MODULATION OF BANK SHEAR STRESSES DUE TO BEDFORMS IN A MEANDERING RIVER: FIELD SCALE RESULTS FROM THE WABASH RIVER, IL-IN, USA

Substantial past work has demonstrated how bedforms can modulate bed roughness, flow field structure and sediment transport rates in straight channels, with recent research demonstrating that migrating bedforms can produce temporal and spatial peaks in shear stress along the outer bank of an experimental meandering channel. These shear stresses were estimated to be about 50% larger than the bank shear stresses present in the absence of bed forms. Since bank erosion rates are typically linearly related to the applied shear stress, these peaks in shear stress caused by the presence of bedforms may act to increase bank erosion and thus bend migration rates, and perhaps contribute to the evolution of more complex bend planform shapes. However, this hypothesis has never been tested at the field scale.

This paper will describe results from an integrated field and numerical modelling study of flow within a meandering channel in the presence of migrating bedforms. Detailed field measurements of the hydrodynamics (using an acoustic Doppler current profiler, aDcp), bed morphology (using multibeam echo sounding) and bank morphology (using LiDAR) of a large actively-migrating meander bend were conducted within Maier bend, Wabash River, IL-IN, USA. The bed morphology exhibited different scales of bed forms from ripples to dunes and unit bars. Wavelet analysis was used to discriminate the bed morphology, and thus it was possible to separate the small, medium and large scale dunes, this providing a baseline case for the numerical modelling. The high-resolution maps of the bend with these bedforms will be discussed, with an analysis of their size, location and orientation within Maier bend.

Using a fully three-dimensional Reynolds-Averaged Navier-Stokes (RANS) numerical model, two cases have been simulated using the Maier bend morphology and flow fields: [1] a bend with bedforms, and [2] a bend without bed forms, in order to test the above hypothesis. Our results show that the three-dimensional flow field compares well to that observed in the field measurements where bedforms were present. Furthermore, peaks in shear stress along the outer bank of the bend that are correlated to the location of the bedforms within the bend are observed, confirming the potentially important role of bedform kinematics in bend erosion and evolution.