Field experiment on alternate bar development in a straight sand-bed stream

Abstract

[1] Alternate bars in rivers and streams develop as a result of differences in length scales involved in the adjustment of flow and sediment transport to irregularities of the bed. The amount of field evidence supporting theoretical insights is highly limited. Here, we present results from a large-scale field experiment in a 600 m long straight reach. Over a period of almost 3 years, the channel was allowed to evolve autogenously from initially flat bed conditions, subject to discharge variation. Alternate bars developed within 8 months from the start of the experiment. The initial stages of bar development included bar growth, both in wavelength and amplitude, and bar migration. The latter was too limited to classify the bars as being migrating bars; therefore, we classify the bars as nonmigrating bars. Toward the end of the experiment, the regular alternate bar pattern evolved into an irregular pattern and bar amplitude started to decrease. From the start of the experiment we observed a declining channel slope, from 1.8 m km 21 initially to 0.9 m km 21 halfway the experiment, after which it stabilized. We applied two bar theories to establish their predictive capacity. Both bar theories predicted the development of alternate bars under the constructed channel conditions. In response to the declining channel slope, both theories predicted a decreasing likelihood for the development of alternate bars. Our study shows that under field conditions, the applied bar theories may predict the initial stages of bed development.Citation: Eekhout, J. P. C., A. J. F. Hoitink, and E. Mosselman (2013), Field experiment on alternate bar development in a straight sand-bed stream, Water Resour. Res., 49,[8357][8358][8359][8360][8361][8362][8363][8364][8365][8366][8367][8368][8369]

Bed Load Sediment Transport and Morphological Evolution in a Degrading Uniform Sediment Channel Under Unsteady Flow Hydrographs

“…Bedforms also develop as a fundamental instability response to perturbations in the spatial and temporal scales of flow and sediment transport processes [e.g., Tubino, ; Eekhout et al, ; Martin & Jerolmack, ; Redolfi et al, ]. This is again highly pertinent in regulated rivers where unsteady flows are generated through controlled dam releases, natural flood events, or for managed stream restoration (Venditti et al, ).…”

“…The formative conditions for alternate bars in unsteady flows can be predicted through application of an existing theoretical model by Tubino (; see section S5 in the supporting information for details). This model has been widely reported (e.g., Eekhout et al, ; Welford, ; Welford, ) to predict the occurrence of alternate bars in field studies, but, until now, has not been applied to zero sediment feed scenarios. In Tubino's () model, the formative conditions for bar development are defined primarily by a parameter û (equation S5 and section S5), representing the time scale ratio between flow unsteadiness (equation S6, i.e., different from unsteadiness parameter Γ HG ) and morphology instability.…”

Numerical Study of Alternate Bars in Alluvial Channels With Nonuniform Sediment

“…This definition caused problematic ambiguity and vagueness, because it did not distinguish between forcing over the full length of a bar, as in the case of point bars that cannot be described by linear stability analysis, and forcing in a single cross section, leading to a dynamic response of nonmigrating bars that can be described using linear stability analysis. Eekhout et al () and Rodrigues et al () discussed this ambiguity of using the term “forced bars” for two types of bars, albeit without proposing a new terminology. To resolve the old ambiguity, the Delft School introduced the term “hybrid bars,” which was then gratefully adopted by Duró et al (), Le, Crosato, Mosselman, and Uijttewaal (), Le, Crosato, and Uijttewaal (), and Scorpio et al ().…”

Numerical simulation of large‐scale bed load particle tracer advection‐dispersion in rivers with free bars

“…Extra randomness associated with morphodynamics at different scales may affect the heaviness of the waiting time, possibly pushing the pattern of dispersion from superdiffusive to subdiffusive. Additionally, the migration speed of free bars in nature tends to be relatively slow, even in straight channels, and free bars may in some cases stop migrating [Crosato et al, 2011;Eekhout et al, 2013;Rodrigues et al, 2015]. The retention of tracers in a quasistatic bed morphology would constrain tracer motion, eventually resulting in subdiffusion and advective slowdown as all tracer particles eventually become trapped and stop moving.…”