Modelling Regime Changes of Dunes to Upper-Stage Plane Bed in Flumes and in Rivers

Abstract

In this paper we derive a new morphological model, with an extended version of the sediment transport model for the mean step length (the average distance travelled by sediment particles), in which this mean step length depends on the mean bed shear stress. This model makes the step length increase with increasing flow, in line with previous experimental results. To account for suspension and the large-scale turbulent structures in rivers, the step length also depends explicitly on water depth. This approach enabled modelling of the transition from dunes to the upper-stage plane bed. It was shown that by increasing the step length, the lag between shear stress and bed load transport rate increases, and the dunes eventually become smoother and lower, until finally the dunes wash out. The newly adopted model approach is tested successfully with a synthetic data set from the literature, where plane bed conditions are indeed reached in the model, similar to the results of a more advanced model. It is shown that with increasing discharge, the flow increases, which leads to higher step length and to the washing out of the dunes. Although the present model still overestimates the dune height for river cases, the potential of the model concept for river dune dynamics, including the transition to upper-stage plane bed, is shown. The model results indicate that, if a transition to upper-stage plane bed occurs in a realistic river scenario, a reduction of the water depth of approximately 0.5 m can occur.

The Effect of Sediment Transport Models on Simulating River Dune Dynamics

“…Many models have been proposed to determine the value for α (Sekine & Kikkawa, 1992;Shimizu et al, 2009). In this paper the model as proposed by Van Duin et al (2021) is used, because it has been adjusted for deep rivers. The value of α is defined as:…”

“…where α min is the base value of the non dimensional step length, equal to 50, and h ref is a calibration factor set at 0.1166. Value τ* = 0.5 is related to the transition from dunes to USPB (Van Duin et al, 2021).…”

“…DuDe is derived from a process-based morphodynamic sand wave model as developed by Németh et al (2006) and Van den Berg et al (2012), which again is based on the numerical model of Hulscher (1996). Van Duin et al (2017) and Van Duin et al (2021) have extended the model to simulate USPB.…”

“…(2017). This model has been validated for high flow situations (Paarlberg et al., 2010) and upper stage plane bed [USPB] (Van Duin et al., 2021), but this model was not yet extended or tested for low flow conditions.…”

“…In this study, we adopted the dune development model developed by Paarlberg et al (2009) and extended by Van Duin et al (2017). This model has been validated for high flow situations (Paarlberg et al, 2010) and upper stage plane bed [USPB] (Van Duin et al, 2021), but this model was not yet extended or tested for low flow conditions.…”