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

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.