There is a ‘rule of three‘ in statistics that provides a rapid method for working out the confidence interval for flood occurrence.

From Wikipedia:

If a certain event did not occur in a sample with *n* subjects, the interval from 0 to 3/n is a 95% confidence interval for the rate of occurrences in the population.

For example, if a levee hasn’t been overtopped since it was built 100 years ago, then it can be concluded with 95% confidence that overtopping will occur in fewer than 1 year in 33 (3/100). Alternatively the 95% confidence interval for the Annual Exceedance Probability of the flood that would cause overtopping is between 0 and 3/100 (3%). Of course you may be able to get a better estimate of the confidence interval if you have other data such as a flow record, information on water levels and the height of the levee.

The rule of 3 provides a reasonable estimate for *n* greater 30.

### Like this:

Like Loading...

*Related*

Luke PHi Tony,

Is there any similarity in basis between this and the recommendation by Chow (1953) for the suggested limit of extrapolation for frequency analysis (three to four times the duration of the observed record) in that fact that ‘we know it didn’t go larger than this over this period’?

Thanks

tonyladsonPost authorHi Luke,

I don’t think these are related. Extrapolating out to 4 times the record sounds risky. There is some discussion of extrapolation of flood frequency curves in the new ARR. Book 3, Chapter 2.2.4. Also see Book 3, Chapter 2.8.1 link to the web-based version of ARR

Cheers