Development of Indexes to Evaluate the Effectiveness of Low Water Level Control in Irrigation Ponds – A Case Study of Irrigation Ponds in Tottori, Japan

Abstract

The spillways of most irrigation ponds in Japan need additional repairs to safely pass the 200-year flood event, which is the design criteria set by the government. At the same time, low water level control (LWLC) in irrigation ponds is currently recognized as one of the countermeasures for mitigating floods and avoiding dam collapse. However, the pond managers do not know to what extent the water level of the pond should be lowered. In this study, the flood mitigation function of LWLC in each pond was quantitatively evaluated, and indexes were proposed to determine whether it is worth practicing LWLC in each pond or whether the spillway needs to be repaired. Seventy-two irrigation ponds in Tottori prefecture were selected for analysis and the water balance of each pond was calculated with 10-, 50-, and 200-year rainfall events. The results showed that half of the ponds cannot safely cope with a 10-year rainfall event even if they are empty prior to the event. The ratio of the catchment area to water surface area at full capacity in such ponds often exceeds 50 to 1. On the other hand, ponds with a catchment to water surface area ratio of less than 50 show a high flood mitigation function due to LWLC. In addition to this, priority for repair work can be evaluated in terms of whether the spillway can pass peak runoff or not, because the size and the type of spillway are different among ponds. The effectiveness of LWLC can be evaluated in terms of the ability of the spillway to pass peak runoff from the catchment when the pond is full. The three indicators, namely, the ratio of catchment area to water surface area at full capacity, difference of peak flood reduction rate, and peak discharge ratio, are simple and useful indexes for LWLC to prevent dam collapse.