ON THE ENERGY DISSIPATION OF HIGH OVERFLOW DAM WITH FLIP BUCKET AND ESTIMATION OF DOWNSTREAM LOCAL EROSION

This paper deals with the problem of energy dissipation of high overflow dam with flip bucket and the estimation of downstream local erosion. Besides the kinetic energy carried away by the tailwater flow, the three ways of energy dissipation, i.e. (1) along the surface of overflow dam, (2) within the trajectory of jet and (3) in the effected water cushion, are discussed firstly. Thereby, the amount of energy, △E_(3-4), dissipated in the water cushion composed of the tailwater and the scour pit eroded by the fallen jet is estimated. By knowing the rate of energy dissipation per unit volume of the water cushion, η, the volume of water cushion to fulfill the requirement of energy dissipation is obtained as V=△E_(3-4)/η. Furthermore, the maximum depth of water cushion, T, is determined from the relationship between V and T. Since η is proportional to H~(1/2), so the expression of T obtained by the above procedure is T=K_3q~(0.5)H~(0.25). In the stage of planning and preliminary design, K_3 may be considered as 1.25 for the quick estimation of local erosion. Prototype observation and model test data of erosion pit of eight overflow dams are compared with the corresponding estimated erosion depth, and their basic agreement is acquired. In the another part of this paper, the horizontal distance of trajectory is discussed. It indicates that the elevation of flip bucket plays important role as well as the lip angle of the bucket. So that, the lip angle of the bucket corresponding to the maximum trajectory distance is usually less than 45°. The rear slope of the erosion pit is used as the criterion of the effect of local erosion against the stability of dam, and the critical value of this slope is suggested.