It will sound like a rumble of thunder.

Blasting is set to resume near Old Ellicott City next week as Howard County crews start digging a massive tunnel to redirect flood waters away from the historic mill town’s Main Street.

After some test blasting in December and January, daily blasting was supposed to begin in March, according to the Howard County Department of Public Works.

County spokeswoman Safa Hira confirmed Monday that underground blasting is set to resume at the site at 8800 Frederick Road on or around April 14.

Hira said there will be a single blast every few days for the next several weeks. Blasting will take place during daylight hours, but not on Sundays. No impacts to traffic are anticipated.

Residents in proximity to the tunnel site should anticipate hearing air horns prior to the blast, Hira said.

Blasts are expected to sound like “a brief rumble of thunder” and may be accompanied by momentary vibration, she said.

Crews are monitoring vibrations from the blasting with seismographs and are conducting pre- and post-construction inspections on all properties near the site. Private property owners were notified of the inspections via certified mail.

Inspectors are documenting the condition of walks, foundations, exterior walls, porches, trim and external parts of the building or structure. Gauges may be installed on some existing cracks with a property owner’s permission.

Construction on what’s known as the North Tunnel began last year and has gained momentum in 2025. In February, officials invited journalists to view a manmade crater along Frederick Road where the tunnel entrance is planned.

The county will use a tunnel-boring machine, which residents affectionately named “Rocky,” to dig the mile-long, 18-foot-wide tunnel about 150 feet underground through rock from near Papillon Drive’s intersection with Frederick Road to the Patapsco River.

Rocky is expected to arrive from Ohio later this fall.

The blasting is necessary to clear solid rock to make way for the boring machine.

When the $141 million tunnel is completed around 2027, it will be the most expensive public infrastructure project in Howard County history. It is the primary undertaking in the county’s sweeping plan, called “Ellicott City Safe and Sound,” to mitigate deadly flash floods like the ones that devastated the town in 2016 and 2018.

Officials hope that the tunnel, with the capacity to channel 26,000 gallons of water per second, can absorb and redirect the brunt of such floods from intense storms that send runoff pouring down the steep sides of the valley where the town lies at the bottom.

The plan is behind schedule and over budget thanks to an expanded project scope, the COVID-19 pandemic and rising costs for construction materials and labor.

Once completed, Howard County Executive Calvin Ball said, it will position Ellicott City as a model for climate resiliency.