Water Leakage through Embankment of an Overage Irrigation Tank : A Case Study to Identify Leaky Transections

Small-scale irrigation tanks have been an important source of irrigation in Japan. These contribute nearly 10% of the total irrigated area, controlling the ravages of drought. Since many of the total 246,158 tanks are centuries-old, their rehabilitation and/or modernization are in emerging need. This paper is associated with one of the serious issues envisaged, i. e., excess leakage of impounded water through an earthen embankment. As a case study, an attempt is made to identify leaky embankment transections for an overage tank in Matsuyama, Ehime Prefecture, Japan, which is in real need of rehabilitation to prevent a calamity of the subsequent embankment-break. Using the up-to-date data (tank water level, water level and temperature in the boreholes installed, leaked water amount, etc.) obtained from long-term in-situ observations, their relational analyses are carried out to investigate the hydro-geological aspects of the embankment and find a transection of major leakage-path. The results demonstrate that the leaky transection with extremely high permeability (1.0 × 10^<-2> cm/s) exists closely to the right-bank of the tank, and the flow seeping along the toe of the embankment is indeed a result of leakage of the impounded water.