A 500 YEAR FLOOD IS A FLOOD THAT SHOULD OCCUR ONCE EVERY 500 YEARS; 2015: HOUSTON HAS MEMORIAL DAY FLOODING(500 YEAR). 2016: HOUSTON HAS TAX DAY FLOODING (500 YEAR). 2017: HOUSTON HAS HURRICANE HARVEY FLOODING (1000 YEARS). CONCLUSION: EACH YEAR IN TEXAS EQUALS 500 NORMAL YEARS EVERYWHERE ELSE.