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A new one-dimensional (1-D) numerical model for calculating flow and sediment transport in steep mountain streams is developed. 3ST1D, which stands for Steep Stream Sediment Transport 1-D model, is applicable to unsteady flow conditions that occur over transcritical flow stream reaches such as flows over step-pool sequences. 3ST1D consists of two coupled components, the hydrodynamic and the sediment transport. The flow component is addressed here by solving the unsteady form of the Saint-Venant equations. The Total Variation Diminishing Dissipation (TVD)-MacCormack scheme, which is a shock-capturing scheme capable of rendering the solution oscillation free, is employed here to approximate the hydrodynamic solution over transcritical flow stream reaches. The sediment component of the model accounts for multifractional sediment transport and incorporates a series of various incipient motion criteria and frictional formulas applicable to mountain streams. In addition, sediment entrainability is estimated based on a state-of-the art formula that accounts for the bed porosity, turbulent bursting frequency, probability of occurrence of strong episodic turbulent events, and sediment availability in the unit bed area. The model at the end of each time step predicts the flow depth, velocity and shear stress distribution within a cell and calculates changes in bed evolution and grain size distribution. The overall performance of the model is evaluated by comparing its predictions with observations from two flume studies, two field investigations and against the predictions of the quasi-steady model of Lopez and Falcon developed for mountain streams. A sensitivity analysis is performed to assess the effects of cell size and Manning's roughness coefficient in the predictive ability of the model. RÉSUMÉUn nouveau modèle numérique uni-dimensionnel (1-D) de calcul d'écoulement et de transport de sédiment dans les torrents rapides de montagne a été développé: 3ST1D, mis pour Steep Stream Sediment Transport 1-D model (modèle de transport 1-D de sédiment dans les torrents à forte pente), est applicable aux écoulements instationnaires qui se produisent dans les biefs critiques tels que les séquences de marches. 3ST1D se compose de deux composantes couplées, l'hydrodynamique et le transport de sédiment. Pour la composante écoulement on a recours à la forme instationnaire des équations de Saint-Venant. Le schéma TVD de MacCormack (diminution de la variation totale de dissipation) qui est un schéma absorbant les chocs capable de rendre la solution libre d'oscillations , est utilisé pour approcher la solution hydrodynamique dans les biefs du torrent où l'écoulement est transcritique. La composante de sédiment du modèle tient compte de sédiments composites et incorpore une série de critères pour le début d'entraînement et de formules de frottement applicables aux torrents de montagne. En outre, l'entraînement de sédiment est basé sur l'état de l'art qui tient compte de la porosité de lit, de la fréquence des bouffées turbulentes, de la ...

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