Form drag of subaqueous dune configurations

Fredrik Huthoff, Andries Paarlberg, Manfred Jellesma, Suzanne Hulscher, Jord Warmink, Alice Lefebvre

Wednesday 1 july 2015

11:00 - 11:15h at Oceania (level 0)

Themes: (T) Sediment management and morphodynamics, (ST) River morphodynamics

Parallel session: 9B. Sediment - River

In hydrodynamic modeling of river flows 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 modeled and empirical flow data is achieved. However, from existing bathymetric measurements it is known that bed forms take on different shapes during the passing 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. Using a numerical Reynolds-averaged 2DV flow model we investigate how typical dune configurations (changes in shape, size and placements) affect the form drag, and, therefore, how dune evolution under passing of a discharge wave manifests itself as a dynamic bed roughness. The results of this research provide insight in the roughness-variability during passing of flood waves and indicate potential for improved flood modeling if non-steady roughness parameters were used.