HR: 14:35h
AN: H53L-04    [Abstracts]
TI: Implications of Bed-Schematization in Morphological Models with Graded Sediment
AU: * Sloff, K
EM: kees.sloff@wldelft.nl
AF: Delft Hydraulics and Delft University of Technology, PO box 177, Delft, 2660 MH, Netherlands
AU: Mosselman, E
EM: erik.mosselman@wldelft.nl
AF: Delft Hydraulics and Delft University of Technology, PO box 177, Delft, 2660 MH, Netherlands
AB: For modeling morphology in river with graded-sediment beds, generally methods are used that subdivide the sediment into separate size fractions, and that subdivide the bed into discrete layers. In the past years we have applied and analyzed numerical models using these methods for various river cases, including the full range of time-dependent conditions and spatial variations. It has been found from these applications that particularly the schematization of the bed into discrete layers and the interactions with the transport rate have a serious influence on the outcomes and predictability of these models. The thickness of the active-layer or mixing layer on the bed surface directly determines the time scale of bed- composition changes. Choosing a very thin active layer will lead to very fast adjustments of bed composition to changes or gradients in conditions, such that (generally slow) morphological responses will be cancelled. A more physically sound choice of the layer thickness (e.g., on basis of bed form dimensions), leading to a thicker layer, causes a deceleration of bed-composition changes and propagation. Applications to the Rhine River have shown that this causes the time-scales of morphological changes and bed composition changes to approach each other leading to a stronger interaction between grain size and morphological changes. Tracer experiments in the gravel bed of the German Rhine indicate that a relative thick active layer is required to simulate the slowly migration (2 to 5 km/year) of the granite tracers downstream. Note that both prototype data and model results are showing a dispersion of bed composition disturbances, as propagation speed for different fractions is different. In the model approach sediment-transport rates are coupled to availability of sediment fractions in the active layer. Transport models and hiding and exposure models strongly react to the composition and evolution of the layer, and analysis of the equations reveals some unconventional model responses. For instance, in a case of the Rhine River it has been shown that by removing the coarsest fraction from the bed mixture resulted in halving of mean grain size, a doubling of transport rate, but without a change of the propagation speed or time scale of morphological developments. This feedback between layer composition, transport rate and time scales has been confirmed by theoretical analyses. The feedback mechanism between bed and transport in graded-sediment models complicates further refinement, calibration and validation of the layer-approaches. For instance recent introduction of vertical sorting models (related to bed forms) by means of exchange fluxes between active layer and substratum under-layers result in fining of the active layer and an increase of transport rates in a similar way as shown above. However, the response of morphology and bed composition (sorting evolution) is modified considerably. Further research focuses on unraveling the complex interactions between the bed schematization, transport rates and morphological development. Here fore laboratory and field data are essential ingredients.
DE: 1825 Geomorphology: fluvial (1625)
DE: 1847 Modeling
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: 2007 Fall Meeting