Author(s): Joey Voermans; Wim S. J. Uijttewaal
Linked Author(s): Wim S.J. Uijttewaal
Keywords: Shallow flows; Turbulence; Wakes; Roughness; Laboratory experiments
Abstract: Flows in rivers and inundated flood plains can often be considered as shallow, implying that the horizontal distribution of velocities is strongly influenced by local depth and roughness. Roughness heterogeneity can be caused for example by differences in land use, variation in soil composition, and variation in vegetation coverage. Particularly in cases with a large heterogeneity of roughness the flow pattern can be complex and flow resistance is difficult to predict. Yet, the accurate modeling of conveyance in flood plains and inundation of polders is of great importance for river engineering and flood safety measures. In order to better understand the response of a shallow flow to disturbances by roughness patches, a series of laboratory experiments has been performed in a large shallow water flume with a schematized configuration of a roughness patch. Experimental data were collected regarding the turbulent free-surface flow field downstream of the patch, which can be considered as a wake due to the velocity deficit caused by the roughness. In the shear layers bounding the wake, the flow structure is rather complex as momentum transfer can be governed by three mechanisms: shear stress due to 3D isotropic turbulence, shear stress due to large scale quasi 2D eddies, and advective transport due to secondary circulation. It is clear from the data that all three mechanisms mentioned above do play a significant role. This implies that a proper representation of the flow field and its effective resistance requires a 3D modeling approach or a smart parameterization of the observed effects. .