Understanding What a 1,000-year Flood Means: The Ellicott City Tale (credit: NOAA NWS Baltimore, MD)

DISCUSSION: On Sunday, May 27th, 2018 Ellicott City in Maryland experienced a devastating flood that tore through the heart of the town. Cars were washed away by what looked like a raging river as water rushed down main street. The water rose fast and high in some locations above the first story of houses. When all was said and done, an estimated eight inches of rain fell over the city in less than 12 hours, with the majority of the rain falling over a three-hour time span. Analysis of the flood determined that Ellicott City was hit with a 1,000-year flood. However, this was not the first 1,000-year flood to hit the city. On July 30, 2016, Ellicott City experienced a similar flooding event, with approximately six inches of rain falling over a two-hour period. Like the most recent event, the 2016 flood left the city in ruins.
 
But how is it possible to have two 1,000-year floods less than two-years apart from each other?  The answer is in what a 1,000-year flood actually means. The term itself is misleading, suggesting that such a flood should happen only once every 1000 years. However, 1,000-year flood represents a hydrological probability defining the likelihood or rarity of a flood occurring. A 1,000-year refers to a 1 in 1000 chance or a 0.1% chance of the flooding event occurring any given year. Similarly, a 100-year flood has a 1 in 100 chance or a 1% chance of occurring each year while a 10-year flood has a 1 in 10 chance or a 10% chance of occurring each year. The U.S. Geological Survey, also known as USGS, refer to this probability as the one percent Annual Exceedance Probability, or the AEP. Along rivers, gauges measure the height of the water and the quality of the waters’ flow across America. The information from the gauges is then statistically analyzed for any given location and the flood probabilities are determined. However, other factors play a role in how likely a flooding event is, which can not necessarily be included in the statistical analysis. For example, the accuracy of the incoming data and changes in how the land is used in any given year. These, combined with generally unpredictable changes in yearly weather patterns, lead to rare events, like a 1,000-year flood, happening in the same location less than two-years apart.
 
Information provided by the USGS and the NWS in Baltimore (LWX)
Image from the Hydrometeorological Design Studies Center ftp://hdsc.nws.noaa.gov/pub/hdsc/data/aep/201805_Ellicott_City/AEP_Ellicott_City_May2018.jpg

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©2018 Meteorologist Sarah Trojniak