Three-dimensional river bed forms

Colombini, M.; Stocchino, Alessandro

doi:10.1017/jfm.2011.556

Cited by 52 publications

(68 citation statements)

References 28 publications

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“…Unfortunately, simplified theoretical relations to predict dimensions from Fr , C , and τ * have not been produced, which limits our ability to test linear stability analysis predictions against our observations directly. Nevertheless, the overall pattern of bedform change with Fr and τ * is consistent with some aspects of theoretical predictions (Colombini, ; Colombini & Stocchino, , ). For example, the theory suggests that dune length initially decreases with increasing Fr to a minimum and then increases until an upper stage plane bed develops.…”

Section: Discussionsupporting

confidence: 88%

Transport Scaling of Dune Dimensions in Shallow Flows

2019

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Dune dimensions in sand‐bedded rivers are often thought to scale with flow depth (h), with height (H) scaling as 1/6h and length (L) as 5h, even though substantial scatter about the relations has been observed. Transport stage has been shown to affect bedform geometry, but this control is often ignored in favor of depth scaling relations. Here, we use a series of flume experiments to systematically test controls on dune dimensions and variability. Experiments involved three sets of runs under five constant transport stages, ranging threshold to washout conditions, at three different flow depths. The mobile bed was repeatedly scanned during a 10‐hr equilibrium period to derive mean values and quantify the variability. The results show that dune‐depth scaling is not consistent because of a transport stage effect. Dune height increases with transport stage until a point when H decreases. Length remains nearly constant with transport stage until further increases in transport stage leads to lengthening. In general, dunes grow higher when significant bedload transport occurs but become flatter and longer in the presence of substantial suspension. Ultimately, dunes scale with transport stage, which is a function of slope, grain size, and h. The results are used to derive transport stage relations to guide predictions of dune dimensions in rivers and reconstructions of paleoflows based on dimensions estimated from cross strata. The relations incorporate the nonlinear response of dune dimensions with transport stage and provide metrics of uncertainty to include in predictions.

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Section: Discussionsupporting

confidence: 88%

Transport Scaling of Dune Dimensions in Shallow Flows

2019

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“…The main objective of this section is to provide novel insights into the hydrodynamic processes that drive the complex morphological features we discussed in the previous section. Specifically, we seek to probe questions that have been at the centre of extensive debate in the literature, including, among other things: the linkage between coherent structures in the turbulent boundary layer and the initial bed instability (Colombini 2004;Venditti & Church 2005;Venditti et al 2006;Chou & Fringer 2010;Omidyeganeh & Piomelli 2013a,b), the role of flow separation off sand wave crestlines as a means for sand wave migration (Kostaschuk 2000;Schindler & Robert 2004;Best 2005;Colombini & Stocchino 2012) and the origin of low-frequency coherent structures rising to the water surface, also known as surface 'boils' (Jackson 1976;Yalin 1992;Venditti & Bennett 2000;Best 2005).…”

Section: Coupled Bed-flow Interactionsmentioning

confidence: 99%

“…For instance, in their pioneering stability theory-based models, Kennedy (1969), Hayashi (1970) and Jain & Kennedy (1974) assumed quasi-steady potential flow to simulate the bed evolution mechanism of microscale sand ripples. Others, including Fredsoe (1974), Richards (1980), Coleman & Fenton (2000), Colombini (2004), Coleman et al (2006) and Colombini & Stocchino (2012), have also used a similar modelling framework to analyse microscale ripples. The assumptions inherent in linear stability models are valid only during the initial stages of sand wave formation during which the amplitude of sand waves is very small and rolling of sediment particles within the bedload layer is the major mechanism for transporting sediment and destabilizing the initially flat bed.…”

Section: Introductionmentioning

confidence: 99%

Numerical simulation of sand waves in a turbulent open channel flow

2014

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We develop a coupled hydro-morphodynamic numerical model for carrying out large-eddy simulation of stratified, turbulent flow over a mobile sand bed. The method is based on the curvilinear immersed boundary approach of Khosronejad et al. (Adv. Water Resour., vol. 34, 2011, pp. 829-843). We apply this method to simulate sand wave initiation, growth and evolution in a mobile bed laboratory open channel, which was studied experimentally by Venditti & Church (J. Geophys. Res., vol. 110, 2005, F01009). We show that all the major characteristics of the computed sand waves, from the early cross-hatch and chevron patterns to fully grown three-dimensional bedforms, are in good agreement with the experimental data both qualitatively and quantitatively. Our simulations capture the measured temporal evolution of sand wave amplitude, wavelength and celerity with good accuracy and also yield three-dimensional topologies that are strikingly similar to what was observed in the laboratory. We show that near-bed sweeps are responsible for initiating the instability of the initially flat sand bed. Stratification effects, which arise due to increased concentration of suspended sediment in the flow, also become important at later stages of the bed evolution and need to be taken into account for accurate simulations. As bedforms grow in amplitude and wavelength, they give rise to energetic coherent structures in the form of horseshoe vortices, which transport low-momentum near-bed fluid and suspended sediment away from the bed, giving rise to characteristic 'boil' events at the water surface. Flow separation off the bedform crestlines is shown to trap sediment in the lee side of the crestlines, which, coupled with sediment erosion from the accelerating flow over the stoss side, provides the mechanism for continuous bedform migration and crestline rearrangement. The statistical and spectral properties of the computed sand waves are calculated and shown to be similar to what has been observed in nature and previous numerical simulations. Furthermore, and in agreement with recent experimental findings (Singh et al., Water Resour. Res., vol. 46, 2010, pp. 1-10), the spectra of the resolved velocity fluctuations above the bed exhibit a distinct spectral gap whose width increases with distance from the bed. The spectral gap delineates the spectrum of turbulence from the low-frequency range associated with very slowly evolving, albeit energetic, coherent structures induced by the migrating sand waves. Overall the numerical simulations reproduce the laboratory observations with good accuracy and elucidate the physical phenomena governing the interaction between the turbulent flow and the developing mobile bed.

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“…Traditionally, the stability analysis of the sediment-fluid interface has largely been studied using two important simplifications: (i) the use of classical linear stability theory based on normal modes and (ii) the hypothesis of steady base conditions, i.e., no discharge variations [15][16][17][18][19][20]. The modal (or normal) approach constitutes a powerful mathematical tool in stability theory and has been extensively used in * alice.caruso@polito.it fluid mechanics for more than a century.…”

Section: Introductionmentioning

confidence: 99%

River bedform inception by flow unsteadiness: A modal and nonmodal analysis

et al. 2016

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River bedforms arise as a result of morphological instabilities of the stream-sediment interface. Dunes and antidunes constitute the most typical patterns, and their occurrence and dynamics are relevant for a number of engineering and environmental applications. Although flow variability is a typical feature of all rivers, the bedform-triggering morphological instabilities have generally been studied under the assumption of a constant flow rate. In order to partially address this shortcoming, we here discuss the influence of (periodic) flow unsteadiness on bedform inception. To this end, our recent one-dimensional validated model coupling Dressler's equations with a refined mechanistic sediment transport formulation is adopted, and both the asymptotic and transient dynamics are investigated by modal and nonmodal analyses.

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Three-dimensional river bed forms

Cited by 52 publications

References 28 publications

Transport Scaling of Dune Dimensions in Shallow Flows

Transport Scaling of Dune Dimensions in Shallow Flows

Numerical simulation of sand waves in a turbulent open channel flow

River bedform inception by flow unsteadiness: A modal and nonmodal analysis

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