Charleston, SC - Dredging in many ports is a never-ending battle. But that could soon change in one area of the Port of Charleston, thanks to a new system that eliminates routine dredging alongside marine terminals.
The Ports Authority Board today approved the purchase of equipment for a new sediment suspension system for the Columbus Street Terminal in downtown Charleston.
"This project will save millions of dollars," said Bernard S. Groseclose Jr., president and CEO of the South Carolina State Ports Authority, "so we certainly appreciate the support we have received in moving forward with this proven technology." Groseclose noted that S.C. Rep. Wallace Scarborough (R-James Island) was especially supportive.
It has happened for thousands of years -- fine silts and clays originating from tributaries to the Cooper River make their way downstream to Charleston Harbor. During times of slack water between the tides, heavier silts and clays fall out of the water column and settle to the bottom.
A lot of this material comes to rest in front of berths 1 and 2 at the Columbus Street Terminal, just north of the South Carolina Aquarium. The fluid mud is currently settling at a rate of one to two feet per month.
To maintain adequate depth and ensure that ships can safely dock, the Ports Authority must hire dredging contractors every four to five months to suck up and dispose of the material. The cost of maintaining the depth has averaged $750,000 annually.
With state and federal permits now in hand, the Ports Authority is preparing to install the new sediment suspension system to eliminate routine maintenance dredging at a portion of Columbus Street Terminal. The system consists of ten water jets, three-feet wide and equally spaced along the front of the berth.
During each day's four slack tides, the system will route water from the upper portion of the water column, down through pipes and out of oscillating jets at the desired depth. This prevents the heavier material from settling, while not actually impacting the bottom.
To operate effectively, the system is managed by a desktop computer that automatically monitors a series of sensors and uses a NOAA-developed tide-predicting algorithm.
The system will cost about $1.9 million to install and another $50,000 a year to operate. But the Ports Authority estimates it will save $7.5 million in dredging costs, a net savings of approximately $5 million over the 10-year project life.
While new to Charleston, the concept was first developed by the Navy two decades ago. Marine terminals in other states, including Georgia and North Carolina, have since installed versions of the system.
In addition, environmental impacts of sediment suspension systems are negligible. While agitation dredging removes sediments that have already accumulated, the new system has less flow and never allows the material to deposit on the bottom. A monitoring plan will evaluate effectiveness and any impacts.
The system will also have safeguards to minimize the chance or impacts of an accidental release of hydraulic fluid. The computer controller will constantly monitor the system and fluid levels, triggering a shutdown if there are any problems with leaks or seals. The system also will use a bio-degradable, vegetable-based hydraulic fluid that is non-toxic.
The Ports Authority applied for permits for the new system in March 2003. The South Carolina Department of Health and Environmental Control issued its permit in March of this year, and the U.S. Army Corps of Engineers offered its approval in May.
Installation is expected in about two years, following modifications to the dock, construction and installation of equipment. In the meantime, maintenance dredging will continue.