The Ellicott City, Maryland Flash Flood of 30 July 2016: Where Meteorology, Hydrology, and Geography Collide

A statistically-rare (<0.1% annual exceedance probability) heavy precipitation event developed during the evening hours of 30 July 2016, affecting an area of Central Maryland from Montgomery County through Howard County to the City of Baltimore. While significant impacts were observed throughout the impact zone, the most noteworthy impacts occurred in the historic district of Ellicott City, Maryland, with two fatalities, damage of numerous buildings and infrastructure, and necessitating the closure of much of Main Street in Ellicott City for a period of weeks.

Although the rainfall – nearly six inches in two hours – would have created major impacts in almost any populated area, unique hydrologic and geographic features exist in Ellicott City, including multiple small streams feeding into a large river, and significant elevation changes.  These create a complex situation, and played a substantial role in the severe damage which occurred.

The event will be reviewed through a timeline of known events, with specific focus on the complex hydrologic elements in play and the geography of this very small watershed.