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
To learn more about other flooding-related stories and topics from around the world, be sure to click on the following link: https://www.globalweatherclimatecenter.com/flooding!
©2018 Meteorologist Sarah Trojniak
Subtropical Storm Alberto indirectly brought torrential downpours and thunderstorms to the Mid-Atlantic Region. While Alberto did not go straight over the region, a flow of moisture was moving northeast coming from Alberto. The northeast flow moved towards a stationary front that was sitting over the Tennessee Valley and the Mid-Atlantic region. These were the ingredients for the destructive flooding that occurred in Maryland, Virginia and North Carolina.
Ellicott City in Maryland was under water on Sunday, May 27, 2018 when a nearby river overflowed its banks and into the streets of local homes and businesses. Many people needed to be rescued from buildings and vehicles as the force of the water pushed vehicles down the streets. One person lost his life when he was attempting to help a person in need. The above image is from Syria, Virginia where a woman and a young girl were swept away in the flooding. The young girl was quickly found; however, the woman has yet to be found as of the time this article was posted. The image on the top used to be a bridge where people could walk, but the strength of the flooded waters pushed the bridge off and you can see it in the image on the bottom. On Route 33, in Central Virginia, a landslide covered the highway prompting several road closures and detours. If we continue south, North Carolina had its share of landslides and flooding. One landslide compromised the integrity of the Lake Tahoma Dam in North Carolina prompting evacuations. Some areas have received over 6 inches of rain and the threat of rain continues. Many roads have been closed throughout the Mid-Atlantic due to debris and flooding.
Always remember; if you see high water, turn around, don’t drown. It only takes 6 inches of water to stall a vehicle while a foot of water will float many vehicles. Once the water level reaches two feet, your vehicle will likely be carried away. Flooded waters are typically muddy so any debris lurking in the water may not be visible.
Alberto is no longer a threat, but rain will persist through the weekend. With already saturated soil, any additional rainfall will produce more flooding. After the first weekend of June dry weather will make its way back into the area.
Stay tuned for more updates on the flooding situation in the Mid-Atlantic by clicking www.globalweatherclimatecenter.com/flooding
ⓒ 2018 Meteorologist Brandie Cantrell