What is the 100-Year Floodplain?

A 100-year floodplain or flood zone is a zone that has a one percent chance of flooding on an annual basis. There are areas on maps specifically signaled to be 100-year flood zones because that area should expect a flood at least once every 100 years — thus the one percent chance. These are areas that are darkly shaded on the Flood Insurance Rate Map, or FIRM. On these areas, buildings being built must have specific permits, environmental regulations, and insurances such as flood insurance.


However, even these specific floodplains may have a higher mathematical chance of experiencing a flood than the 1 percent originally thought. According to mathematical calculations, some of these areas have a 63.4% chance of flooding in a hundred year period. This is significantly higher than expected; however, there is also an uncertainty factor involved that could also make this number even higher. Overall, it is dangerous to build on these floodplains, and buildings that are there must meet specific requirements and qualifications.


In order to receive flood insurance for a building, you must first obtain an elevation certificate. For this to happen, the elevation of the building must be determined by a surveyor. This elevation certificate will determine the cost of flood insurance for that building. The higher the risk of a building being flooded due to its elevation, the higher the cost of the insurance.


If you live on a 100-year floodplain, you may have never seen a flood happen, but the chances of it happening could be higher than you think. Having a land surveyor evaluate your land could be a prudent decision that will help you properly prepare for the worst.