On the Depth of Scour Caused by Flow Downstream of an Overfall from Sabo-dam (II)

The case of using the jet producing device for experiment of scour

Abstract

In this study, the author considers on the depth of scour caused by flow downstream of an overfall from Sabo-dam. It is confirmed by experiment of scour that the jet by overfalling nappe impinges slant on the bottom boundary of scour and the successive flow produces the boundary-layer along the bottom boundary. The maximum velocity outside of the boundary-layer, u0max, is defined to be a power function of the ratio (D/η), in which D is the thickness of the jet, η is the distance of the mixing zone. The coefficient of its exponent, p, is obtained as p=3/4 by the experlment. On the other hand, non-dimentional function defining the terminal depth of scour is derived by using the Karman integral relation for the boundary-layer. α and β used as the powers in this function are expressed as α=2p-1 and β=1/(2p+γ). The special device of jet flow is made for purpose of this experiment of scour. From the experimental results of scour with this device, the value of α and β are obtained as α=1/2, β=1/2 for D/d≥1, in which d is diameter of grain. Substituting the value of α and β into the foregoing equations yeilds further p=3/4, γ=1/2. This value of p agrees with the value of previous power (p) on the u_0max, and the value of γ also satisfies approximately the assumption used in the theory of boundary-layer.