Over the years, as the peninsula settled, sea level rose, and the conveyance system became full of sediment, an ever increasing amount of energy was and is needed to remove the stormwater that accumulates on the surface (see "Why Does It Seem Like Charleston Always Floods When It Rains" on the Department homepage). As per the aforementioned website and accompanying presentation, the best way to quickly remove accumulated stormwater is to pump it out.
First Drainage Improvement Project
In 1999, the City of Charleston undertook its first drainage improvement project involving drop shafts, a deep tunnel system, and a stormwater pump station when it executed the Calhoun Street East Drainage Improvement Project. The project consisted of an 8-feet diameter tunnel under Calhoun Street from Marion Square to Concord Street, a 5.5-feet diameter tunnel under Meeting Street from Mary Street to Marion Square, large and small drop shafts along Meeting and Calhoun Streets, and a stormwater pump station on Concord Street with 3 pumps each capable of pumping water in excess of 30,000 gallons per minute.
Division I of the Market Street Drainage Improvement Project
In 2006, the City executed Division I of the Market Street Drainage Improvement Project involving surface collection improvements on Concord Street, upgrading the controls of the pump station to allow for remote monitoring and control, and the installation of an additional pump giving the pump station a total pumping capacity in excess of 120,000 gallons per minute in anticipation of connecting the Market Street system to the existing pump station.
Division II of the Market Street Project
Division II of the Market Street project, which was completed in 2014, consists of:
140-feet deep, 25-feet diameter main working shaft near the intersection of Market and Concord Streets on State Ports Authority property
Series of 54-inch drop shafts between the Market buildings on State, Anson, and Church Streets
Emergency outfall adjacent to the existing brick arch outfall on Concord Street
10-feet diameter tunnel that runs under Market Street from the working shaft to Church Street (approximately 1,200 feet)
Another 10-foot diameter tunnel that runs under Concord Street from the working shaft to the existing pump station (approximately 2,800 feet).
Concord Street Tunnel
The Concord Street tunnel connects to the wet well of the existing Concord Street Pump Station. The tunnels need to be as deep as they are because the material immediately below the surface contains lots of silt and sand and is very poor for tunneling and structural support. Below that, however, is the Cooper marl, a material very conducive to tunneling (so much so that the first carrier tunnels constructed by CPW for water and wastewater conveyance pipes were not lined). It is in the Cooper marl that these tunnels will be constructed.
Division III of the Market Street Project
Division III of the Market Street project is currently in final design and is expected to start construction sometime in 2018. It will consist of constructing a new, larger surface collection and conveyance system and tying it to the drop shafts. Additionally, the City of Charleston is taking the opportunity to perform a streetscape project on the Market from Meeting Street to East Bay Street.