Search

The morphology of tide‐dominated systems is progressively influenced by human activities and climate change. Quantitative approaches aiming at understanding or forecasting the effects of interventions and climate change are often aggregated, thereby simplifying or schematizing the investigated area. In this work, we advance on the knowledge of sediment transport processes shaping tidal systems and on methodologies translating schematized model output into physically realistic variables. In terms of improved physics, we systematically evaluate the influence of sand‐mud interaction processes. Most tidal systems are shaped by a mixture of sand and mud. Morphological models typically compute transport of sand and mud independently, despite studies clearly demonstrating that their physical behavior is mutually dependent. We investigate the effects of two interaction mechanisms (erosion interaction and roughness interaction, applied with varying mud erodibility) with a schematized process‐based morphodynamic model. We convert model output into metrics that describe the meso‐scale configuration of the modeled systems, allowing a quantitative comparison of scenarios. Modeled patterns and intertidal flat shape, size and composition widely vary with mud erodibility settings, but equally depend on the evaluated sand‐mud interaction mechanisms (with erosion interaction having a larger effect than roughness interaction). Sand‐mud interaction thus needs to be accounted for from a physical point of view, but also to improve predictions of tidal basin evolution models, particularly the (bimodally distributed) sediment composition of intertidal flats. Plain Language Summary: Human interventions and climate change are progressively influencing the state of coastal systems. Their impact on coastal evolution takes place on decades to centuries and the present‐day modeling tools have limitations to study these impacts. Models with highly schematized configurations have often been used in the past to increase our understanding of coastal evolution. We advance on this, by determining the effect of sand‐mud interaction on the modeled long‐term evolution of tidal basins. The bed of most tidal basins is composed of two sediment types: sand and mud. From previous research, it is known that sand and mud do not behave independently. We investigate the effects of two interaction mechanisms (erosion and roughness interaction) using a model with a schematized configuration in terms of topology and forcing, but including the most relevant physical processes. To quantitatively compare the output of different model scenarios, we translate the results into a number of metrics. Our results reveal that sand‐mud interaction processes (particularly erosion interaction) influence tidal basin evolution. Including sand‐mud interaction can significantly improve modeling results, especially the sediment composition of intertidal flats. We present guidelines that can be followed to improve models depending on the real‐life system they should compare to. Key Points: Sand‐mud interaction largely impacts long‐term morphodynamic evolution of intertidal areas in tidal systemsAccounting for sand‐mud interaction in morphodynamic models is crucial to reproduce the observed bimodal distribution of the mud contentSchematized model results should be parameterized for validation and comparison with real systems [ABSTRACT FROM AUTHOR]

Abstract: Cohesive sediment is an important component of aquatic environments, which must be monitored and managed for environmental, engineering, and human health reasons. While the hydrodynamic aspects of sediment erosion and transport are well understood, the erodibility of cohesive sediment has proved more difficult to address and predict. Erodibility is the propensity for the sediment to be eroded, and is represented typically as an erosion threshold or erosion rate. It is an attribute of the sediment itself, and is dependent on the sediment properties that dictate the resistive forces in the sediment, such as gravity, friction, cohesion, and adhesion. This paper reviews recent findings from a range of disciplines to create a comprehensive picture of the physical, geochemical and biological properties that influence the erodibility of cohesive sediment. By identifying these key sediment properties, we provide the background for a discussion on how changes in and interactions between the properties generate significant spatial and temporal variations in erodibility. We discuss the development of a predictive model of erodibility, and emphasize the need for more comparable field and laboratory data. [Copyright &y& Elsevier]

Abstract: The selective processing of sediment fractions (sand and mud; >63μm and ≤63μm median grain size) by macrofauna was assessed using two size classes of inert, UV-fluorescent sediment fraction tracers (luminophores). The luminophores were applied to the sediment surface in 16m2 replicated plots, defaunated and control, and left to be reworked by infauna for 32 days. As the macrofaunal assemblage in the ambient sediment and the control plots was dominated by the common cockle Cerastoderma edule, this species was used in an additional mesocosm experiment. The diversity, abundance and biomass of the defaunated macrobenthic assemblage did not return to control values within the experimental period. Both erosion threshold and bed elevation increased in the defaunated plots as a response to the absence of macrofauna and an increase in microphytobenthos growth. In the absence of macrobenthos, we observed an accretion of 7mm sediment, containing ca. 60% mud. Image analysis of the vertical distribution of the different luminophore size classes showed that the cockles preferentially mobilised fine material from the sediment, thereby rendering it less muddy and effectively increasing the sand:mud ratio. Luminophore profiles and budgets of the mesocosm experiment under “no waves–no current” conditions support the field data very well. [Copyright &y& Elsevier]

