Shinwol rainwater storage and drainage system is a large-scale urban underground facility for flood protection to supplement insufficient drainage capacity of the existing cities. For the smooth flow of flood water into the main tunnel, several conditions are considered to determine the type of energy reduction facility to be installed below the drop shaft. In this study, a hydraulic model experiment was conducted to calculate the most appropriate depth of plunge pool for Shinwol rainwater storage and drainage system. The experimental scenarios were designed with three depths of plunge pool and six discharges. It is found that the maximum pressure on the bottom surface increased for all three depths of plunge pools when the inflow discharge increased. The change in the location with the maximum and minimum pressure according to the increase in the inflow discharge for each scenario was considered to be due to a rotatory force in the upper drop shaft. This study found the scenario B as the optimal depth for Shinwol rainwater storage and drainage system.