A CASE STUDY ON RECONSTRUCTION EFFECT FOR SMALL IRRIGATION TANK

Abstract

: Irrigation tanks are indispensable water facilities for agriculture and flood control in Japan.
However, most irrigation tanks in Japan have deteriorated and are suffering from damage such as water leaks and
collapses due to heavy rain and earthquakes. Aging, a declining population, and current economic conditions are
large hurdles for constructing new irrigation tanks. Therefore to prevent the occurrence of failures, the
reconstruction of these deteriorated irrigation tanks is an urgent task. In this study, the reliability of the
reconstruction work is analyzed by calculating the probability of failure caused by the infiltration of deteriorated
Irrigation Tank H due to heavy rain. The probability of failure is calculated from the probable rainfall amount and
the sliding failure. The annual probability of failure during heavy rain is obtained by considering both the safety
factor which includes the variation in soil parameters and the probability of the occurrence of rainfall. Moreover,
the benefit-cost ratio (B/C) is calculated to evaluate the reconstruction effect. As a result, B/C is found to exceed
1 when the social discount rates are 0~3% of half of the durable years of the irrigation tank. B/C is approximately
1 when the social discount rate is 4% of the durable years.