Bedform response to flow variability

Abstract

Laboratory observations and computational results for the response of bedform fields to rapid variations in discharge are compared and discussed. The simple case considered here begins with a relatively low discharge over a flat bed on which bedforms are initiated, followed by a short high-flow period with double the original discharge, during which the morphology of the bedforms adjusts, followed in turn by a relatively long period of the original low discharge. For the grain size and hydraulic conditions selected, the Froude number remains subcritical during the experiment, and sediment moves predominantly as bedload. Observations show rapid development of quasi-two-dimensional bedforms during the initial period of low flow with increasing wavelength and height over the initial low-flow period. When the flow increases, the bedforms rapidly increase in wavelength and height, as expected from other empirical results. When the flow decreases back to the original discharge, the height of the bedforms quickly decreases in response, but the wavelength decreases much more slowly. Computational results using an unsteady two-dimensional flow model coupled to a disequilibrium bedload transport model for the same conditions simulate the formation and initial growth of the bedforms fairly accurately and also predict an increase in dimensions during the high-flow period. However, the computational model predicts a much slower rate of wavelength increase, and also performs less accurately during the final low-flow period, where the wavelength remains essentially constant, rather than decreasing. In addition, the numerical results show less variability in bedform wavelength and height than the measured values; the bedform shape is also somewhat different. Based on observations, these discrepancies may result from the simplified model for sediment particle step lengths used in the computational approach. Experiments show that the particle step length varies spatially and temporally over the bedforms during the evolution process. Assuming a constant value for the step length neglects the role of flow alterations in the bedload sediment-transport process, which appears to result in predicted bedform wavelength changes smaller than those observed. However, observations also suggest that three-dimensional effects play at least some role in the decrease of bedform wavelength, so incorporating better models for particle hop lengths alone may not be sufficient to improve model predictions.

Computational Modelling of Fluvial Flat Bed Regime

“…Moreover, the Giri and Shimizu 10) model is calibrated by adjusting the step length, which reduces its predictive capability. Although this study confirms the capability of the model to describe two-dimensional bedform behaviour in time-varying flow, more advanced models are needed to predict this behaviour 5) .…”

“…Moreover, the side walls as well as the secondary flow had a significant effect on the bedforms. Recently, Nelson et al 5) carried out laboratory experiments with only two discharge levels to replicate rising and falling flow stages, and used the experimental observations to validate the numerical model proposed by Shimizu et al 6) .…”

Morphodynamics of river dunes : suspended sediment transport along mobile dunes and dune development towards upper stage plane bed

“…During this flood, the dune heights were observed to increase drastically from 0.20 m up to 1.8 m. In addition, during floods in several rivers (e.g., the Elkhorn, Missouri, Niobrara, and Rio Grande), dunes are initially observed to grow rapidly as flow strength increases, followed by a transition regime that eventually resulted in washing out of the dunes towards upper stage plane bed [Raslan, 1994 andreferences therein, Ashworth et al, 2000;Prent and Hickin, 2001]. This morphological evolution of dunes to upper stage plane bed (USPB) is the strongest bedform adjustment during time-varying flows and is associated with a significant change in hydraulic roughness and water levels [Nelson et al, 2011]. This drastic change in hydraulic roughness during the dune transition to upper stage plane bed is also illustrated in Figure 1.1.…”

“…This morphological evolution of dunes to upper stage plane bed (D-USPB) is the strongest bedform adjustment during time-varying flows and is associated with a significant change in hydraulic roughness and water levels [Nelson et al, 2011]. In addition to flow and sediment parameters, hydraulic roughness due to the presence of bed forms is directly related to bedform height Δ and length λ [Yalin, 1964;Van Rijn, 1984b;Karim, 1999;Van der Mark, 2009].…”

“…In addition, the recent study of Nelson et al [2011] showed that these two processes determine the response of bed forms to flow variability. However, for the prediction of the dimension of dunes during the D-USPB regime, predominantly empirical relations based on a limited number of mainly flume data sets with relatively high Froude numbers can be found in the literature [Yalin, 1964;Allen, 1978;Ranga Ruju and Soni, 1976;Van Rijn, 1984b;Julien and Klaassen, 1995;Karim, 1995].…”