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Intense flash flood inundates US city

Ben Domensino, Tuesday May 29, 2018 - 15:40 AEST

Residents in Ellicott City are assessing the damage after the second intense flash flood in three years inundated their streets and buildings on Sunday.

The usually picturesque town, located to the west of Baltimore, was transformed by debris-riddled torrents of water cascading down its narrow streets on the weekend. The deluge was caused by thunderstorms training over the area, meaning multiple thunderstorms dumped heavy rain on the one location in a short space of time.

A rain gauge near Ellicott City registered 240mm of rain in 24 hours from this event, although there are reports that most of this fell within a matter of hours on Sunday.

Image: 24 hour rainfall observation for Catonsvill, near Ellicott City, via NOAA NWS. Conversion from 9.47in to 240mm.

This latest flooding event comes just two years after Ellicott City was inundated by a similar flash flood in 2016, which dumped more than 160mm of rain in just three hours.

According to an exceedance probability analysis carried by the National Weather Service, the 2016 flood event was deemed to have a probability of occurrence of about 1/1000. This means that there is statistically a 0.1 per cent chance of this rainfall occurring in any given year. These findings makes it even more shocking that a similar event has occurred just two years later.

According to historical records, there have been at least 15 significant flood events in Ellicott City since 1768, two of which have occurred in the last three years.

Ellicott City is prone to floods of this nature because it lies in a natural funnel of land, where several streams converge before flowing into the Patapsco River. The impervious surfaces of the city itself also enhance the amount of runoff during heavy rainfall, further contributing to flooding.

- Weatherzone

© Weatherzone 2018

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