On transverse variation of velocity and bed shear stress in hydraulic jumps in a rectangular open channel

Summary

This paper is concerned with an experimental investigation on the transverse variation of the velocity and bed shear stress in hydraulic jumps in a rectangular open channel. The velocity is measured by means of a Pitot tube and small propeller-type flowmeter, while the bed shear stress is determined by the Preston's method. The experimental parameter is the supercritical Froude number$Fr = U_0 /\sqrt {gz_0 }$, whereU ₀ is the supercritical velocity,g the acceleration due to the gravity andz ₀ the supercritical water depth.

It is found that hydraulic jumps can be divided into two groups broadly at the supercritical Froude number Fr≊2.5. In case of the first group, no surface roller is formed and only a little amount of air is entrained into the jump. A series of swelling-depression sequences are formed on the surface along the channel axis, and thus the bed shear stress experiences a sinusoidal variation. On the other hand, in case of the second group a definite roller is formed in the jump region and a considerable amount of air is entrained into the roller. The surface height increases rapidly from the toe line to end line. As the result, the bed shear stress decreases rapidly with increasing the distance from the toe line, but the decay rate is decreased with increasing the supercritical Froude number.

It is concluded that in hydraulic jumps the surface height, velocity and bed shear stress are dependent on all of the three spatial coordinates. For example, the bed shear stress is greatly decreased with increasing the transverse distance from the channel axis.