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Form drag of subaqueous dune configurations

Jellesma, Manfred (2013) Form drag of subaqueous dune configurations.

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Abstract:In hydrodynamic modelling of river ows a key model parameter is the hydraulic roughness of the river bed, which is related to grain characteristics of the bed and the geometries and spatial distribution of bed forms. This parameter is commonly used for model calibration, where stationary roughness coefficients are chosen such that an accurate match between modelled and empirical ow data is achieved. However, from existing bathymetric measurements it is known that bed forms take on different shapes during the passage of a discharge wave. These observations of changing bed forms therefore indicate that bed roughness is not a stationary variable (as assumed in many hydrodynamic river models), but rather that bed roughness responds to hydrodynamics. It appears that during the rise of a discharge wave the bed forms tend to grow in height and length, and that bed forms tend to merge and decay during the falling stage of a discharge wave. The objective of this research is to investigate the form drag of several idealized dune con- figurations in order to provide a better understanding of roughness variability during ood waves. Therefore the software package OpenFOAM is used to set up a non-hydrostatic numerical 2DV (two dimensional vertical plane) flow model for flow over dunes. The form drag of the several dune configurations is determined from this model, which is then used to describe the roughness variation during ood waves. In total three, on measurements based, dune configurations are investigated: (1) secondary dune, (2) primary dune and (3) primary dune consisting of merged secondary dunes. The model is validated by theoretical logarithmic straight bed ow and by measurements of single dune shape laboratory experiments. The straight bed case fits the logarithmic velocity profile well and the roughness parameter based on the ow properties of the model is in accordance to the input of the model. For the dune shapes, a calibration and validation case are carried out. The calibration case estimates the grain roughness of the concrete material used in the experiments, while the validation case shows the performance of the model. Both modelled dune cases show high agreement to measured velocity profiles, ow separation zone and total roughness parameters. Form drag of the investigated dune configurations appears to increase for increasing dune length and height. The form drag increases as well for the same primary dunes but consisting of merged secondary dunes. It appears the form drag of primary dunes consisting of merged secondary dunes is slightly higher than the summation of the form drag of each separate bed form. In perspective to the varying bed forms during passing of a discharge wave, it is concluded the form drag of the river bed increases during the rise of a discharge wave. For the falling stage of a discharge wave it is concluded the form drag may still increase. Secondary dunes merged on primary dunes add a significant amount of form drag, but the effect of decay of primary dunes in this stage is not determined. It depends on actual primary and secondary bed form shape and dimensions if form drag increases or decreases during the falling stage.
Item Type:Essay (Master)
HKV Consultants
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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