Validity of Uniform Flow Hypothesis in One-Dimensional Morphodynamic Models
Publication: Journal of Hydraulic Engineering
Volume 137, Issue 2
Abstract
Local-uniform-flow (LUF) hypothesis is a simplification of the governing equations describing river morphodynamics, which is needed to determine the evolution of the bed profile and bed-material composition in the case of large time and space scales. This paper presents a rigorous analysis of the full one-dimensional river hydrodynamic and morphodynamic mathematical model compared to its LUF approximation. The analysis establishes two criteria to assess the validity of the LUF hypothesis: (1) a criterion for rivers in equilibrium and (2) a criterion for evolving rivers (i.e., in nonequilibrium). The first criterion is based on the concept of the morphological box. Variations of the river bed longer than the box length are adequately reproduced by the LUF hypothesis, whereas only spatially averaged values are resolved within the box. The second criterion is based on the concept of an evolution window. Temporal variations represented by wave periods larger than the evolution window can be adequately reproduced by the LUF hypothesis, whereas variations with shorter periods are averaged within this window. The minimum size of morphological box and evolution window that limit the error introduced by the LUF hypothesis increases when the Froude number decreases. Further, the minimum size of the evolution window increases for decreasing sediment concentration and increasing mixing layer thickness (i.e., for larger bed forms). The LUF hypothesis is therefore best applied to small mountain rivers for which both the minimum size of the morphological box and the evolution window is relatively small, so that spatial and temporal variations can be resolved in more detail. Applications using the LUF hypothesis for large watersheds (including the lowland portion of the fluvial network) are possible, but are limited to simulations over larger spatial and temporal intervals.
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Acknowledgments
This work was funded by: Autorità di Bacino Alto Adriatico ADBVE (Watershed Agency of Northern Adriatic Sea), the PRIN project “Control and mitigation of storage sedimentation: sediment management in the watershed and in reservoirs” of the Italian Ministry for Education and Scientific Research, and by the “Fondazione ing. Aldo Gini” fellowship.UNSPECIFIED
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Published In
Journal of Hydraulic Engineering
Volume 137 • Issue 2 • February 2011
Pages: 183 - 195
Copyright
© 2011 ASCE.
History
Received: Feb 19, 2009
Accepted: Jun 11, 2010
Published online: Jun 15, 2010
Published in print: Feb 2011
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