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Ellicott City’s Historic District suffered extensive damage from two devastating flash floods, one in 2016 and another in 2018, that caused widespread destruction. In response, the Department of Public Works, alongside its consultant team, initiated the development and implementation of a series of flood mitigation efforts.
A central element of the EC Safe and Sound Plan, this project involves constructing a deep bedrock tunnel approximately 5,800 feet in length and 15 feet in diameter. It includes two drop shafts and an outfall structure. The tunnel begins near the 8800 block of Frederick Road and discharges into the Patapsco River north of Parking Lot B. It will pass beneath an active CSX Transportation railway that runs parallel to the river and is designed to intercept stormwater flows from the Hudson Branch and a nearby unnamed tributary, while allowing base flow to follow existing drainage patterns.
Known as the North Tunnel, this one mile long, 18 foot diameter conduit will be excavated through solid granite roughly 150 feet below ground using a tunnel boring machine named Rocky. This method minimizes disruption at the surface while achieving the necessary hydraulic performance.
Construction is progressing steadily. Excavation of both Papillon shafts is complete, with the Mining Shaft fully lined in concrete and the Drop Shaft currently being lined. Two underground chambers have been excavated, with one remaining. About seven rounds remain to complete the final drill and shoot chamber, followed by construction of the permanent invert slab in preparation for TBM assembly. TBM launch is anticipated in late April.
When completed in 2027, the Ellicott City North Tunnel will have the capacity to convey 26,000 gallons of water per second, providing long term flood protection for the historic district.