There used to be a calculator by NOAA: http://www.srh.noaa.gov/epz/?n=wxcalc_floodperiod http://www.srh.noaa.gov/epz/?n=wxcalc_floodperiod that is now here:

There used to be a calculator by NOAA: http://www.srh.noaa.gov/epz/?n=wxcalc_floodperiod http://www.srh.noaa.gov/epz/?n=wxcalc_floodperiod that is now here: https://www.weather.gov/epz/wxcalc_floodperiod https://www.weather.gov/epz/wxcalc_floodperiod According to it, the probabilities of a 500 year event in the next 500 years is 63.2%. What I like to call flippism point, i.e. when you have the same chances as flipping a (perfect) coin is next 346 years. A three consecutive years series does seem a bit "queer".