The Ellicott City Flash Floods of July 2016 and May 2018

From a U.S. Geological Survey fact sheet of 2021.

On July 30, 2016, and again on May 27, 2018, intense, short bursts of rain flooded downtown Ellicott City, Maryland. People died, buildings, roads and infrastructure were badly damaged, and hundreds of vehicles were washed away. The 2016 storm dropped 6.60 inches of rain in 3 hours; the 2018 storm, 6.56 inches in 3 hours.

After each storm, USGS crews measured the peak flow of the three streams that run through downtown into the Patapsco River — Hudson Branch, Tiber Branch and New Cut Branch.

The place

Ellicott City, an unincorporated community in Howard County about 4.6 miles southwest of Baltimore, lies in the eastern Piedmont. Its population was 65,834 in 2010, up 9,437 since 2000, and the area averages about 45 inches of rain a year. It sits on the Ellicott City Granodiorite, crystalline rock some 420 to 470 million years old.

• The Tiber Branch watershed, which holds the town, covers about 3.68 square miles at its confluence with the Patapsco, with about 300 feet of relief. It is about 31.6 percent forest and 65.2 percent developed, with 30.9 percent impervious surface. Steep streams, big relief and large paved areas make its streams rise and fall fast in storms — the "flashy" behaviour typical of urban streams.
• The sub-watersheds are urban or suburban; Hudson Branch is the most developed and most impervious.

• The Patapsco River drains about 294 square miles at Ellicott City; since July 1954 its flow has been regulated by Liberty Reservoir, about 15 miles upstream.

Downtown sits at the bottom of a hill where the three streams meet and flow into the Patapsco, which runs along the town's east side. It can flood from the streams, from the river overtopping its banks, or both. In 2016 and 2018, Main Street became an overflow channel for the three streams: the upper street floods only from Hudson and Tiber Branches, the lower street from any of the streams or from the river.

A long history of floods

Floods are recorded from 1768, just before the Ellicott brothers founded the town in 1772. Before 2016, the largest known floods were those of July 1868, June 1972, September 1923, September 1975, May 1952 and July 1956. Flood markers downtown recorded the river's peak above street level for all but 1956 — from 21.5 feet in July 1868 to 7.0 feet in May 1952; the 1868 flood stood 7.0 feet higher than that of Tropical Storm Agnes in June 1972. The markers were destroyed in the 2016 flood and had not been replaced as of April 6, 2021.

Flood markers at the bottom of Main Street at the railroad trestle, Ellicott City. Photograph by John C. Hammond, U.S. Geological Survey, December 5, 2014.

At the USGS gage on the Patapsco at Hollofield (station 01589000), about 4 river miles upstream, with records from May 1944:

Every historical flood since 1944 marked downtown came from the Patapsco, affecting only lower Main Street. The causes, frequency and risk of flash floods are less well documented and understood.

The two storms

July 30, 2016. 6.60 inches fell from 6 to 9 p.m., 5.96 inches of it between 6:50 and 8:50 p.m., at NOAA's Ellicott City gage. The National Weather Service estimated a chance of 1 in 1,000 or less in any year — 0.1 percent. It was a "back-building" event, thunderstorms striking in quick succession. The worst flooding came between 8:05 and 9 p.m.: all three streams flash-flooded, two people died, and lower Main Street also flooded from the Patapsco.

The approximate high-water level and damaged sidewalks on Main Street after the July 30, 2016, flood. Photograph by Eric L. Boyd, U.S. Geological Survey, August 5, 2016.

May 27, 2018. 6.56 inches fell from 3:15 to 6:15 p.m., 5.00 inches between 3:53 and 5:53 p.m. — nearly the same total and the same 0.1-percent chance. All three streams flash-flooded again, and one person died. The river peaked more than 3 feet lower than in 2016, but likely still backed water up and slowed drainage from Main Street. There were two rounds of heavy rain rather than one, and the heaviest fell farther south, outside the NOAA gage's area. USGS high-water marks confirmed it: in two comparable places on New Cut Branch, peaks were 1.3 to 1.7 feet higher than in 2016.

Measuring the peaks

USGS crews made indirect measurements of the peaks. They flagged high-water marks on each stream just outside downtown, where the flow was confined to the valley and could be traced to one watershed; surveyed the marks and the channel cross-sections (and, on Hudson Branch, a culvert); and estimated the channel's roughness. Tiber and New Cut Branches were computed by the slope-area method; Hudson Branch by flow through a culvert with road overflow.

Peaks were higher on all three streams in 2018 — by about 9 percent on Hudson Branch, 28.5 percent on Tiber Branch and 85.5 percent on New Cut Branch — consistent with the storm centring farther south, over New Cut Branch. A Howard County flood-warning gage on Hudson Branch put its peak at about 8:10 p.m. in 2016 and 5:53 p.m. in 2018.

In historical perspective

The USGS compared the six Ellicott City peaks with those at 75 streamgages and 6 other sites in Maryland and Delaware during the storms of August and September 1971, plotting peak flow per square mile against drainage area alongside the Myers extreme-flood rating curves.

Peak unit discharge against drainage area, floods of August–September 1971 in Maryland and Delaware and of July 2016 and May 2018 in Ellicott City (modified from Carpenter, 1974). Credit: U.S. Geological Survey.

The Ellicott City peaks sit at the high end, but are comparable to some of 1971's most extreme — notably at stations in northeastern Baltimore City and Baltimore County hit by the storm of August 1–2, 1971, which dropped 12.6 inches in 12 hours at White Marsh, and at sites in central Maryland and northern Delaware in the storm of September 11–13, 1971 (6 to 10 inches). These sites fall roughly in a line running southwest to northeast through central Maryland and northeastern Delaware.

That pattern matches a national study by O'Connor and Costa (2003), which found the largest U.S. floods are not randomly distributed but reflect regional climate and landscape. Maryland along the eastern edge of the Appalachians has a high density of them: local topography, with subtropical moisture from the Gulf of Mexico and the Atlantic, often triggers convective storms there. So although the 2016 and 2018 floods were rare, the region has had storms producing comparable runoff before.

Sources

• Doheny, E., 2021, Storms and floods of July 30, 2016, and May 27, 2018, in Ellicott City, Howard County, Maryland: U.S. Geological Survey Fact Sheet 2021–3025. https://doi.org/10.3133/fs20213025
• The fact sheet draws on NOAA storm reports, Carpenter (1974) and O'Connor and Costa (2003).
• Corrected: the source dates the 2018 Patapsco peak to "May 30, 2018"; the storm and flood were on May 27.
• The two photographs and the graph are taken from the fact sheet's PDF; its location maps, on an Esri basemap, are not reproduced.
• Rewritten in hubnx's own words.