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Edition:Final report; Jun 1985-Oct 1988
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Abstract:Scour in supercritical flow is one extreme aspect of the effects of velocity on scour. Analysis of the case of scour in a long contraction shows that if all other independent variables are kept constant (1) some finite velocity is necessary to have any scour, (2) as the velocity is increased, the scour increases as long as there is no sediment movement in the wide, approach reach, and (3) as sediment movement in the approach increases with further increase in the velocity, the scour decreases a modest amount. The analysis does not indicate that there should necessarily be a change in behavior in supercritical flow--although the definition of scour needs to consider velocity head changes and energy losses. Rather than velocity, the variable of interest should be the ratio of the particle shear to the critical tractive force. Adaption of the long-contraction solution to the case of the pier or abutment indicates that the scour at a pier or abutment should display the same behavior: scour increaisng with velocity for the clear-water condition and decreasing slightly for sediment-transporting flow. Experiments agree with the analysis for both geometries. No instability of flow or other "strange" behavior was noted in the supercritical flow, possibly because of the simplicity of the geometries, or because the equipment could not achieve high enough Froude numbers.
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