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DEFINING FLOOD RISK

METEOROLOGIST JEFF HABY

One of the greatest risks from storms is flooding. When a flood occurs, water rises at locations that are occupied by buildings, roads and where people are located. It is important to make people aware of the relative flood risk of locations such as at the locations of homes, parks, businesses and roadways. Many people are willing to accept the risk, especially if the land is valuable and/or the yearly risk is low. Flood risk is often defined as the probability of a particular flood height occurring in a given year. One common parameter is the 100-year flood. To be a 100 year flood, there is a 1% chance of a location getting a particular flood height in a given year. A 10 year flood would have a 10% chance of occurring each year. The larger the number of particular flood heights expected in a 10 or 100 year period, then the greater the flood risk. Since it is a probability, it is not a guarantee of how many floods will occur every 10 or 100 years but rather is a guide for the typical number of floods that can be expected or the percent chance of a particular flood height each year (the actual flood heights or percentage of years with a particular flood height can end up being higher or lower than the theoretical value). Locations that are typically at greater flood risk are along the coast, in low lying area, near rivers/streams and where water ponds since it can only slowly drain away. It is important to understand the flood risk of a location since it determines insurance rates and the risks to property and life.