Two important points emerge:
The probability of a 100 year flood occurring in 100 years is NOT 100%! (See below)
The probability of a 100 year flood occurring in 30 years (the lifetime of the average home mortgage) is 26.0%!
Flood Hazard Mapping
Food hazard mapping is used to determine the areas susceptible to flooding when discharge of a stream exceeds the bank-full stage. Using historical data on river stages and discharge of previous floods, along with topographic data, maps can be constructed to show areas expected to be covered with floodwaters for various discharges or stages.
FloodMap.GIF (21479 bytes)
In constructing such maps aerial photographs and satellite images of prior floods are studied to help to determine the areas that would be covered. The illustration above shows a possible hazard map based on estimated discharges or river stages for a hypothetical 10-year flood, 50-year flood, and 100-year flood.
In addition, scale models are often constructed of areas prone to flooding. Such models only work if they are kept up to date with current flood prevention structures and drainage modifications.