| Overflow earth-rock cofferdam has superiority that saves investments, shortens construction period. From domestic and international experiences, we can see that the design of downstream protection slope of overtopping earth-rock cofferdam is the key question of safety. Concrete slope protection is broadly used in hydropower engineering for its stability is good and its construction is simple.In order to design the arrangement and size of the concrete protection rationally, it is need to confirm the worst flood discharge during overtopped earth-rock cofferdam first. The paper aims directly at the research of joint discharge through tunnel and overtopped cofferdam during construction. Based on system analyses the main factors which affecting overflow operating condition of overflow earth-rock cofferdam, the condition of overflowing is measured by the current velocity head of cofferdam downstream slope blacks. Then, through analyzing the uncertainty of the flood and the discharge process of diversion tunnel, a risk model for the downstream slope of overtopped earth-rock cofferdam during construction diversion and calculating method are presented.According to the hydraulic condition of the downstream slope, the strength of the concrete block is analyzed, and the mechanism and fashion of losing stability is studied. And consider that it is fluctuating pressure and seepage pressure that causes concrete slope protection to be destroied.The stability of the concrete block and that of the bedding layer restrict each other. In fact, the pressure under the board can be cut down greatly after bedding layer has been design properly. The stability of the concrete block under the bedding layer is eroding is analyzed. At the same time, the paper analyses sensitiveness of downstream slope angle and board thickness that influence the stability of concrete block, which offer the theory reference for concrete block protection design.
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