Extensive rainfall can lead to the discharge of untreated mine drainage from abandoned mines. The Ministry of Economy, Trade and Industry aims to promote environmental impact assessments in scenarios where untreated mine drainage discharges may occur due to extensive rainfall. This study aims to develop a risk assessment method for these events, focusing on 26 abandoned mine drainages without responsible parties. We estimated metal concentrations at discharge points under the assumption that untreated drainage is released. A tiered system was used for the risk assessment: Tier 0 evaluates risks using river flow rates during droughts, while Tier 1 uses simulated post-rainfall river flows via AIST-SHANEL. In the Tier 0 assessment, two mine drainages (No. 5 and No. 8) exhibited Cd, Pb, As, Cu, Zn, Fe, and Mn concentrations below environmental standards, indicating that high dilution effect contributed to a low-risk discharge even after a certain period following extensive rainfall. In the Tier 1 assessment, even for mine drainages assessed as high-risk in the Tier 0 assessment, some mine drainages, which initially assessed as high-risk in Tier 0, showed metal concentrations below environmental standards for up to 7 days following peak rainfall, suggesting that high flow rates can mitigate discharge risks. For practical application, simulating river flow during extensive rainfall would allow predictions of heavy metal concentrations at discharge and water use points. Our approach would enable the setting of appropriate discharge periods and volumes, even in situations where untreated discharges are unavoidable, contributing to minimize environmental impacts downstream.