What is a 100-year-flood?

Flood in North LIttle Rock June 2019


By Marc Johnson, PE, CFM, and Linda Johnson, PE, CFM

The tragic floods that have struck Arkansas and nearby states this year have caused tens of millions of dollars in damages to homes, businesses, farms, and infrastructure. They have brought grief and despair to many citizens who weren’t prepared to handle a tragedy of this magnitude.

Some long-time residents of the Arkansas River Valley have said they have never seen flooding of this magnitude.

It has been described as a 100-year-flood.

Engineers, hydrologists, and other experts sometimes use this term, but it is somewhat misleading. Members of the general public wrongly assume that description should mean that another flood of this magnitude should not strike for another 100 years, or a 500-year-flood should typically only come every 500 years.

The simple truth is that these types of catastrophic floods can occur every year or multiple times during a year.

The term “100-year-flood” actually means that in any given year there is a “one percent” chance of that flood occurring (1 chance out of 100 or 1/100). A 200-year-flood would mean that there is a “0.5 percent” chance (1/200) of that type of catastrophe occurring in any one year. When we refer to a 500-year-flood, that translates into a 0.2 percent chance (1/500) of that event happening in any one year. People understand percentages – like a 20% chance of rain – and it’s a better way to describe the risk.

If a homeowner lives in the same location in the floodplain for the duration of a 30‑year mortgage, there is a 26-percent chance that the 1‑percent annual chance flood will occur at some point over the life of that mortgage based on cumulative probabilities (i.e., like throwing the dice 30 times).

For decades, FTN has worked with FEMA throughout Arkansas and other states developing floodplain maps. Under federal law, the purchase of flood insurance is required for all federal or federally-backed mortgages for the purchase and/or construction of buildings in FEMA high-risk flood areas. If the property is not in a high-risk area, but instead in a moderate-to-low risk area, federal law does not require flood insurance; however, a lender can still require it.

FTN recommends that not only people with property that is in a high-risk flood area purchase flood insurance but also recommends people even with property near a high-risk flood area also purchase it since historically 25% of flood claims come from these moderate-to-low risk areas. Moreover, the flood insurance in the areas outside the high-risk flood area is relatively cheap.

Water resources services, including floodplain permitting and mapping, master drainage projects, and other related services have represented a large portion of FTN’s work during our 38 years of doing business. If flooding has caused you problems, we may be able to help.

Please give us a call in Little Rock at (501) 225-7779 or email us at mcj@ftn-assoc.com or lsj@ftn-assoc.com.