Date: Mon, 01 Aug 2016 13:36:14 EDT
Summary: Extreme rainfall associated with localized thunderstorm activity caused flash flooding in Ellicott City, MD and surrounding vicinity on the evening of Saturday, July 30, 2016
Rainfall totaling up to 6 inches in a two-hour time period caused flash flooding in Ellicott City, MD and surrounding vicinity on the evening of Saturday, July 30, 2016. The most recent National Weather Service statistics describing the expected frequency of a precipitation event of that magnitude and duration indicates it was a 0.1-percent (1,000-year) event. USGS streamflow gages in the affected area recorded peak streamflows with preliminary estimates of 50-percent chance to 20-percent chance (2-year to 5-year) return intervals. Three USGS gages were inundated and ceased to function and so can not be evaluated immediately for return period, and at least one short-term gage reported a record peak. Affected watersheds include portions of the Little Patuxent River, Patapsco River, and Gwynns Falls.
USGS responded on Sunday, July 31, 2016 by collecting discharge measurements at some affected streams in the area (two-person field crew), and identifying high-water marks at several other gages whose stage-discharge ratings were exceeded (one-person field crew).
The USGS response continues today (Monday, August 1, 2016) with twelve staff (six crews) in the field to assess gage damage, collect crest-stage gage data, identify high-water marks, and collect water-quality samples from sites affected by Saturday's storms.