The mouth of the Seine River estuary (France) has undergone marked morphological evolution over several decades mainly due to engineering works aimed at improving access to Rouen and Le Havre harbours. The intertidal areas are decreasing in size and the lower estuary is accumulating sediment and prograding. In order to understand and better describe the major morphological behaviours of the estuary, a morphodynamic numerical model was developed within the Seine-Aval program. At the end of the 1st part of the research program, a validated fine sediment transport model (3D) was available (Le Hir et al., b). As the present morphological study addresses medium-term issues (a few decades), and because of the need to investigate impacts of local structures or events, we chose to use the so-called “process-based approach” starting from the existing model. First, the existing model was upgraded to account for (suspended) sand transport, and to achieve coupling between morphological changes and sediment transport. Erodability of the sediment accounts for the respective proportions of mud and sand. Simulations starting from an arbitrary surficial sediment cover show that the model is able to reproduce realistic sediment patterns. For example, it is able to change the sediment nature on the intertidal flat near Le Havre from sand to mud. Observed structures of suspended sediment are also reproduced: fine particles mainly follow the turbidity maximum whereas significant concentrations of sand grains in suspension are found where the hydrodynamic stresses are intense. Concerning morphodynamics, simulations with real forcing over one year are discussed. The effect of waves on the bathymetric evolution of the mouth is shown and the sensitivity of morphodynamics to the coupling procedure is tested. [ABSTRACT FROM AUTHOR]

Scientific research on sediment dynamics in the coastal zone and along the littoral zone has evolved considerably over the last four decades. It benefits from a technological revolution that provides the community with cheaper or free tools for in situ study (e.g., sensors, gliders), remote sensing (satellite data, video cameras, drones) or modelling (open source models). These changes favour the transfer of developed methods to monitoring and management services. On the other hand, scientific research is increasingly targeted by public authorities towards finalized studies in relation to societal issues. Shoreline vulnerability is an object of concern that grows after each marine submersion or intense erosion event. Thus, during the last four decades, the production of knowledge on coastal sediment dynamics has evolved considerably, and is in tune with the needs of society. This editorial aims at synthesizing the current revolution in the scientific research related to coastal and littoral hydrosedimentary dynamics, putting into perspective connections between coasts and other geomorphological entities concerned by sediment transport, showing the links between many fragmented approaches of the topic, and introducing the papers published in the special issue of Water on “Sediment transport in coastal waters”. [ABSTRACT FROM AUTHOR]

Abstract: Mixed sediments are constituted of cohesive and non-cohesive materials with distinct behaviours that numerical models traditionally manage separately. This paper first introduces a rapid state of the Art in sediment transport modelling in order to point out the specific requirements for process-based models applied to mixed sediments. Based on a preliminary study by , which showed the validity of the advection approach to compute fine sand transport, a complete modelling strategy is described: it is applied to the suspended transport of sand and mud mixtures, and accounts for consolidation of mixed sediments. Special care is paid to the realistic representation of the structure and density of sand and mud mixtures, and to the segregation in consolidating sediment layers. The model state variables are the different classes of particles, generally classified according to their size, and grouped into categories that are either transported as bedload or in suspension. The bed is described as thin layers characterised by a distribution of these classes. The erosion law for fine sands and for sand and mud mixtures is a function of the excess shear stress calibrated against measurements in a small flume. The transition between cohesive and non-cohesive behaviours is parameterised through a critical mud fraction that depends on the sand grain size: the coarser the sand, the higher the mud content before the sediment becomes cohesive. The consolidation module is based on Gibson equation formulated for each class, and modified to account for segregation. Constitutive relationships are calibrated by means of laboratory settling tests. In the deposition module, new deposits may be managed in different ways (creation of a new layer or integration into the existing surficial layer) depending on the mud fraction and its relative concentration. When deposited material is mixed with the surficial sediment, pores between coarser particles are first filled up with finer particles before increasing the layer thickness. The new modelling frame has first been used to simulate laboratory settling tests with mixed sediments. When the initial mixture density is low, sand particles can settle through the mud and form a dense sandy layer on the bottom. In a second application, the model is used to describe sorting processes when tidal currents re-suspend a sand and mud mixture. A sand layer is then likely to form within the sediment, while the surficial layers are muddier. A dynamic bed armouring process is shown: although sand is easily resuspended, eroded grains in the sand layer settle rapidly, reducing the erosion of underlying sediment. Resulting suspended sediment concentration is strongly reduced, as well as sediment fluxes. The application demonstrates the model ability to simulate layering processes and time-variations of sediment erodibility. [Copyright &y& Elsevier]