Risk is defined as a combination of the probability of an event occuring and the consequences if it does occur. For example, the probability of a flood occuring in a low-lying area next to a river might be quite high, but if it is a natural setting with no negative consequence from a flood aside from the deposition of some sediment, then the the risk is low. If, on the other hand, a town is next to the river, then there would be negative consequences for it and its residents should a flood occur. Risk would then be high.
The probability of a flood occuring in the 100-year floodplain is one in 100 or 1 percent each year. It does not mean that a flood happens every 100 years. With a 1-percent probability, floods could happen two years in a row. Likewise, the 500-year floodplain is the area where the probability of flooding is one in 500 or 0.2 percent each year.
Boulder, Colorado is at the foot of the Rocky Mountains. Several narrow valleys lie to the west of the city. When rainstorms drop large amounts of water in one of these valleys, it flows downhill into the city. Often thunderstorms in the area are isolated to a particular region, meaning that flooding could happen in a creek that comes from one valley where the rainstorm has stalled and not another. However, in September 2013, a very large rainstorm parked itself over the entire region for several days causing widespread flooding of all the waterways that pass through the city.
The probability of flooding can also be affected In various areas of the world by differences in topography, rapid snowmelt, ice jams in rivers, the permeability of the land, and proximity to coasts that are vulnerable to flooding during storms like hurricanes.