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

Olav J M Van Duin,Jan S Ribberink,Suzanne J M H Hulscher

doi:10.3390/app112311212

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.

Highlights

Hydraulic roughness values play an important role in correctly predicting water levels in rivers [1,2,3], which is critical for flood management purposes
The model clearly shows hysteresis effects; the relation between discharge and water depths is as significantly different for the rising limb of the hydrograph as it is for the falling limb
The dune evolution model is applied to only the main channel; interactions with the floodplains are not taken into account here

Summary

Introduction

Hydraulic roughness values play an important role in correctly predicting water levels in rivers [1,2,3], which is critical for flood management purposes. River dunes increase the hydraulic roughness significantly: their shapes cause form drag Because of their significant impact on hydraulic roughness, water level forecasts during a high river water discharge depend on accurate predictions of the evolution of river dune dimensions. One aspect of this is the correct prediction of a transition to upper-stage plane bed conditions. The goal is to model river dune behaviour under varying discharges between the lower-stage plane bed to the upper-stage plane bed. Such a model should work under flume and field conditions, and should predict transitions at the appropriate discharges

Results

Discussion

Conclusion