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Numerical simulation of sediment mixture deposition part 1 ... - LTHE

Numerical simulation of sediment mixture deposition part 1 ... - LTHE

Numerical simulation of sediment mixture deposition part 1 ... -

Numerical simulation of sediment mixture deposition part 1: analysis of a flume experiment Simulation numérique du dépot sédimentaire en granulométrie étendue 1: reproduction et analyse d'un essai de laboratoire Ph. BELLEUDY, Laboratoire d'étude des Transferts en Hydrologie et Environnement, E.N.S d' Hydraulique et de Mécanique de Grenoble , INPG, France SOGREAH, Société Grenobloise d'Etudes et d'Applications Hydrauliques, Grenoble, France ABSTRACT This paper reports the numerical simulation of previously published laboratory experiments concerning deposition of a sediment mixture in a flume. The results of the simulation are analyzed and their interdependencies are discussed with reference to flume observations and previously published analyses. Cross-comparison of different parameters of the system (deposition rate, sediment transport, grain size) allows some validation of the system of equations of our modeling system. It may also give some clues and directions for further measurements and numerical experiments. RÉSUMÉ Cet article présente une simulation numérique d'un cas de dépôt de sédiments dans un canal de laboratoire. Les observations publiées par les auteurs des essais ainsi que les analyses qui en ont été faites sont commentées à partir des résultats de la simulation. L'analyse comparée des différents paramètres du système (dépôt, débit solide, granulométrie) permet de valider l'assemblage d'équations qui constituent notre système de modélisation. Elle donne aussi quelques perspectives pour la réalisation d'essais ultérieurs en laboratoire ou pour la modélisation. 1 Introduction This introduction is not intended to situate the importance of phenomena related to graded sediment in rivers, but rather to stress the connections between some basic investigations (here an exercise of mathematical modeling of a laboratory experiment) and the final (?) goals of our science. Those goals apply in areas ranging from environmental to civil engineering and consist in the understanding of river behavior and the ability to predict river evolution (natural or artificial) and the morphological impact of "training" the river. On the other hand basic scientific research is involved in theoretical investigations, performing experiments and producing formulations and models. The author’s experience in application of numerical simulation to engineering has been reported in Belleudy (1992) and Belleudy and Schüttrumpf (1994). It has demonstrated the necessity of modeling, because more and more complex questions are asked, and more and more precise answers are required. For that purpose, modeling systems have been developed whose complexity results from the complexity of our area of study. The risk of such sophistication of models is the possibility that the modeler can produce everything which is needed by turning the right "tuning knob" (Belleudy, 1994). Among the requirements of such modeling is the maintenance of close connections to basic research for development of better formulations of physical processes and validation of the models. Summary of SAFL experiments and analysis The starting point of the present work is a description by Seal et al. (1997) of experiments which were run in a flume at St. Anthony Falls Laboratory (SAFL). A mixture of sand and gravel was fed into a flume under constant flow conditions. The sediment mixture was released at a constant rate near the upstream end of the flume. The paper relates the formation of a deposit onto the bottom of the channel, with special attention to its grain size distribution. Analysis of these experiments by their authors and other scientists seems already complete and pertinent. From direct and indirect measurements of grain size distributions, Toro-Escobar et al. (1996) give some clues about the process of exchange of material between transported material (bed-load), the surface of the deposit, and the sub-surface in this case of aggradation. This paper was complemented by a discussion by Di Silvio and Marion (1997) relating their experience of multi-layered models. Numerical simulation of these experiments was also performed at SAFL. Cui et al. (1996) have developed models including almost classical formulations and also original ideas set up at SAFL. Direct conclusions from the laboratory experiments were also included in the models. The main objective of these experiments was a comparison of coupled and uncoupled simulation algorithms with the conclusion that uncoupled solutions have nearly identical performance to coupled solutions, even near critical flow conditions. Objectives The objective of the present paper is to continue the analysis and the discussion about SAFL experiments and in general about grain sorting during deposition processes. The analysis is performed using a numerical simulation of the experiments with the SEDICOUP modeling system. In a companion paper (Belleudy, 2000), referred to hereafter as 'Part 2', we take advantage of the software modularity for testing different modeling options and sensitivity of the system to these options. We then give a discussion on modeling options (not only those in SEDICOUP but also other ideas), which is based on the application of the software in simulation of SAFL experiments. Revision received July, 1999. Open for discussion till June 30, 2001. JOURNAL OF HYDRAULIC RESEARCH, VOL. 38, 2000, NO. 6 417

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