In THE PERFECT STORM: The Science Behind Subrogating Catastrophic Flood Losses, hydrology expert witness Richard Van Bruggen writes on the 100-Year Storm versus the 100-Year Flood:
Two sets of terminologies which are confused more than any other involve confusing the 100-year storm with the 100-year flood. These are two distinct and different events. Floods are classified according to their frequency and depth. For instance, there are 10-year, 25-year, 50-year, 100-year, and 500-year floods. A 100-year flood occurs less frequently than a 10-year flood, but because it has a larger volume and greater depth of water, it has far more destructive power, causes more damage, and is a more serious threat to human safety. We do not necessarily get a 100-year flood from a 100-year rainfall. This is where man comes into play. God may send a 10-year rainfall, but it is man that transforms it into the 100-year flood. Whether or not a 100-year rainfall causes a 100-year flood depends on the “time of concentration,” of the watershed, which itself, depends on watershed size, runoff characteristics, and other geological conditions.