Multipeaked breakthrough curves in karstic rivers:effects of a diffluence-confluence system

In karstic environments, it is not unusual for an underground river to split into two or more streams (diffluence) and merge back together downstream (confluence). This kind of behavior can generate multipeaked breakthrough curves (BTCs) in dye tracing at a sampling site located downstream of the confluence(s). It is also possible that such a phenomenon is difficult to highlight with dye tracing if the tracer clouds coming from the different streams reach the sampling locations at the same time. In this study, an attempt at quantifying the importance of different criteria in the occurrence of a multipeaked BTC is done by performing a dye tracing campaign in a two-tributaries diffluence-confluence (DC) system and using a one-dimensional solute transport model. The results from both field data and the solute transport model suggest that a double-peaked BTC occurs downstream of a DC system if the following conditions are met: (1) the injection is done close enough to the diffluence, (2) the sampling point is located not too far from the confluence, and (3) the two (or more) streams have sufficiently contrasted travel times from the diffluence to the confluence. The paper illustrates that, even if a diffluence occurs in a karstic river, multipeaked BTCs are not necessarily observed downstream of the confluence if these three conditions are not met. Therefore, characterizing a DC system using dye tracing is a real challenge. This could explain why publications that report studies involving multipeaked BTCs are quite rare.</p

Experiments of dam-break wave over a triangular bottom sill

The IMPACT project addressed the assessment and reduction of risks from extreme flooding caused by natural events or the failure of dams and flood defence structures. It was funded by the European Commission (EC), started in November 2001 and lasted for 3 years. This paper presents a dam-break flow experiment in a channel with a triangular bottom sill (the bump), that was part of the IMPACT benchmarking programme (WP3: flood propagation). This test case highlights some key issues linked to the propagation of dam-break waves on dry bed in the presence of a bed slope. Two types of measurement devices were used to obtain the evolution of the water depth. First, three water level gauges were used to obtain the time evolution of the water level at three different locations around the bump. Then, using high speed digital cameras, the flow was filmed through the glassy walls of the channel, resulting in a continuous experimental surface profile by combining the images acquired at different locations along the channel. An automated procedure was designed to measure the water depth from the digital images. This procedure has been successfully validated by comparison with the gauge records. Finally, all those measurements form a data set that can be used for the validation of numerical models

Review of imaging-based measurement techniques for free surface flows involving sediment transport and morphological changes

In order to validate the numerical methods aimed at the simulation of fast transient flows involving sediment transport and morphological changes, data are required. However, field data are scarce, or, if existing, are often inaccurate or incomplete, due to the difficulty of taking reliable measurements in such difficult flow conditions. Laboratory experiments constitute a good alternative to obtain validation data for numerical models. When performing simplified experiments, a limited number of well-identified flow features can be highlighted if appropriate measurements are taken. Advances in experimental techniques in the last decades have significantly enlarged the field of possible data acquisition, especially thanks to the development of non-intrusive techniques such as digital imagery. Non-intrusive techniques are of paramount importance when considering sediment transport because a measurement device interacting with the flow would also modify the observed morphological features. In this paper, several imaging-based techniques are presented for water-level and bed evolution measurements. The key features and advantages are discussed but also the drawbacks of those techniques. The discussion is illustrated by different examples that have resulted in data sets commonly used by scientists all over the world to test their numerical simulation tools

Closing parameters of morphodynamical models without trial-and-error

The widely used shallow-water equations are insufficient to model appropriately complex river flows highly laden with sediment such as dam-flushes. We propose a 2D finite-volume two-phase/two-layer model able to do so and to cope with the sediment concentration. The objective is to use this model as a predictive and optimization tool for future dam flushes, to avoid environmental disasters, among other things. The problem is that this model is more complex than a simple shallow-water model. It has more parameters to calibrate. For some of them, no closure formulation exists in the literature and a river-specific trial-and-error calibration based on past-obtained data must be achieved. Hence, the model cannot be used as a predictive tool. To circumvent this trial-and-error calibration, we propose a closure procedure combining a numerical experiment and machine learning. That procedure can theoretically be used to close the last unclosed parameter of any morphodynamical model. The model, with its parameters closed by the presented procedure, gave satisfactory results for two different dam-break test cases. Yet, its performance in the case of dense suspension still needs to be addressed

