In natural gravel-bed streams, the complex topography of the bed can cause variation of flow resistance and sediment transport. Previous studies have shown that in addition to grain resistance (skin friction), flow resistance is also caused by well known sources, such as bank roughness, channel bars, bed undulations and channel curvature. The sediment transport is similarly influenced by the complex topography and the transport rate can vary in the longitudinal and transversal directions. Despite the considerable computational effort required, only 3D numerical models allow one to give a detailed description of the flow field and of the distribution of bedload along the pooland-riffle structure typical of alternate bars in gravel bed rivers. The aim of this work is to quantify the difference in hydraulic resistance and sediment transport on river bars, comparing the results of the 3D model with those of an equivalent 1D case, which refers to flat bed conditions, but otherwise corresponding to identical average velocity and bed slope. The 3D numerical morphodynamic model is applied to different bed topographies of alternate bars, considering the typical harmonic content of the different types of bars that can be observed from measurements in the laboratory and in the field. The contribution of grain resistance is estimated with the local values of the bed shear stress, while bar resistance results from the overall deviation of the flow field with respect to that occurring in the flat bed configuration. The local sediment transport is computed with the local Shield stress and local bed inclination in both the longitudinal and transverse directions. The main outcome of the work is the definition of a function allowing for the correction of the total sediment transport and resistance in a cross-section due to 3D effects of alternate bars, to be incorporated in 1D models for practical application. This is done by averaging the results of the 3D model over the bar pattern (e.g. one wavelength). The results also quantify how local structure in the flow and bed modify the sediment transport rate and bed resistance at macroscopic (baraveraged scale). We term the resulting relations “morphologically averaged” sediment transport and resistance equations.
ON THE EVALUATION OF SEDIMENT TRANSPORT IN GRAVEL-BED RIVERS WITH ALTERNATE BARS / Francalanci, Simona; Solari, Luca; Toffolon, Marco; Parker, Gary .. - ELETTRONICO. - (2010), pp. 0-0. (Intervento presentato al convegno Gravel-Bed Rivers 7 tenutosi a Tadoussac, Canada nel 6-10 settembre 2010).
ON THE EVALUATION OF SEDIMENT TRANSPORT IN GRAVEL-BED RIVERS WITH ALTERNATE BARS
FRANCALANCI, SIMONA;SOLARI, LUCA;
2010
Abstract
In natural gravel-bed streams, the complex topography of the bed can cause variation of flow resistance and sediment transport. Previous studies have shown that in addition to grain resistance (skin friction), flow resistance is also caused by well known sources, such as bank roughness, channel bars, bed undulations and channel curvature. The sediment transport is similarly influenced by the complex topography and the transport rate can vary in the longitudinal and transversal directions. Despite the considerable computational effort required, only 3D numerical models allow one to give a detailed description of the flow field and of the distribution of bedload along the pooland-riffle structure typical of alternate bars in gravel bed rivers. The aim of this work is to quantify the difference in hydraulic resistance and sediment transport on river bars, comparing the results of the 3D model with those of an equivalent 1D case, which refers to flat bed conditions, but otherwise corresponding to identical average velocity and bed slope. The 3D numerical morphodynamic model is applied to different bed topographies of alternate bars, considering the typical harmonic content of the different types of bars that can be observed from measurements in the laboratory and in the field. The contribution of grain resistance is estimated with the local values of the bed shear stress, while bar resistance results from the overall deviation of the flow field with respect to that occurring in the flat bed configuration. The local sediment transport is computed with the local Shield stress and local bed inclination in both the longitudinal and transverse directions. The main outcome of the work is the definition of a function allowing for the correction of the total sediment transport and resistance in a cross-section due to 3D effects of alternate bars, to be incorporated in 1D models for practical application. This is done by averaging the results of the 3D model over the bar pattern (e.g. one wavelength). The results also quantify how local structure in the flow and bed modify the sediment transport rate and bed resistance at macroscopic (baraveraged scale). We term the resulting relations “morphologically averaged” sediment transport and resistance equations.
I documenti in FLORE sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
