Documents disponibles écrits par cet auteur

How fast can a river flow over alluvium? / Michel A. Verbanck in Journal of hydraulic research, Vol. 46 extra issue (2008) 

Public ISBD [article]  in Journal of hydraulic research > Vol. 46 extra issue (2008) . - p. 61.71 Titre : How fast can a river flow over alluvium? Titre original : Y'a t'il une vitesse maximum a laquelle peuvent couler les rivières alluviales? Type de document : texte imprimé Auteurs : Michel A. Verbanck, Auteur Article en page(s) : p. 61.71 Note générale : Hydraulique Résumé en Français Langues : Anglais (eng) Mots-clés : Alluvial  regime  Flow-sediment-morphology  system  Mobile  bedforms  Stationary  inphase  waves  Antidunes  Vortex  drag  model  Separated  and  reattaching  flow  Movable-bed  resistance  Sediment  transport  capacity Index. décimale : 627 Ingénierie des cours d'eau naturels, des ports, des rades et des cotes. Installations de navigation, de dragage, de récupération et de sauvetage. Barrages et centrales électriques hydrauliques Résumé : A new “vortex-drag” approach is proposed to compute the rate of dissipation of mean-motion energy in alluvial streams flowing over mobile bed forms. 1D-routing exercises strongly suggest that, for a given depth and energy-slope, there exists a certain flow velocity limit an alluvial stream is unlikely to exceed. This remarkable flow condition would actually correspond to a fundamental mode in terms of vortex Strouhal frequency (to which only higher harmonics can eventually be associated). Interestingly, the maximum possible velocity is not attained over a plane bed, but occurs when the wavy shape of the water-surface remains in phase with the sand wave. The model treats flow over sandy alluvium as a combination of boundary-attached and -detached flow features, both necessary to explain how well-marked protrusions can develop spontaneously in the bed profile and be maintained in response to extreme water-sediment discharge and stream power conditions. Through the vortex-drag concept and its athematical development, a lot is learned about the nature of the mutual interactions existing between the various alluvial processes: bedform development, alluvial channel resistance, sediment transport capacity and turbulence-damping. Bedform types occurring in the upper alluvial regime are at the center of the analysis. Based on the model and on confrontation with available experimental evidence, it is suggested that the inphase wave [IPW] flow configuration is associated with minimum possible drag, creating low-turbulence high-velocity conditions ideal to evacuate extreme river discharges. Occurrence of IPW conditions is therefore important in terms of flooding alleviation, but will develop only if the right type of sediment is maintained available on the stream bed. The boundary-attached component of the model introduces a generalized Froude number formulation in alluvial hydraulics, to represent topographically-forced gravity waves occurring in non-shallow environments such as ripple fields. For the boundary-detached component of the model, we introduce a “control factor m” (m ≥ 1) which reflects a complex feedback-control loop process active in the separation cell immediately downstream of the bedform crest. Lift-up of bed particles (under the action of the corresponding vortices shedding out of the reattachment region) implies that control factor m should also play an important role in the further development of bursting-based conceptual models of non-cohesive sediment transport. DEWEY : 627 ISSN : 0022-1686 En ligne : http://www.journalhydraulicresearch.com [article] How fast can a river flow over alluvium? = Y'a t'il une vitesse maximum a laquelle peuvent couler les rivières alluviales? [texte imprimé] / Michel A. Verbanck, Auteur . - p. 61.71. Hydraulique Résumé en Français Langues : Anglais (eng) in Journal of hydraulic research > Vol. 46 extra issue (2008) . - p. 61.71 Mots-clés : Alluvial  regime  Flow-sediment-morphology  system  Mobile  bedforms  Stationary  inphase  waves  Antidunes  Vortex  drag  model  Separated  and  reattaching  flow  Movable-bed  resistance  Sediment  transport  capacity Index. décimale : 627 Ingénierie des cours d'eau naturels, des ports, des rades et des cotes. Installations de navigation, de dragage, de récupération et de sauvetage. Barrages et centrales électriques hydrauliques Résumé : A new “vortex-drag” approach is proposed to compute the rate of dissipation of mean-motion energy in alluvial streams flowing over mobile bed forms. 1D-routing exercises strongly suggest that, for a given depth and energy-slope, there exists a certain flow velocity limit an alluvial stream is unlikely to exceed. This remarkable flow condition would actually correspond to a fundamental mode in terms of vortex Strouhal frequency (to which only higher harmonics can eventually be associated). Interestingly, the maximum possible velocity is not attained over a plane bed, but occurs when the wavy shape of the water-surface remains in phase with the sand wave. The model treats flow over sandy alluvium as a combination of boundary-attached and -detached flow features, both necessary to explain how well-marked protrusions can develop spontaneously in the bed profile and be maintained in response to extreme water-sediment discharge and stream power conditions. Through the vortex-drag concept and its athematical development, a lot is learned about the nature of the mutual interactions existing between the various alluvial processes: bedform development, alluvial channel resistance, sediment transport capacity and turbulence-damping. Bedform types occurring in the upper alluvial regime are at the center of the analysis. Based on the model and on confrontation with available experimental evidence, it is suggested that the inphase wave [IPW] flow configuration is associated with minimum possible drag, creating low-turbulence high-velocity conditions ideal to evacuate extreme river discharges. Occurrence of IPW conditions is therefore important in terms of flooding alleviation, but will develop only if the right type of sediment is maintained available on the stream bed. The boundary-attached component of the model introduces a generalized Froude number formulation in alluvial hydraulics, to represent topographically-forced gravity waves occurring in non-shallow environments such as ripple fields. For the boundary-detached component of the model, we introduce a “control factor m” (m ≥ 1) which reflects a complex feedback-control loop process active in the separation cell immediately downstream of the bedform crest. Lift-up of bed particles (under the action of the corresponding vortices shedding out of the reattachment region) implies that control factor m should also play an important role in the further development of bursting-based conceptual models of non-cohesive sediment transport. DEWEY : 627 ISSN : 0022-1686 En ligne : http://www.journalhydraulicresearch.com