 Their proposal included a permanent flood wall and temporary gap structures to protect downtown Alton from future floods. A 500-year flood has a 0.2% chance of occurring in a given year. The law of probability states that both 100-year floods and a 500-year flood could occur in the same year. The floods of 1993 and 2019 were considered 100-year floods, meaning they have a 1% probability of actually occurring in any given year.