所謂 500年"一遇"，我猜是統計學中的"return period" (= 500 years)，i.e. "任何一年度內, 發生的機率是 1/500"，並不代表真的要500年才能發生。
http://en.wikipedia.org/wiki/Return_period
"... This does not mean that a 100 year flood will happen regularly every 100 years, despite the connotations of the name "return period". In any given 100 year period, a 100 year event may occur once, twice, more, or not at all..."
http://www.dsd.gov.hk/TC/Files/flood_prevention/flood_protection_standards/cstandard02.pdf
