Risk Evaluation for Earth-Fill Dams by Response Surface Method

There are about 1.6 million earth-fill dams in Japan, which 70% of them were built before the year of 1850. The government has designated disaster prevention priority earth-fill dams and taking measures, but it is impossible to repair all of them. We propose a method to prioritize repairs based on risk to effectively prevent levee damage due to heavy rain. In this study, 31 earth-fill dams in Okayama and Hiroshima in Japan are selected as research subjects. First, we propose a method to easily calculate the potential damage costs caused by flooding in the downstream area due to the breach from the surrounding land use situation. Second, flood analysis that using the finite volume method is used to calculate the cost of submergence damage in the downstream area of the earth-fill dam. Since this work requires a large amount of calculation cost, to simplify this calculation, the response surface method is taken into consideration. The optimum response surface is determined by comparing the damage cost by the response surface method and the detailed method with flood analysis for the earth-fill dams in the studied area. In order to calculate the risk, it is necessary to determine the probability of the overflow on the levees due to heavy rain. The breach is assumed to occur when the inflow to the reservoir due to heavy rainfall exceeds the outflow capacity of the spillway. Finally, the risk by the breach of levees is calculated from the probability of the breach and the estimated damage cost. The accuracy of the response surface method is evaluated by comparing the risk ranking that means the priority of the repair by the detailed and the response surface methods. In conclusion, it has been clarified the proposed response surface can evaluate appropriate risk ranking.

Keywords: Earth-fill dams, Response surface method, Risk evaluation, Levee breach