The proposed tunnel under Ellicott City offers both potential to eliminate future flooding on lower Main Street, as well as challenges.

The largest and potentially most-effective aspect of Howard County’s $140 million plan to minimize flooding along Main Street in Maryland’s historic Ellicott City is a still-to-be-designed tunnel.

“The intent is to divert as much water off of lower Main Street as quickly as possible,” James Irvin, director of public works for Howard County, told WTOP.

“The water will go in the tunnel on the north side of Main Street and be discharged into the (Patapsco) river.”

County Executive Calvin Ball announced the plan May 13, which will include razing four buildings on lower Main Street. An earlier plan by former County Executive Allan Kittleman would have called for the destruction of 10 buildings, to widen the channel that carries water toward the Patapsco River during heavy rains.

The county will seek to construct the tunnel as part of a public-private partnership, involving a request for proposals, or RFP.

Ball, and other county officials have said the specifics of how the tunnel will look and its exact route will be determined based upon the suggestions and solutions offered in the RFPs.

Ellicott City was devastated by floods in 2016 and 2018 — two people died in the 2016 event, one died in 2018. Founded in 1772, the former mill town’s topography has made it perpetually prone to flooding.

The town’s watershed includes several streams flowing downhill along Main Street. The Tiber Branch, Hudson Branch, Autumn Hill Branch, and New Cut Branch all converge to enter the Patapsco River.

“The normal stream flows will always be there, on a typical day,” said Irvin. “During storm events, when the streams can no longer handle the water, they’ll be diverted into the tunnel to bypass lower Main Street.”

Irvin said some basics of the tunnel have been determined.

“It’ll be a concrete structure, roughly 15 feet in diameter,” said Irvin. “One of the issues is finding an available tunnel-boring machine that meets our needs.”

Making way for the tunnel through the hilly town will be one of the most challenging parts of the job.

“What you see in Ellicott City, it’s pretty much granite or hard rock all the way through,” said Irvin.

Irvin believes the intake for the tunnel will be located in the vicinity of current Parking Lot F near the intersection of Ellicott Mills Drive and Main Street. The flood-swollen Tiber Creek washed away a portion of Ellicott Mills Drive, which recently reopened with a sturdier, larger concrete waterway beneath the road.

“It will probably drop down anywhere from 25 to 50 feet below the surface, and that’s where the (tunnel) structure will start, and continue in an easterly direction until you come to the Patapsco River, where it will come back up to daylight.”

Irvin said the tunnel is expected to run to the north of Main Street, and does not believe construction will affect travel, business, or pedestrians on Ellicott City’s prime location.

“We don’t anticipate going under Main Street directly,” he said. “It will probably (run) below some properties, we don’t know exactly which ones because the alignment hasn’t been picked.”

Irvin said the county will make sure construction doesn’t negatively affect homes and properties above the tunnel’s route.

“It’ll hopefully be deep enough so it won’t have an impact on the structures that it crosses underneath, and that’s all part of the design process, to make sure we do that properly, and those properties will be monitored during the construction process.”

In announcing the plan, Ball said the plan would not completely eliminate water on Main Street, but would leave a maximum of 3 feet of water if the 2016 storm were to happen again.

The county will seek additional outside funding, or use bond money in future capital budgets. Ball said the 2025 timeline is “aggressive,” but doable.