed on hydrate-bearing pressure-core sediments recovered from the Krishna–Godavari Basin (offshore of India) during the India National Gas Hydrate Project Expedition 02 (NGHP02). The numerical simulation reproduces the tests in which the sediment is loaded and unloaded prior and after hydrate dissociates via depressurization at constant effective stress. Our results successfully capture sediment collapse when hydrate dissociates at a mean effective stress above that of the host sediment consolidation curve. The mechanical constitutive model Hydrate-CASM also allows reproducing the experimentally observed changes in sediment swelling index with changes in hydrate saturation.

Drainage of soft cohesive sediment with and without <i>Phragmites australis</i> as an ecological engineer

Conventional drainage techniques are often used to speed up consolidation of fine sediment. These techniques are relatively expensive, are invasive and often degrade the natural value of the ecosystem. This paper focusses on exploring an alternative approach that uses natural processes, rather than a technological solution, to speed up drainage of soft cohesive sediment. In a controlled column experiment, we studied how Phragmites australis can act as an ecological engineer that enhances drainage, thereby potentially promoting sediment consolidation. We measured the dynamics of pore water pressures at 10 cm depth intervals during a 129-day period in a column with and without plants, while the water level was fixed. Water loss via evaporation was measured using Mariotte bottles and the photosynthetic processes – including plant transpiration – were measured with a LICOR photosynthesis system. The results show that several processes initiated by P. australis interfere with the physical processes involved in sediment drainage and consolidation. Phragmites australis effectively altered the pore pressure gradient via water extraction, especially between 40 and 60 cm from the bottom of the column. In this zone, daily cycles in pore pressures were observed which could directly be linked to the diurnal cycle of stomatal gas exchange. On average, water loss via evaporation and transpiration of leaves of P. australis amounted to 3.9 mm day−1, whereas evaporation of bare soil amounted on average to 0.6 mm day−1. Moreover, the depth-averaged hydraulic conductivity increased on average by 40 % in presence of P. australis. The results presented in this study provide information needed for predictive modelling of plants as ecological engineers to speed up soil forming processes in the construction of wetlands with soft cohesive sediment.

LONG-TERM SUBSIDENCE OF BEACH NOURISHMENT AND DUNE RESTORATION IN CAMINADA HEADLANDS, COASTAL LOUISIANA

Coastal barrier islands are the first line of defense for protecting wetlands, inland bays, and mainland regions from direct effects of wind, waves, and storms. Rosati (2006) indicate that 20 to 40% of the total sand volume can be sequestered and lost from the sandy barrier island through consolidation. As a result, predicting long-term subsurface sediment consolidation is integral to determining the ability of barrier islands to provide coastal protection and resilience to future hazards, such as relative sea level rise, sediment erosion, and hurricanes. This study uses the Caminada Headlands geotechnical investigations and monitoring data to determine empirical correlations for deltaic sediment compressibility and develop a validated and calibrated consolidation and subsidence numerical model for future barrier island restoration projects. With this calibrated model, differential settlements associated with sand fill placement can be estimated to design placement elevations to maintain post-construction topography for ecological habitat and restoration requirements and can be used for future beach restoration projects along barrier island shorelines.