Abstract: In a series of experimental tests on a model of pumped-storage power station, this study has found
that, a free-surface flow tunnel can generally be in normal use in flood period, but at certain phases of the
water releasing process it generated a hydraulic jump that was moving along the flow. This jump normally
moved upstream with its strength weakening if the reservoir stage was falling, while it moved downstream
with its strength growing if the stage was rising. Its development in such movements could lower tunnel
safety, because behind the jump is a water depth great enough to fully block the tunnel passage. In addition,
at the tunnel outlet the jet flow was rushing forward onto the fending dike downstream. To avoid
unfavorable effects of a moving hydraulic jump in the tunnel and improve the flow downstream, we
suggested a modification on the flip bucket: adding both a turn angle and a side weir. Thus, by adjusting the
direction of jet nappe closer to the river mainstream, taking advantage of the flow circulation on the flip
bucket, and controlling flow diversion by the side weir, strong hydraulic jumps could be suppressed. Our test
results show that in the period of transition between storage and drainage, the side weir can effectively curb
both water depth at the outlet and intensity of hydraulic jump. Thus, tunnel blocking is avoided completely,
and the jet angle and river flow around jet plunging are also improved.