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Title: Modulation of the flow structure by progressive bed forms in the meandering Wabash River

Author
item ABAD, JORGE - University Of Pittsburgh
item FRIAS, CHRISTIAN - University Of Pittsburgh
item KONSOER, KORY - University Of Illinois
item BEST, JAMES - University Of Illinois
item RHOADS, BRUCE - University Of Illinois
item Langendoen, Eddy
item GARCIA, MARCELO - University Of Illinois

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 7/15/2014
Publication Date: 8/27/2014
Citation: Abad, J.D., Frias, C., Konsoer, K.M., Best, J.L., Rhoads, B.J., Langendoen, E.J., Garcia, M.H. 2014. Modulation of the flow structure by progressive bed forms in the meandering Wabash River. In: Proceedings of River Flow 2014 September 3-5, 2014, Lausanne, Switzerland. A.J. Schleiss, G. De Cesare, M.J. Franca, & M. Pfister (eds.) pp. 1113-1117.

Interpretive Summary: Calculations of bank erosion rate in meandering streams generally use boundary shear stress estimates that are only affected by river geometry at the reach scale, such as channel curvature and the point bar located on the inside of a meander bend. However, laboratory experiments have shown that bank shear stress may be increased by as much as 50% locally by migrating bed forms. Scientists of the USDA-ARS National Sedimentation Laboratory in collaboration with researchers of the Universities of Pittsburgh and Illinois, have developed a computer simulation of the three-dimensional flow, sediment transport, and bed morphology of a meander bend in the Wabash River, Illinois/Indiana to test if this observation is also valid for large rivers. The computer simulation was validated using detailed, high resolution flow and bathymetric data. Wavelet analysis was used to discriminate the effects of bed forms on boundary shear stresses from those of the reach scale mean bed topography. Increases in bank shear stresses near bed forms were found to be as large as 150% relative to sections without the presence of bed forms, which is even larger than found in the laboratory experiments. More importantly, the findings show that in the case of large rivers bank erosion rates are strongly related to river bed evolution.

Technical Abstract: Recently, it was observed that migrating bed forms produce temporal and spatial peaks of shear stresses along the outer bank of an experimental meandering channel. These stresses are about 50% larger than the shear stresses exerted by the mean near-bank flow. Because of this increase in bank shear stress the migration rate of the bend may be significantly increased. However, this hypothesis has never been tested in the field, where bed forms could be more complex than those found in experimental cases. Herein, only fluvial erosion is considered, while geotechnical processes occurring at the outer bank are not accounted for. Detailed measurements of hydrodynamics (using acoustic current Doppler profiler), bed morphology (using multibeam echosounder and RTK GPS) and bank morphology (using laser scanner) were conducted at two bends on the Wabash River along the Illinois and Indiana Stateline. The bed morphology exhibited different scales of bed forms, ranging from dunes to ripples. Using Wavelet analysis it was possible to separate the ripples and dunes structures resulting in a bed without bed forms, which shows the typical erosion (outer bank)/deposition (inner bank) arrangement in meandering channels. Using a fully three-dimensional Reynolds-Averaged Navier-Stokes (RANS) numerical model, two scenarios are simulated: [1] bend with bed forms, and [2] bend without bed forms to test the above hypothesis. The results show that the three-dimensional flow field compares well to that observed for both scenarios. Further, peaks in shear stresses along the outer bank are indeed observed, which are correlated to the location of the bed forms with respect to the bend. Further conclusion and its importance for long-term morphodynamics of meandering channels are described.