If you are like most people, when you hear the term "100-year flood", you think it means a kind of flood that happens only once every 100 years. The truth is a 100-year flood could happen again before the next century, or even within the same year. It could also not happen again for 150 years.
Here's what it really means
The term 100-year flood refers to the likelihood that a flood of a certain severity will occur in any given year. Think of it as a "what are the odds?" question. For example:
In 2003, Hurricane Isabel struck Virginia, devastating many communities in Southwestern Virginia with heavy flooding. While not officially categorized, we'll call it a 100-year flood occurrence. It is a 100-year flood because, according to historical data about rainfall and stream stage, the probability of the flooding reaching the levels it did was 1 in 100 or 1%.
In 1995, a 500-year flood reshaped Paine Run in Augusta County, the Staunton River in Madison County, and Moorman's River in Albemarle County. It was labeled a 500-year flood because the water and flow speed reached levels anticipated once every 500 years, or a .2% chance.
If a 100-year flood occurs in 2021, it does not increase or decrease the odds that another 100-year storm won't happen in a month or two years. A 100-year flood is a statistical evaluation based on likelihood.
Location, location, location
Classifying a flood by 100 or 1000 years is also based on location.
From 2015 to 2017, Houston, Texas, endured two 500+-year floods and one 1000-year flood. When Hurricane Harvey made landfall in 2017, it dumped more than 30 inches of rain over an area of more than 11,000 square miles. That's nearly 3 feet of water over an area the size of Maryland. Geologists say no similar flood has ever been recorded in the U.S., yet the likelihood of another flood of its kind in Houston is 1 in 1000 or 0.1%.
Virginia experienced two 100-year floods, just three years apart, when hurricanes Camille (1969) and Agnes (1972) dumped massive amounts of water over parts of the state. Some Geologists believe a 1000-year flood occurred at Ft. Belvoir in August 2001 when Tropical Storm Lee dropped 7" of rain in just 3 hours.
Comparing 30" of rain in Houston to 7" of rain in Ft. Belvoir shows how location greatly determines a flood's XXX-year description.
It isn't likely that Virginians will ever experience a Houston-like 1000-year flood because of its topography, continental location, rivers, and coastal formation. However, to put it into perspective, you could say that the type of flood Houston experienced would be more like a 10,000-year flood in Virginia.
100-Year Floodplain Maps
The term "100-year flood" did not enter the American lexicon until 1973, when the federal government first defined which land would fall under new flood control regulations and which would not. Using statistical research and modeling, they determined that 100-year floods would be the basis for creating floodplain maps.
To generate these maps, to this day, the USGS (United States Geological Survey) utilizes sensors, called streamgages, to measure the depth and velocity of rivers and streams across the state. They combine the data the sensors provide with a potpourri of other information, such as the shape of the river and the number of dams and islands, to come up with estimated flow rates, measured in cubic feet per second, and depth averages associated with the flow rates.
Over the years, the USGS has used these flow estimates to determine normal water flow and depth versus flooding water flow and depth around rivers and streams. When these measurements are applied to maps, floodplains are quickly identified.
FEMA (the Federal Emergency Management Agency) uses these maps to administer the federal flood insurance program. Local governments use them to set building and housing density codes and insurance companies to establish rates.
As an example, in most cases, a home built in a 100-year Floodplain would have to be elevated to meet federal and state building codes, resulting in higher construction and insurance costs.
How accurate are floodplain maps?
As with all weather-related predictions, floodplain maps should be viewed as "best guesses." Mother nature has a mind of her own and has shown that she'll contradict even the best scientific reasoning. Case in point? Fifteen to twenty percent of insured flood claims happen outside areas identified as high risk on floodplain maps. Homes built within the floodplains, however, are 26% more likely to be inundated at least once over the course of a 30-year mortgage.
You can't put all of the blame on Mother Nature
A major factor that makes floodplain maps unreliable is how humankind is changing the landscape. Urban development is occurring so quickly that geologists cannot keep up with how they affect flooding. Flattened hills, paved roads and driveways, and filled-in wetlands significantly affect water flow and soil absorption. Places that absorbed water three years ago may not do so today, and landscaping that slowed or diverted water flow no longer affects where underwater and surface water go.
Flooding risks are changing all of the time, yet the estimates on that risk are not keeping up.
The Virginia Flood Risk Information System has an interactive map you can use to see how close your home is to a 100-year floodplain.
Sources: ABC News, FEMA, FiveThirtyEight, FEMA, Virginia Flood Risk Information System, Virginia Places.org