Three‐dimensional flow structure and bed morphology in large elongate meander loops with different outer bank roughness characteristics

Abstract

Few studies have examined the three‐dimensional flow structure and bed morphology within elongate loops of large meandering channels. The present study focuses on the spatial patterns of three‐dimensional flow structure and bed morphology within two elongate meander loops and examines how differences in outer bank roughness influence near‐bank flow characteristics. Three‐dimensional velocities were measured during two different events—a near‐bankfull flow and an overbank event. Detailed data on channel bathymetry and bed form geometry were obtained during a near‐bankfull event. Flow structure within the loops is characterized by strong topographic steering by the point bar, by the development of helical motion associated with flow curvature, and by acceleration of flow where bedrock is exposed along the outer bank. Near‐bank velocities during the overbank event are less than those for the near‐bankfull flow, highlighting the strong influence of the point bar on redistribution of mass and momentum of the flow at subbankfull stages. Multiple outer bank pools are evident within the elongate meander loop with low outer bank roughness, but are not present in the loop with high outer bank roughness, which may reflect the influence of abundant large woody debris on near‐bank velocity characteristics. The positions of pools within both loops can be linked to spatial variations in planform curvature. The findings indicate that flow structure and bed morphology in these large elongate loops is similar to that in small elongate loops, but differs somewhat from flow structure and bed morphology reported for experimental elongate loops.

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

“…Differences in roughness are examined when the banks are inundated and not inundated. Previous work on the two bends has examined the mechanisms that lead to mass failure of the outer banks (Konsoer et al ., ) and the spatial pattern of flow structure through elongate bends at different stages (Konsoer et al ., ), The present article extends this previous work by providing insight into spatial variation in characteristics of outer bank roughness for elongate bends along large meandering rivers and differences in bank roughness characteristics for subaerial versus subaqueous conditions. This knowledge supports efforts to link roughness characteristics with near‐bank flow conditions, both in the field and in numerical models of large meandering river systems.…”

“…This material is capped by 1–2 m layer of fine silty‐sand loam that produces a nearly vertical bank face. In the downstream part of the bend, erosion along the outer bank has exposed a local outcrop of interbedded shales and sandstones of the Carboniferous Pennsylvanian Bond Formation (Konsoer et al ., ). Vegetation along the outer bank is confined mainly to the top of the bank.…”

“…At low flow, about 4 to 6 m of nearly vertical outer bank is exposed, which consists mostly of fine sediments, with more than 70% silt/clay content at many locations. The nearly vertical, fine‐grained, portion of the outer bank is underlain by a basal layer of coarse sand and gravel that varies in thickness around the bend (Konsoer et al ., ). The downstream portion of Horseshoe Bend has eroded into shale units of the Carboniferous Pennsylvanian Mattoon Formation, which has thus restricted downstream migration.…”

“…In recent years, geomorphic investigations of river banks and bank erosion have showcased the impressive capabilities of terrestrial laser scanners and the relative ease with which detailed, high‐resolution datasets can be obtained. These studies have been used to estimate rates and spatial patterns of bank erosion (Resop and Hession, ; O'Neal and Pizzuto, ; Konsoer et al ., , ; Leyland et al ., ), compare the accuracy of terrestrial laser scanning (TLS) surveys to more traditional techniques (O'Neal and Pizzuto, ), and relate large‐scale (tens of meters) topographic bank variability to near‐bank flow patterns (Hackney et al ., ; Leyland et al ., ). However, despite the amount of high‐resolution information of bank morphology provided by TLS surveys, very few studies have actually explored in detail the statistical characteristics and length scales of bank surface roughness.…”

Distinct patterns of bank erosion in a navigable regulated river

“…Uniformly vegetated banks can significantly reduce near-bank velocities along entire river bends (Konsoer et al, 2016a). At a smaller scale, Pizzuto et al (2010) suggested that the joint effect of nearby trees on flow detachment could reduce erosion rates similarly to small-scale roughness (Kean and Smith, 2006), based on short-term observations of sequential abutments on sandy-loams.…”