CHARLESTON — Even as the Port of Charleston and its fierce competitor Savannah., Ga., compete to get federal money to deepen their harbors, the little-seen work of dredging to maintain their shipping channels at current depths continues.
The largest American hopper dredge is off Charleston this week. It is removing silt from the shipping channel and depositing at an offshore disposal site. The Glenn Edwards is working to maintain the channel depth at its currently authorized 45 feet.
By week's end the vessel will be off Savannah, doing similar work in the channel leading to the Savannah River and the Georgia ports.
Both Charleston and Savannah are pursuing harbor deepening projects that could cost more than $900 million combined to enable the ports to handle larger vessels expected to routinely call once the Panama Canal is deepened in two years.
The Glenn Edwards, operated by the Manson Construction Co. of Seattle, has been operating off Charleston since late last month.
The vessel has pipes that act like a giant vacuum cleaner to suck material off the channel floor. It can load 13,500 cubic yards of silt. Once full, the vessel travels several miles to an offshore disposal site where the material is dropped by opening doors in its hull that are 14 feet by 21 feet.
During its stay, the vessel, contracted by the U.S. Army Corps of Engineers, has removed 600,000 cubic yards of silt while working around the clock.
Glenn Edwards Capt. Bill Anderson said removing silt and transporting it to a disposal site is easier than if the dredge were being used to gather material to rebuild a shorefront beach. In such a project, a barge and pipes leading to the shore must be attached to the dredge.
"Bottom dumping is easier, but a beach job is a little more fun," he said, adding that with a beach project, the vessel and its crew of 18 can see progress. "You can see day to day that you are building the beach."
Each year, about 2.1 million cubic feet of sediment is removed from the Charleston Harbor shipping channel at a cost of upward to $13 million, said Lisa Metheney, assistant chief of programs and project management for the Charleston District of the U.S. Army Corps of Engineers.
The entrance channel, the area farthest out to sea, needs to be maintained about every two years depending on currents and how much sediment builds up.
On the vessel at all times are observers, on the watch to make sure dredging doesn't hurt endangered right whales or endangered sea turtles. The corps also monitors the type of sediment removed to make sure it's compatible with the area where it is being deposited.
If the channel deepening projects are approved, the Glenn Edwards likely won't do the work, Anderson said.
The hopper dredge can easily clear silt that has washed into shipping channels. But going deeper could require cutting through clay or harder sediments.
Generally, to deepen a channel, a pipeline dredge with a cutter wheel is used. The wheel cuts the material and then the dredge removes it.