RISK OF EXTREME EVENTS UNDER NONSTATIONARY CONDITIONS

The concept of the return period is widely used in the analysis of the risk of extreme events and in engineering design. For example, a leve e can be designed to protect against the 100-year flood, the flood whi ch on average occurs once in 100 years. Use of the return period typic ally assumes that the probability of occurrence of an extreme event in the current or any future year is the same. However, there is evidenc e that potentia; climate change may affect the probabilities of some e xtreme events such as floods and droughts. In turn this would affect t he level of protection provided by the current infrastructure. For an engineering project, the risk of an extreme event in a future year cou ld greatly exceed the average annual risk over the design life of the project. An equivalent definition of the return period under stationar y conditions is the expected waiting time before failure. This paper e xamines how this definition can be adapted to nonstationary conditions . Designers of flood control projects should be aware that alternative definitions of the return period imply different risk ender nonstatio nary conditions. The statistics of extremes;md extreme value distribut ions are useful to examine extreme event risk. This paper uses a Gumbe l Type I distribution to model the probability of failure under nonsta tionary conditions. The probability of an extreme event under nonstati onary conditions depends on the rate of change of the parameters of th e underlying distribution.