Measurement of Fluctuating Pressures on Coarse Bed Material
Publication: Journal of Hydraulic Engineering
Volume 131, Issue 9
Abstract
Bed protections are usually characterized by low-mobility transport conditions and nonequilibrium turbulence profiles. As the present knowledge of the influence of turbulence on stability of cover layer units is minimal, an in-depth investigation was undertaken regarding the influence of turbulence on the stability of rough granular beds. Detailed measurements of (fluctuating) pressures on a bed element are used to evaluate certain concepts that are often used in modeling the entrainment of bed material from hydraulically rough beds. Three pressure transducers are placed in a cube that is part of a rough granular bed under open-channel flow, and velocities are measured using laser Doppler velocimetry. The measurements show that the magnitude of the fluctuating pressure at a certain point of the cube is a function of the exposure relative to the stones upstream of the cube. A quadrant analysis reveals that the drag force is not only directly dependent on the horizontal near-bed velocity, but on the vertical velocity as well. Further, the effect of small-scale eddies shedding from the stone during large-scale increases of longitudinal velocity is shown. The fact that large-scale velocity fluctuations create a large part of the pressure (or force) variance indicates that downstream of a roughness transition these fluctuations have to be taken into account in order to evaluate the stability of the bed.
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Acknowledgments
The research has been financially supported by the Road and Hydraulic Engineering Division of the Ministry of Transport, Public Works, and Water Management, (Contract No. UNSPECIFIEDDWW-1700) and Delft Cluster, under the theme Coast and River.
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Published In
Journal of Hydraulic Engineering
Volume 131 • Issue 9 • September 2005
Pages: 770 - 781
Copyright
© 2005 ASCE.
History
Received: Jun 25, 2003
Accepted: Dec 22, 2004
Published online: Sep 1, 2005
Published in print: Sep 2005
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