Length scales and statistical characteristics of outer bank roughness for large elongate meander bends: the influence of bank material properties, floodplain vegetation and flow inundation

Authors: KONSOER, KORY - Louisiana State University; RHOADS, BRUCE - University Of Illinois; BEST, JAMES - University Of Illinois; Langendoen, Eddy; Ursic, Michael - Mick; ABAD, JORGE - University Of Engineering And Technology, Lima, Peru; GARCIA, MARCELO - University Of Illinois

Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/6/2017
Publication Date: 10/9/2017
Citation: Konsoer, K., Rhoads, B., Best, J., Langendoen, E.J., Ursic, M.E., Abad, J.D., Garcia, M.H. 2017. Length scales and statistical characteristics of outer bank roughness for large elongate meander bends: the influence of bank material properties, floodplain vegetation and flow inundation. Earth Surface Processes and Landforms. 42(13): 2024-2037.

Interpretive Summary: Rates of bank erosion are influenced by the near-bank flow field, mechanical properties of the bank materials, and the roughness of the channel banks. Bank roughness consists of spatially varying topography produced by action of the flowing water and mass failures, and in protruding bank vegetation. These irregularities affect shear stresses acting on the channel boundary, near-bank flow structure, sediment transport near the bank toe, and therefore rates of erosion of the bank itself. Scientists of the USDA-ARS Watershed Physical Processes Research Unit, Oxford, MS, in collaboration with researchers of the University of Illinois and Louisiana State University employed high-resolution acoustical and optical measurement techniques to map subaerial and subaqueous bank features for two meander bends (one vegetated and one unvegetated) on the Wabash River, Illinois/Indiana. Results showed that the characteristics of roughness along unvegetated banks composed primarily of non-cohesive sediment vary with bank elevation and exhibit a dominant range of roughness length scales. On the other hand, banks composed predominantly of cohesive material along a forested floodplain have relatively uniform roughness characteristics over the vertical extent of the bank face and do not exhibit a dominant roughness length scale. Additionally, roughness decreases considerably when the banks are submerged. The findings highlight the complexity and spatial variability of bank roughness on a large meandering river, and therefore the need for improved process description in computer models of river morphologic adjustment.

Technical Abstract: This paper explores the scales and characteristics of form roughness along the outer banks of two bends on a large meandering river through investigation of irregularities in bank contours and local topographic variability on the bank face. The analysis also examines how roughness varies over the vertical height of the banks and when the banks are both exposed subaerially and inundated during flood stage. Detailed topographic data on the subaerial outer banks were obtained using terrestrial LiDAR during low flow conditions and subaqueously using multibeam echo sounding (MBES) during near-bankfull conditions. Scales of roughness for subaerial conditions were evaluated for different elevation contours on the bank faces using Hilbert-Huang Transform (HHT) spectral analysis, and the statistical characteristics for discrete areas on the bank faces were determined by calculating the root-mean-square of elevation variations. Results of the HHT analysis show that the characteristics of roughness along banks composed primarily of non-cohesive sediment, and eroding into cropland, vary with bank elevation and exhibit a dominant range of roughness length scales. On the other hand, banks composed predominantly of cohesive material and carved into a forested floodplain have relatively uniform roughness characteristics over the vertical extent of the bank face and do not exhibit a dominant roughness length scale or range of length scales. Additionally, comparison between local surface roughness for subaerial versus subaqueous conditions shows that roughness decreases considerably when the banks are submerged, most likely because of the removal of vegetation and eradication of small-scale erosional features in non-cohesive bank materials by flow along the bank face. Thus, roughness appears to be linked to the hydraulic conditions affecting the bank, at least relative to conditions that develop when banks are exposed subaerially.