What is a 100-Year Flood?

Written by: Sara Dennis

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The recent Hurricane Florence dumped massive amounts of water in parts of North Carolina, in what many news sources are saying could be categorized as a 500-year or 1000-year flood event. But what does a 1000-year, 100-year, 50-year, etc. flood actually mean? It describes the probability of a specific level of flooding that occurs over a 24-hour period in a certain area, but despite the nomenclature, the term has nothing do to with long cycles encompassing years. Thus, a 100-year flood describes a flood that has 1 in 100 chance (1%) of occurring in a year. And since this is related to chance, the odds reset each year, meaning that a 100-year flood could happen in the same area two years in a row. Because the terminology is confusing, some hydrologists refer to these probabilities as recurrence intervals, or their annual exceedance probability.

The table below shows the likelihood that a certain flood event will happen and how likely the flood event will occur over a span of 30 years.

  FEMA: NFIP Floodplain Management Guidebook

FEMA: NFIP Floodplain Management Guidebook

So why do we talk about 100-year floods?

 Flooding in New Orleans after Hurricane Katrina

Flooding in New Orleans after Hurricane Katrina

The calculations for 10-year floods, 25-year floods, and 50-year floods, (and other recurrence intervals) are often used in construction and design. For example, roads have to be built to withstand certain amounts of flooding, depending on their size and location. The design standards in North Carolina require major arterial roads (interstate highways, US Routes, North Carolina Routes) to be built to both withstand and drain 50-year floods. Their smaller roads and rural routes are held to a smaller standard and only have to be built to withstand 25-year floods. The amount of rain and flooding produced by Hurricane Florence was much greater than a 50-year flood, and in many areas the roads and surrounding areas are still flooded and impassable, days later.

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Knowing the calculations for an area’s 100-year flood event also allowed the Federal Emergency Management Agency (FEMA) to identify 100-year Floodplain areas, which are susceptible to being inundated by flood waters with a 1% chance of annual recurrence. Within these areas, FEMA also identifies 100-year Floodways, which are the areas in a floodplain that are the most likely to have the deepest and fastest moving water. When developing in a FEMA 100-year Floodplain there are certain regulations you must abide by. For example, in Columbus you cannot fill an area of the flood plain without compensation, which is the removal of an equivalent volume of material from the area. In other words, there has to be a net zero gain of material within the floodplain (see image below for more information). Additionally, it’s not permitted to develop within the FEMA 100-year Floodways, with a few exceptions for structures like bridges.

FEMA: NFIP Floodplain Management Guidebook

Determining 100-year flood events and other information is incredibly important in civil engineering and it impacts our everyday life. We’ll go into more details about how to mitigate stormwater and flooding in coming articles.