Nishimura, Shin-Ichi

doi:doi

Risk Evaluation and Reliability-Based Design of Earth-Fill Dams for Overflow Due to Heavy Rains

Abstract eng:
Many earth-fill dams have been built for farm ponds in Japan, particularly in the Setouchi area which is surrounded by the Inland Sea of Japan. Some of the dams are getting old and decrepit, and have weakened. Every year, a number of them are damaged by heavy rains and earthquakes, and in a few worst cases, the dams are completely destroyed. To mitigate such disasters, improvement work is conducted on the most decrepit earth-fill dams. Since there is a recent demand for low-cost improvements, the development of a design method for optimum improvement work at a low cost is the final objective of this research. A reliability-based design method is introduced here in response to this demand. In this research, the risk to earth fills during heavy rains is evaluated. The rainfall intensity is dealt with as a probabilistic parameter, and a statistical model for the annual maximum rainfall intensity is determined from the rainfall data record in Okayama City for 45 years. Based on the statistical model, the probability of overflow is calculated. If the water from a reservoir overflows onto the earth-fills, it is assumed here that the embankment will be ruined. This assumption is supposed to be on the safety side, since the embankments is not necessarily destroyed by the overflow. Based on the estimated probability of failure, the risk to the downstream area of the earth-fill dams is evaluated. The damage costs in the downstream area is estimated from the area predicted to be submerged by the flooding. When the maximum overflow head on the spillway bed, h_p becomes greater than the design overflow head, h_d, the overflow occurs. Then, the probability of overflow is defined by Equation (1) as the times of h_pd in the iterations of the Monte Carlo simulation. P_f=Prob[h_d‹h_p] The expected total cost is given by the following equation. C_T=C₀+C_f.E[n] in which CT is the expected total cost n is the frequency of overflows within lifetime span t (years), C₀ is the cost of the improvement, and C_f is the cost of failure due to flooding. In this research, the improvement of the spillway is considered, and the improvement brings about a drastic increase in the discharge ability of the spillway. Based on a comparison of the expected total costs, the effect of the improvement work for the spillway could be evaluated. The effects of the improvement works had been represented as the difference in the total costs between the current and the improved states.

Contributors:

Fujisawa, Kazunori

Publisher:

Research Publishing, No:83 Genting Lane, #08-01, Genting Building, 349568 SINGAPORE

Conference Title:

Proceedings of the 5th Asian-Pacific Symposium on Structural Reliability and its Applications

Conference Title:

5th Asian-Pacific Symposium on Structural Reliability and its Applications

Conference Venue:

Singapore (SG)

Conference Dates:

2012-05-23 / 2012-05-25