Predicting the effective diffusivity across the sediment–water interface in rivers
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Hyporheic Zone
Dimensionless quantity
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Seafloor Spreading
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Ripple marks
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Two series of experiments were conducted in a new Large Oscillating Water Tunnel of Delft Hydraulics with the main objectives to study the behaviour of bedforms and the transport of sand in controlled wave conditions simulated at full scale (1:1). The main results are:
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The length, height, depth and asymmetry of 3386 individual bedforms were measured from high-resolution grids derived from a series of multibeam surveys conducted in the San Francisco Bay coastal system. Powerful tidal currents that can exceed 2.5 ms-1 generate bedforms that measure up to 317 m in wavelength, and up to 10 m in height. Detailed analysis of bedform shape indicates that there are regions where ebb-directed or flood-directed bedload transport is predominant, revealing important sediment transport pathways. The increased mapping resolution of multibeam technology over the past decade allows for the gathering of highly detailed, quantitative information regarding bedforms and thus advances understanding of coastal sediment transport processes and provides valuable information to coastal managers for more efficient sediment resource management.
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Abstract A numerical model is used to determine changes in the bed morphology resulting from the bed load and suspended transport of sand. The ability of the model to predict mean and turbulent quantities and the suspended transport of sand is assessed by appeal to laboratory flume experiments and field observations. The model is then applied to investigate changes that may be expected to occur in an initial bedform corresponding to asymmetrical sandwaves characteristic of those found in the coastal and estuarine environment. It is found that suspended transport may frequently dominate over bed load transport in producing changes in the bedform structure. The predicted evolution of the bedform is reasonably consistent with observations made of sediment fluxes and sandwave migration in the Taw Estuary, England.
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Advances in both seabed imaging capabilities and numerical models of shelf sea dynamics means new approaches are required to study those seas which surround us. Shelf sediment transport pathways are still principally derived from bedform and sediment indicators. A wealth of newly acquired data requires novel tools for extracting meaning in order to better understand shelf sediment dynamics. A new objective and quantitative method for extracting bedform dimensions has been developed and tested against synthetic and real data. New bedform analysis results have produced the largest global bedform measurements collection and extended the analysis range by an order of magnitude. To explore sediment transport at both tidal bank ( 10 km) scales, a new well-calibrated unstructured grid model of the European shelf has been developed, and model results using uniform and observation-derived bed roughness compared. Temporal changes in shelf sediment transport over both the recent past (100 years) and during the Holocene marine transgression (last 8 ka BP)are further explored. Marine and aeolian bedforms are compared through a range of statistical analyses and are shown to share the same morphology. Variability in medium to large marine bedforms (Ashley, 1990) is greater, and due to sensitivity to relatively larger changes in flow; larger bedforms are found to be more stable. Deviations from the derived global relationships are shown, through the palaeomodelling, to be due to relict bedforms. Variable bed roughness derived from the bedform observations and grain size has produced hydrodynamic results at least as good as those obtained from uniform roughness. Modern sediment transport on the shelf is found to be more complex in areas where traditional analyses cannot capture the temporal variability. The flexible nature of the model has meant subset bank modelling has benefitted from the well calibrated shelf model, and results have highlighted the differences in sediment transport regimes on the shelf. Sediment transport evolution during the Holocene transgression is shown to have been episodic, with quiescent periods followed by rapid changes. The quantitative analysis and modelling performed here has shown it is possible to take advantage of newly acquired data: the bedform analysis has confirmed the relationship between aeolian and marine bedforms and provided a tool for use in numerical modelling. The unstructured grid shelf model flexibility has produced both individual bank transport as well as pathways across the entire shelf.
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Marine transgression
Marine geology
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