Dam break in channels with 90 degrees bend

In practice, dam-break modeling is generally performed using a one-dimensional (ID) approach for its limited requirements in data and computation. However, for valleys with multiple sharp bends, such a ID model may fail for predicting as well the maximum water level as the wave arrival time. This paper presents an experimental study of a dam-break flow in an initially dry channel with a 90degrees bend, with refined measurements of water level and velocity field. The measured data are compared to some numerical results computed with finite-volume schemes associated with Roe-type flux calculation. The ID approach reveals the expected limits, while a full two-dimensional (2D) approach provides fine level prediction and rather satisfactory information about the arrival time. A hybrid approach is now proposed, mixing the ID model for the straight reaches and local 2D models for the bends. The compatibility of the Roe fluxes at the interfaces requires a careful formulation, but the resulting scheme seems able to capture reflection and diffraction processes in such a way that the results are really good in what concerns the water level

A 2D HLL-based weakly coupled model for transient flows on mobile beds

We propose a finite-volume model that aims at improving the ability of two-dimensional numerical models to accurately predict the morphological evolution of sandy beds when subjected to transient flows like dam-breaks. This model solves shallow water and Exner equations with a weakly coupled approach while the fluxes at the interfaces of the cells are calculated thanks to a lateralized HLLC flux scheme. Besides describing the model, we ran it for four different test cases: a steady flow on an inclined bed leading to aggradation or degradation, a dam-break leading to high interaction between the flow and the bed, a dam-break with a symmetrical enlargement close to the gate and a dam-break in a channel with a 90° bend. The gathered results are discussed and compared to an existing fully coupled approach based on HLLC fluxes. Although both models equally perform regarding water levels, the weakly coupled model looks to better predict the bed evolution for the four test cases. In particular, its results are not affected by an excessive numerical diffusion encountered by the coupled model. Moreover, it usually better estimates the amplitudes of the maximum deposits and scours. It is also more stable when subject to high bed-flow interaction

Comparison of coupled and weakly coupled HLLC based finite-volume schemes for dam-break flows on mobile bed

We hereby compare the performance of a weakly coupled finite-volume model against a fully coupled one on a two-dimensional dam-break test case with movable bed. Both models consider bed-load only and are HLLC based, but the weakly coupled one uses an upwind scheme for the sediment fluxes. It appears that the weakly coupled model is far better at predicting maximum deposition and erosion amplitudes and does not suffer from excessive diffusion regarding bed evolution

A second-order semi-implicit hybrid scheme for one-dimensional Boussinesq-type waves in rectangular channels

A numerical procedure is proposed for the solution of the Boussinesq equations in rectangular channels over a horizontal bed. The Boussinesq equations account for non-hydrostatic effects in free-surface flows. The proposed approach is a predictor-corrector procedure where the hyperbolic part of the equations is treated using a higher-order estimate of the variable slope of the Godunov-type, MUSCL scheme. The proposed slope estimate is third-order accurate on irregular grids and fourth-order accurate on regular grids. The dispersive, non-hydrostatic terms are treated using a semi-implicit discretization. A stability analysis of the proposed predictor-corrector procedure shows that optimal accuracy is achieved when the implicitation parameter is set equal to 0.5. This analysis is confirmed by numerical experiments, whereby the propagation of a solitary wave and the development of an undular bore are reproduced and compared successfully with the available analytical solutions. The proposed method is shown to be of a good level of accuracy without being too sensitive to the numerical parameters. Its applicability in practical problems is illustrated by a comparison with laboratory experiments. Copyright (C) 2008 John Wiley & Sons, Ltd