# The influence of flow acceleration on stone stability

The influence of flow acceleration on stone stability

Author ContributorStive, M.J. (mentor)

Stelling, G.S. (mentor)

Verhagen, H.J. (mentor)

Fontijn, H.L. (mentor)

Hofland, B. (mentor)

2004-08

AbstractThe stability of a bed of stones subject to a flow is often described in terms of a critical velocity or shear stress generated by the flow. These classical design methods like for example Shields, do not take the influence of flow acceleration into account. In experiments and practice, it appeared that when a flow is accelerated, stones start to move at a point where the so-called critical velocity is not reached yet. The movement of stones must have a second cause beside the velocity of flow. Only a little information is known on the influence of flow acceleration on stone stability of the bed. The objective of this thesis is to obtain more insight into the influence of acceleration of flow on the stability of stones. By carrying out experiments in a flume containing a local contraction, the stone stability in an accelerated flow is investigated. In the contraction the stability of two different stone sizes, subject to different velocity-acceleration combinations, is analysed. If the hypothesis is correct, than for some velocity-acceleration combinations movement occurs while for the same velocity combined with a lower acceleration no movement occurs. The shear stress occurring in the accelerated flow is determined using the shear velocity. According to the classical Shields method the shear velocity is responsible for the movement of the stones. Movement is detected for lower shear velocities then expected. According to the hypothesis this is a result of the extra generated force on the stones due to acceleration. After analysing the data it appeared that combinations of the same velocity and different accelerations showed differences in movement. The amount of movement goes up for an increase in acceleration combined with a constant or slightly decreasing velocity. This proves that there is a relation between the stability of the stones and a combination of the velocity and acceleration generated forces. The Morison equation is used to describe the relation between the forces acting on a stone. It combines the force generated by acceleration and the force generated by the peak velocities due to turbulence, as the sum of both forces. The extra force due to acceleration appeared to be of the same order as the force due to the velocity. Therefore, when looking at the stone stability in an accelerated flow, it is important to take the force generated by the acceleration into account. The resulting Morison force acting on a stone is proved to be responsible for the stability of the stones. Finally, a unique relation, valid for both stone diameters, between the force acting on the stone and the entrainment is found. This power relation consists of a dimensionless Morison-Shields parameter representing the force on a stone and a dimensionless entrainment parameter. The relation does not depend on stone size and is therefore expected to be universal in use.

To reference this document use: PublisherTU Delft, Faculty of Civil Engineering and Geosciences, Hydraulic Engineering

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