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Observations and modelling of nearshore sediment sorting processes along a barred beach profile
The understanding of cross-shore sediment sorting is of primary importance for the design of sand nourishments and for assessing the suitability of the seabed to different ecological species. In this paper, sediment sorting processes were investigated by using a combination of physical and detailed numerical modelling. Data from large-scale wave flume experiments were used to validate a 2DV cross-shore Delft3D model. The model solves coupled short-wave averaged equations for flow, sediment transport, bed composition and bed level change. The infra-gravity wave motions were explicitly resolved. In order to investigate sorting processes, eight sediment fractions were used as well as a layered bed stratigraphy. The effects of different wave conditions (high energetic and more moderate energetic waves) on the morphodynamic profile development and sorting processes were investigated. The Delft3D model reproduced the profile development and bar position very accurately. Additionally, model predictions of sediment sorting across the profile fitted very well with the available observations. The numerical model simulations showed the importance of including short-wave grouping and infragravity wave effects in order to reproduce the cross-shore profile development, especially the breaker bar dynamics and sediment sorting processes. Infragravity waves contribute to larger sediment entrainment and more offshore bar development. Besides leading to a better prediction of the bed profile, infragravity waves also lead to a better prediction of the bed composition. Model results are in agreement with experimental data, showing its capabilities in functioning as a tool to predict sorting processes.
Predicting coastal hazards for sandy coasts with a Bayesian Network
Low frequency, high impact storm events can have large impacts on sandy coasts. The physical processes governing these impacts are complex because of the feedback between the hydrodynamics of surges and waves, sediment transport and morphological change. Predicting these coastal changes using a numerical model requires a large amount of computational time, which in the case of an operational prediction for the purpose of Early Warning is not available. For this reason morphodynamic predictions are not commonly included in Early Warning Systems (EWSs). However, omitting these physical processes in an EWS may lead to potential under or over estimation of the impact of a storm event. To solve this problem, a method has been developed to construct a probabilistic Bayesian Network (BN). This BN connects three elements: offshore hydraulic boundary conditions, characteristics of the coastal zone, and onshore hazards, such as erosion and overwash depths and velocities. The hydraulic boundary conditions are derived at a water depth of approximately 20 m from a statistical analysis of observed data using copulas, and site characteristics are obtained from measurements. This BN is trained using output data from many pre-computed process-based model simulations, which connect the three elements. Once trained, the response of the BN is instantaneous and can be used as a surrogate for a process-based model in an EWS in which the BN can be updated with an observation of the hydraulic boundary conditions to give a prediction for onshore hazards. The method was applied to Praia de Faro, Portugal, a low-lying urbanised barrier island, which is subject to frequent flooding.
Predicting the impact of seasonal fluctuations on the potential ecotoxicological risk of multiple contaminants in the River Scheldt discharge into the Western Scheldt estuary
The study site, Land van Saeftinghe, is an area within the Western Scheldt estuary with Special Protection Area (SPA) status for the EU Birds directive. We used a combination of hydrodynamic and water quality monitoring, ecotoxicity modeling and observations on the ecological status to correlate the water quality to the observed ecological status of the study site. The monitoring and modeling results show that the copper concentrations are elevated above the Maximum Tolerable Concentration (MTR) during the whole year, while zinc and cadmium show a spring peak (> MTR). Other metals, and all the measured organic contaminants where below the MTR. The peak in the three metals during springtime translates into an increase in the ecotoxicity, as expressed by the calculated multi-substance Potentially Affected Fraction (ms-PAF). The ms-PAF peaked at 25% for all organisms, and 30% for benthic invertebrates. The observed ecological status for the study site at the time of monitoring (2000) was Maximum Ecological Potential (MEP). This result seems to be in contradiction with the exceedance of the MTR for three metals during spring time. However the calculated ms-PAF during the spring peak of 25% is deemed acceptable based on current policy, therefore potential ecotoxic stress is within acceptable boundaries. The ms-PAF results therefore do not contradict the MEP status. The evaluation of the ecological status versus the water quality within the EU Water Framework Directive (WFD) can be improved by using the calculated ecotoxicity (ms-PAF) instead of the water quality. We propose an additional step for water bodies which are currently ‘at risk’, based on their chemical status. In this additional step, the ms-PAF is calculated for all contaminants in the water phase, not discriminating between priority and non-priority substances. The outcome of this calculation defines if the water body is at risk.
