Study on Threshold Values of Safety Control Indicators for Rapid Drawdown of 200~300 m High Earth-Rockfill Dams

High dams and large reservoirs serve flood control, power generation, water supply and ecological security, where rapid safe drawdown is vital for emergency handling and long-term safe operation. Targeting undefined rapid drawdown control standards for over-200 m ultra-high earth-rockfill dams, this study selects 200 m, 250 m and 300 m earth core rockfill dams for finite element calculations of transient unsaturated seepage and slope stability under varying drawdown rates. It analyzes how drawdown rate, drawdown depth and dam height affect dam safety, and tests the sensitivity of rockfill shear strength indices. Results reveal rapid drawdown produces prominent phreatic line lag, which worsens with faster drawdown and taller dams. Slope safety factors fall then rise during water level decline with a critical minimum, and taller dams respond more sensitively to drawdown rate. The allowable drawdown limits are 10 m/d, 8 m/d and 6 m/d for 200 m, 250 m and 300 m dams, paired with maximum safe drawdown depths of 108 m, 140.4 m and 154.8 m. Rockfill cohesion c is highly sensitive at 0–40 kPa; each 2° rise in internal friction angle φ lifts the safety factor by roughly 7.3%. The findings offer theoretical and technical references for drawdown facility design and emergency regulation of reservoirs with dams exceeding 200 m.