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Re: Probabilistic....

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Thank you for clarifying the difference between probability of exceedance and 
annual probability: 
 
>This annual probability can be expressed in terms of exceedence probabilities 
>for a given life using the following relationship.  This relationship assumes 
>a poisson probability distribution. 
 
>         p = 1 - e^(-LT) 
 
>where:    p = probability of exceedence 
>          L = annual probability of exceedence 
>          T = time period 
 
>Therefore for a 0.002 annual probability of exceedence, p for a 50 year 
period  
>is 0.095 or 9.5%.  For p = 0.1 or 10% and T=50 years, L = 0.00210721 or an  
>equivalent average return period of 474.6 commonly rounded off to 475 years. 
 
My oversimplified linear model wasn't too far off when "LT" was relatively 
small (i.e., when the desired time period is significantly less than the 
"return period").   
 
It still isn't totally clear if there is a difference between "annual 
probability of exceedance" and "annual probability of occurrence" (i.e. "L" in 
the above equation)?  Thus, for a 100-year flood (100-year return period) with 
a 0.01 annual probability of occurrence (exceedance?), the probability of 
being exceeded in 50-years would be approximately 40% (LT=0.01x50=0.50)?  Is 
this correct? 
 
I am currently in a discussion with others on "how conservative is it to 
design for a 100-year flood?"  While I don't see major differences in the 
mathematical results using different terms to express the frequency of 
occurrence, I do feel that expressing the risk in terms of exceedance 
probability over a 50-year period makes it easier to understand risk versus 
"return period".