Tunnels were dug beneath the peninsula during the earlier phases of the project | Provided

A major downtown Charleston stormwater drainage improvement project is well underway, and the completion of its next phase of development, slated to be finished next year, could see to an estimated 500% increase in flow of stormwater in affected areas, reducing flooding and protecting landscapes.

The project has been a long process, beginning with Phase 1 in 2013 that established some surface-level drainage systems, but those only go so far, especially in an area like Charleston.

“Everything works a little funky in the Lowcountry,” said Matt Fountain, the city’s director of stormwater management. “Everything is so flat, water doesn’t have a lot of places to go. That’s what the pipes are for, but there’s only so much water you can move through a relatively flat pipe.”

Matt Fountain | Provided

Before the start of the project, the peninsula lacked proper drainage basins, an area where water can naturally flow. Peninsula basins, one of the most well-known being California’s Baja peninsula, tend to be small, because there aren’t a lot of changes in elevation to give water a path to move. That’s where the tunnel approach comes in, Fountain said.

Digging vertical passageways down into tunnels dug during Phases 2 and 3 of the project tied to the surface pipes provided a path for water to take, and they were strategically placed in areas water commonly gathers and sits. 

Phase 4’s construction primarily comprises the outfall, where stormwater can exit the system, consisting of three 8-foot by 10-foot box pipes extending 500 feet into the Ashley River, hidden from view even during low tide and sized to minimize erosion.

As a result of the new outfall, the system will initially work under gravity, increasing the flow rate by up to seven times its existing capabilities along the project route.

The drainage basin will affect a large area of the Charleston peninsula, starting at the point where Interstate 26 meets Highway 17 on King Street and centered along the Septima Clark Expressway corridor. The tip of the affected area stretches north almost all the way to Hampton Park.

Even with the completion of the outfall and Phase 4 as a whole, there still wouldn’t be a way to get water to flow against the force of gravity, rendering much of the water in the system stuck below the water level of the Ashley. But,
the groundwork would be laid for Phase 5,
a pump station with three mechanisms pumping about 120,000 gallons of water per minute from the underground pipes up into the Ashley River, 10 times the flow rate at the end of Phase 4.

“Those of us who have had the opportunity to go into the tunnels — it gives you a feel of the magnificence of this project and the talent and sheer engineering feat this is accomplishing,” said Charleston City Council member Peter Shahid during a March 23 meeting. 

This sort of project isn’t unique to the Lowcountry, and neither are the problems caused by stormwater runoff. 

“Stormwater runoff picks up pollutants like trash, chemicals, oils and dirt that causes damage to rivers, streams, lakes and coastal waters,” a representative from the federal Environmental Protection Agency (EPA) told the City Paper

But, Charleston is at risk for unique complications, due to rapid growth over the last decade.

“Population growth and the development of urban/urbanized areas are major contributors to the amount of pollutants in the runoff as well as the volume and rate of runoff from impervious surfaces,” reads a 2020 report from the EPA’s National Pollutant Discharge Elimination System. “Together, they can cause changes in hydrology and water quality that result in habitat modification and loss, increased flooding, decreased aquatic biological diversity and increased sedimentation and erosion.”

But, management projects like Charleston’s can protect wetlands and aquatic ecosystems, improve quality both in natural environments and drinking water and control flooding by keeping water moving rather than growing stagnant. 

“You’ll certainly see much better performance at lower tides,” Fountain said. “You’ll see the Crosstown corridor flood less in smaller storms. There’s still some challenge with larger storms until we get the pumps in, but you’ll see it drain much quicker instead of sitting there for a whole day.

“It’s not necessarily customized, but it’s not a super common approach either,” he said, adding that part of the reason this approach has been effective is the peninsula’s geography. “Charleston has a really good geological formation that’s really suitable for tunneling in at depth. It’s a super-overconsolidated clay that holds up well for tunnels and it isn’t as expensive as tunneling through rock.”

Phase 4 is estimated to be completed next year, and Phase 5 is expected to be completed in 2024. Those interested in keeping up with further progress can visit the project website at springfishburnedrainage.com, which is updated in real time with new information, images and videos. 

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