Abstract
One of the most relevant features of alluvial rivers concerns flow resistance, which depends on many factors including, mainly grain resistance and form drag. For natural sand-bed rivers, dunes furnish the most significant contribution and this paper provides an insight on it. To achieve this aim, momentum balance equations and energy balance equations are applied to free flow in alluvial channels, assuming hydrostatic pressure distribution over the cross sections confining the control volume, which includes a reference bed form pattern. The resulting equation in terms of energy grade accounts for an empirical bed form drag coefficient resulting from the actual flow pattern and bed form geometry. The model has been validated using a large selection of field data and it seems somewhat sensitive to the dune geometry and to the Nikuradse equivalent roughness, whereas it is shows greater sensitivity to the adopted grain surface resistance formula (e.g., Manning–Strickler formula).
Highlights
Flow resistance is the result of several factors, including the presence of bends, vegetation, local acceleration due to flow unsteadiness, and channel geometry variations, in addition to bed surface roughness and bed forms drag
Valuable state-of-the-art technologies for river sedimentation and morphology modeling may be found in [1,2]. These conventional morphodynamic models mainly refer to nonlinear shallow water equations, Exner equation, and an empirical formula for sediment transport
In conventional models further uncertainty arises from the lack of understanding of some fundamental mechanics related to sediment transport
Summary
Flow resistance is the result of several factors, including the presence of bends, vegetation, local acceleration due to flow unsteadiness, and channel geometry variations, in addition to bed surface roughness and bed forms drag. Even in the case of steady and quasi-uniform flow, confined within the main active channel and involving non-cohesive sediment, most of the existing resistance formulae (e.g., [8,9,10,11]) commonly provide inaccurate predictions of flow resistance This leads to considerable error in stage-discharge prediction, both in terms of water depth (±20%), flow velocity (±15%) [12] or bed friction, for which the difference between measured and predicted values may be ±50% [11] and, where field data is used, discrepancy is even wider [13,14]). Ferreira Da Silva and Yalin [28] generalized two modes of bed forms drag (by virtue of additivity of losses), taking into account the ripples superimposed on dunes In any case, they applied momentum, energy and mass balance equations to a reference bed form, assuming that the effects of a bed form on the flow are comparable to a sudden expansion of a pipe flow. Since this paper focuses on sand rivers in presence of dunes, field data related to sand streams and large sand rivers are considered, accounting for a large span of different hydraulic and sedimentary conditions
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