Experimental Study On The Overall Flood Discharging Capacity Of The 2nd And 3rd Spillways Relating To Miyun Reservior

As the most important drinking water source of Beijing,Miyun reservoir is a large water conservancy project,including many comprehensive utilization functions of flood control,water supply,power generation,irrigation,aquaculture.At the same time,it’s laden with responsibility flood control safety for thirty million people in the downstream two giant cities of Beijing and Tianjin,Beijing-Shenyang expressway and Daqin railway infrastructure,as well as Beijing,Tianjin and Hebei region.It’s very important strategic position.There are three spillway in the reservoir and the combined discharge capacity of the second and third spillway is about 11040 m3 / s.In the space,the distance between the two spillway is very near and the axis distance is 287 m.However,if the two spillway flood discharge at the same time,the spillway flow structure will change,which inevitably affect the comprehensive discharge capacity.Meanwhile,the second and third spillway are in the cove,the discharge water flow will be poor.Especially the third spillway is blocked by the obverse of Beishan.As a result,the water discharge must turn into the approach channel for the third spillway.Since the topography and geomorphology in front of the spillway is also one of the main factors influencing the discharge capacity,it’s very important to study the discharge capacity of the second,third spillway of Miyun reservoir under the condition of different water level,especially the comprehensive joint flood discharge capacity,to better insure the downstream flood control safety for the regions of Beijing,Tianjin,and Hebei.In this research,based on the hydraulic model test,the flow structure for the second and third spillway in Miyun reservoir under different conditions was studied.And the various complex flow change rules were revealed while the relation curve between water level and discharge were plotted under the consideration of the terrain of Beishan at the downstream,the various open cases of the second and third spillway.The curves between the water level and the discharge relation were also plotted when the second and third spillway opened solely.Then the maximum discharge capacity of the second and third spillway were checked.And the relation curves between the water level and discharge were plotted considering the joint discharge of the second and third spillway under the case of all open drainage condition when the maximum joint discharge capacity of the two spillways.In this research,the flow state rules and the influence of the second and third spillway were also analyzed.The hydraulic model test results showed that the second spillway discharge can not meet the requirement of the design discharge while the third spillway discharge can meet the design discharge and the joint dispatching discharge capacity can meet the design requirements basically.The throw distance calculated results of the second and the third spillway are closer to the measured results while the trial value is slightly greater than the calculated value.As the scouring at the flip bucket downstream,the bottom elevation was reduced,the water tongue increased,the scouring pit at spillway downstream was deeper,which posed a threat to the safety of downstream river bed and bank slope.The research results will provide reliable basic information for flood control and ensure the flood control safety of Miyun reservoir.