Are Bedload Transport Pulses in Gravel Bed Rivers Created by Bar Migration or SedimentWaves?

Bedload transport exhibits considerable spatial and temporal variability, as reflected by large fluctuations of transport rates. Among the various mechanisms proposed for explaining this variability, bedform migration is often cited as the main cause. We took a closer look at this issue by running long-duration experiments in a gravel bed flume using constant water discharge and sediment feed rates. We monitored bed evolution and measured bedload transport rates at the flume outlet using high-resolution techniques. The bed was initially flat, but within a few hours bedforms consisting of alternate bars and pools had developed. The bars exhibited a stick-slip motion: they were stable for long periods but moved episodically (every 10 hr on average). Their downstream migration produced 36% of the sediment volume transported, mostly in the form of intense pulses. Much of the transport was caused by the displacement of sediment waves from pool to pool.

Transport of suspended sediment under the dam-break flow on an inclined plane bed of arbitrary slope

The problem of transport of suspended sediment after the break of a dam on an inclined bed is considered. To that end we use the shallow-water approximation for arbitrary, constant slopes of the bottom, taking into consideration the effect of friction. The numerical technique and the frictional model are validated by comparison with available experimental data and asymptotic analytical solutions, with special attention to the numerical solution near the wetting front. The transport of suspended sediment down the inclined bed is obtained and discussed as a function of the slope of the bed for different values of the parameters characterizing the sediment and its transport properties. For sufficiently large times we always find the formation of roll waves near the water front, which affects the transport of sediments significantly. These strong oscillations are accurately computed with the numerical method used. The relative importance of the bed load (to the suspended load) sediment transport is also discussed as a function of the size of the sediment particles and the slope of the bed for different models on the initiation of sediment suspension from bed load. We also check the dilute sediment approach and characterize the conditions for its failure. Finally, the results of the present simplified model are intended to be used as tests of more complex numerical models.

2008

Study and Numerical Simulation of Sediment Transport in Free-Surface Flow

The flow after the rupture of a dam on an inclined plane of arbitrary slope, and the induced transport of non-cohesive sediment, is analysed using the shallow-water approximation. An asymptotic (analytical) solution is presented for the flow hydrodynamics, and compared with the numerical simulation of the dam-break flood. Differences arise due to the appearance of hydrodynamic instabilities (hereafter called roll-waves) in the numerical solution. These roll-waves point out the unstable behaviour of the dam-break wave. It is found that roll-waves enhance the transport of suspended sediment. The limitations of this simple model to predict the transport of sediment in dam-break floods on steep inclines are discussed. Then, a numerical experiment is designed to analyse the unstable character of the dam-break wave, that constitutes a non-parallel and unsteady base flow. By analysing the linear and non-linear numerical evolution of small perturbations, it is possible to reveal how the nature of the ensuing flow depends not only on the Froude number (as it happens in the classical problem of roll-waves over a uniform and steady flow) but also on the non-parallel and time-varying characteristics of the background flow. Consequently, it is also shown that these effects stabilise turbulent roll-waves and raise the critical Froude number required to achieve an unstable flow. This stability result differs with that obtained with a non-parallel spatial stability analysis, pointing out the strong influence of the base-flow time-dependence in the stability criteria. A novel Continuum Mechanics model is presented to study the transport of sediment in a laminar/turbulent free-surface flow. The mixture equations for non-cohesive sediment transport in turbulent free-surface flow are derived from the ensemble averaged Navier-Stokes equations of the three-phases (water, sediment and air). This model avoids the limitations of traditional shallow-water models, and is suitable to study, for instance, the transport of sediment in non-hydrostatic shallow-water flows over bed of arbitrary bottom slopes. The model developed in this work reveals a mathematical equivalence between the propagation of the volumetric concentration of the sediment and the phase function used to capture the free surface. We take advantage of this fact to formulate an explicit Finite Volume Method (FVM) with the exact conservation property, that is implemented in the open source software OpenFOAM. Finally, this model is applied to solve the problem of local scour around a pipeline and the transport of sediment after the rupture of a horizontal dam. It is demonstrated that one-dimensional models based on depth-averaged variables (e.g. generalisations of the one-dimensional Saint-Venant equations to predict mor- phological changes) are superseded by more sophisticated and accurate procedures valid for hyperconcentrated shallow-water flows over bed of arbitrary bottom slopes (e.g. the model described herein).

University of Malaga, Spain2008

Period and amplitude of bedload pulses in a macro-rough channel

Tamara Ghilardi, Mário Jorge Rodrigues Pereira Da Franca, Anton Schleiss

It is known that bedload fluctuates over time in steep rivers with wide grain size distributions, even under conditions of constant sediment feed and water discharge. Bedload fluctuations, which are a consequence of grain sorting, are periodic and are related to fluctuations in the flow velocity and channel-bed morphology. The presence of large relatively immobile boulders, such as erratic blocks that are often present inmountain streams, has a strong impact on flow conditions and sediment transport. However, their influence on bedload fluctuations has not been studied. Sediment transport fluctuations were investigated in this study in a set of 38 laboratory experiments carried out on a steep tilting flume under several conditions of constant sediment andwater discharge for three different slopes (S= 6.7%, 9.9%, and 13%). Sediment transport, bulkmean flowvelocities, and variables describing the channel-bed morphologywere measured regularly during the experiments. Periodic bedload pulses were clearly visible in all of the experiments, alongwith flowvelocity and channel-bed morphology fluctuations. Correlation analysis showed that the durations of these cycleswere similar, although theywere not necessarily in phase. The pulses were characterized by their amplitude and period as a function of various boulder spatial densities and diameters.We could showthat for higher streampower the fluctuations decrease in cycle duration and in amplitude. The presence of boulders increases the stream power needed to transport a given amount of sediment, thus decreasing the bedload fluctuations.

American Geophysical Union2014

Study and Numerical Simulation of Sediment Transport in Free-Surface Flow

University of Malaga, Spain2008