Development of an Emergency Discharging Device and an Early Warning System for Floods at Irrigation Ponds

Abstract

The spillways of irrigation ponds in Japan should be repaired to safely pass the 200-year flood event, which is the design criteria set by the government. However, extensive time is required to repair the spillways owing to the large number of ponds and the high repair costs. Therefore, we developed a low-cost early warning system (EWS) to reduce the risk to households in case of floods, as well as a low-cost emergency discharging device (EDD) to prevent or mitigate these floods. The Yutani irrigation pond in Tottori, Japan was selected for this study. The pond stores runoff from the catchment area and has no inflow from other basins. A siphon tube with a diameter of 20 cm was employed as the EDD for the pond and the discharge performance was evaluated. First, 60-min rainfalls at return periods of different years were estimated. Then, the peak runoff and subsequent arrival time of each year’s 60-min rainfall and the discharges from the spillway and siphon were estimated. The water balance of the pond was calculated using the peak runoff as an inflow element, and the discharges from the spillway and the siphon were used as the outflow elements. The results showed that the pond will overflow if a 50-year rainfall event occurs under current conditions; however, it will not overflow when the siphon is employed even if a 60-year rainfall event occurs. Since the siphon discharge was not large enough to pass a flood event greater than 60 years, we developed an EWS that informs villagers living near the pond when the water level of the pond reaches these levels through an alarm lamp and an e-mail. Based on the results of the water balance analysis, the EWS provided information to judge whether villagers should be evacuated or not and the timing of the evacuation in the case of heavy rainfall events if necessary.