Morphological changes on meander point bars associated with flow structure at different discharges

Abstract

This study investigates the impact of flow structure of different discharges on meander point bar morphology. We carried out mobile and terrestrial laser scanning campaigns before and after a flood on two sandy‐bed point bars. Between the scans, the flow structure was examined using an Acoustic Doppler Current Profiler at three flow stages. The results indicated that a meander point bar both affects and in turn, is itself modified by the flow at different discharges. The lower flow stages also have a significant effect on point bar morphology, especially on deposition over the bar head. Secondary circulation is responsible for scroll bar formation on the point bar margin beyond the apex. Flow separation at the inner bank, by contrast, does not require secondary circulation, but is dependent on flow depth over the point bar. A sudden increase in depth beyond the point bar top causes decreased stream power over the bar tail. The flow separation and decreased stream power cause a slow flow zone and net deposition over point bar tail. The backwater effect, if evident, may strengthen the process. Thus, filling over the bar tail seems generic for point bars and independent on secondary flow. Chutes and chute bars, scroll bars, bar head filling and bar platform filling, by contrast, require special fluvio‐morphological circumstances discussed in this paper. Whilst this paper confirms that the three‐dimensional flow structure has a major effect on point bar morphology, the flow structure seems to depend on how the point bar affects the flow trajectory which, in turn, depends upon the flow stage. Finally, the shape of the bend and the grain size distribution control the impacts of the flow structure, leading to divergent morphologies of point bars with certain generic features. Copyright © 2012 John Wiley & Sons, Ltd.

Flow Patterns and Morphological Changes in a Sandy Meander Bend during a Flood—Spatially and Temporally Intensive ADCP Measurement Approach

“…The area is well-suited as a test site to investigate fluvial processes and channel changes; hence several fluvio-geomorphological studies have been performed in the same river reach during the last ten years. These studies have shown that vertical changes up to 0.7 m occur annually on the point bars during the spring flood period [42]. According to the previous studies, however, the morphological changes of consecutive years can differ notably from each other, and changes in one bend are not clearly connected to changes in another bend [21,22].…”

“…At lower discharges, the HVC shifts towards the outer bank further upstream compared to higher discharges [6]. Therefore, during low discharges, the current over the bar head remains weak, enabling small particles to fill the point-bar margin [36,42]. Furthermore, the processes of an individual bend may also depend on the conditions downstream, because backwater effects from downstream can influence upstream flow conditions [43].…”

“…Its clear advantage is that it can be operated in deep-water rivers, in areas inaccessible on foot but also in shallow areas. These features have led to increased use of ADCPs by researchers, and a wide range of studies exploiting the ADCP and dealing with flow structure and turbulence have been published [18,42,[48][49][50][51][52], some of them focusing on meandering rivers [25,42,49,52]. In most of these studies, however, the 3D flow distribution have been measured along cross-sections and thus the measurements have not covered the whole bend, facing partly similar problems related to the traditional methods [5,49,52].…”

Influence of Meander Confinement on Hydro-Morphodynamics of a Cohesive Meandering Channel

“…Many river scientists and engineers have studied meandering rivers over the past few centuries (e.g., [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]). Despite the fact that river meandering is an erudite topic with a long literature history, there are still some uncertainties on the source and initiation of the meandering pattern and its migration.…”

Flood Modeling Using a Synthesis of Multi-Platform LiDAR Data

“…In addition, the ground-based bare-earth dataset was used to replace locations within the bare-earth airborne dataset that had become obsolete due to recent construction and restoration projects. While combinations of multi-platform datasets have been attempted in the past (e.g., [9,[32][33][34]), these studies only combined two different platform datasets, typically merging ALS and TLS, or MLS and TLS [35]. To the author's knowledge, the research presented in this paper is unique in that a combination of data from three LiDAR platforms (i.e., airborne, mobile, and terrestrial) was created, therefore capitalizing on the complementary technologies and reducing the data collection weaknesses inherent in each individual system.…”