Dynamisch gedrag van waterbouwkundige constructies, deel C : methoden van rekenen en experimenteel onderzoek
Dynamisch gedrag van waterbouwkundige constructies, deel B : constructies in golven
Dynamic behaviour of hydraulic structures, part C : calculation methods and experimental investigations
Dynamic behaviour of hydraulic structures, part B : structures in waves
Sediment sorting at the Sand Motor at storm and annual time scales
Bed sediment composition, with a focus on the median grain size D50, was investigated at the large-scale nourishment ’Sand Motor’ (Zandmotor) at the Dutch coast. Considerable alongshore heterogeneity of the bed composition (D50) was observed as the Sand Motor evolved over time with (1) coarsening of the exposed part of the Sand Motor and (2) a depositional area with relatively fine material just North and South of the Sand Motor. The alongshore heterogeneity of the measured D50 values was most evident outside the surfzone (i.e. seaward of MSL-4m). Coarsening of the bed after construction of the Sand Motor was attributed to hydrodynamic sorting processes, because the alongshore heterogeneity of the D50 showed a similar spatial pattern as the mean bed shear stresses. The observed alongshore heterogeneity of the D50 and correlation of D50 with modelled mean bed shear stresses, suggest that preferential erosion of the finer sand fractions has taken place. The selective transport of finer sand fractions results in a coarser top layer of the bed at the Sand Motor. The preferential transport is most dominant during mild and moderate conditions when hydrodynamic forcing conditions are close to the critical bed shear stresses for transport. The measurements also show the impact of a storm, which consists of a finer D50 of the offshore bed composition in front of the Sand Motor (i.e. where a considerably coarser bed was in place). Additionally, storms may generate a (temporary) zone with fine bed material at the toe of the deposition profile. This means that the coarsening of the bed is reduced by storms as a result of the mobilization of both coarse and fine sediment and mixing of the bed with the relatively finer substrate.
Sediment-induced buoyancy destruction and drag reduction in estuaries
This paper presents an analysis of drag reduction by buoyancy destruction in sediment-laden open channel flow. We start from the log-linear profile proposed by Barenblatt (1953), extended with a second length scale to account for free surface effects. Upon analytical integration over the water depth, an expression for sediment-induced drag reduction is found in terms of an effective Chézy number, water depth, bulk Richardson number, and Rouse number. This relation contains one empirical/experimental coefficient, which was obtained from a large series of numerical experiments with a 1DV point model. Upon calibration of this model against field and laboratory observations, we tuned the turbulent Prandtl–Schmidt number and found an optimal value of σT = 2, consistent to observations by Cellino and Graf (1999). All numerical results could be correlated with a simple relation which is valid for fine sediment suspensions under conditions typical in open channel flow.
Fine sediment transport by tidal asymmetry in the high-concentrated Ems River : indications for a regime shift in response to channel deepening
This paper describes an analysis of the observed up-river transport of fine sediments in the Ems River, Germany/Netherlands, using a 1DV POINT MODEL, accounting for turbulence-induced flocculation and sediment-induced buoyancy destruction. From this analysis, it is inferred that the net up-river transport is mainly due to an asymmetry in vertical mixing, often referred to as internal tidal asymmetry. It is argued that the large stratification observed during ebb should be attributed to a profound interaction between turbulence-induced flocculation and sediment-induced buoyancy destruction, as a resultof which the river became an efficient trap for fine suspended sediment. Moreover, an asymmetry in flocculation processes was found, such that during flood relative large flocs are transported at relative large flow velocity high in the water column, whereas during ebb, the larger flocs are transported at smaller velocities close to the bed. This asymmetry contributes to the large trapping mentioned above. The internal tidal asymmetry and asymmetry in flocculation processes are both driven by the pronounced asymmetry in flow velocities, with flood velocities almost twice the ebb values. It is further argued that this efficient trapping is the result of a continuous deepening of the river, and occurs when concentrations in the river become typically a few hundred mg/l. This was the case during the 1990 survey analyzed in this paper. We also speculate that a second regime shift did occur in the river when fluid mud layers become so thick that net transport rates are directly related to the asymmetry in flow velocity itself, probably still in conjunction with internal asymmetry as well. This would yield an efficient mechanism to transport large amounts of fine sediment far up-river, as currently observed.