When the natural depth of water cannot accommodate the size ships calling on the port, the U.S. Army Corps of Engineers is called in to make the harbor deeper by removing sediment from the riverbed.
The Corps of Engineers has been dredging sediment from the Savannah River since the 19th century. A crucial requirement for maintaining a deepened harbor is having a designated placement area for sediment. The Corps calls these designated areas “dredged material containment areas” (DMCA). And since the Corps must dredge miles of the Savannah River year after year, large containment areas are required. The Savannah Harbor’s DMCAs spread across 5,500 acres along the South Carolina side of the lower Savannah River.
“The DMCAs are absolutely critical to the ongoing maintenance of the harbor,” said Savannah District Chief of Civil Works, Mackie McIntosh. “Savannah is known for its heavy shoaling rates and heavy siltation rates in the inner harbor. So, we’ve been very fortunate to have all these DMCAs operating basically right along the channel and that has allowed us to have shorter pumping distances and to actively manage the material.”
All the dredge material from Fort Pulaski up to Garden City terminal, roughly 20 miles of harbor, goes into the DMCAs.
About five million cubic yards is dredged each year from the inner harbor channel at a cost of approximately $26 million a year. To put that into perspective, the average dump truck holds about eight cubic yards, which would equate out to 625,000 truckloads of material moving from the river into the DMCAs every year.
Savannah District’s Chief of Navigation, Jason O’Kane explains, “The overall idea is, you pump into it, it gets wet, you allow the water to run out of structures called weirs, or drainage pipes in the dike, and the remaining material sits until it can have new material placed on top of it or be used to raise the surrounding dike.” Dikes are the earthen walls surrounding the DCMAs that holds all the material until the water can be drained.
In order to increase the capacity of a given site the dike is raised roughly six feet at a time. In an average year, approximately $2 million is spent directly maintaining the placement area dikes, gates, roads and flow control structures.
“Since you’re basically working with mush, you have to build it with a wide footprint, you have to give it settling time, you can’t load it too quickly with material and it usually settles down to about five feet,” added O’Kane.
The whole drying process in each of the DMCAs ideally lasts for about two years which is why they’re spread across a total of approximately 5,500 acres. However, without the current DMCAs the Corps would have to seek out alternative methods of placement that could range from offshore disposal to designating a new DMCA.
“If you’ve spent a lot of time in Savannah, particularly on the Georgia side of the river … [you know] that’s very valuable real estate and there’s not a lot of it to choose from,” added McIntosh. “It’s either already developed or it’s low lying wetlands, and if you impact wetlands there are tremendous mitigation processes associated with that.”
According to O’Kane, the current Environmental Protection Agency permitting time for new placement sites is around three years for relatively easy to permit sands (large grain sediment). He said the material dredged from the Savannah River is predominately silts and clays (fine grain sediments) and permission from the resource agencies, such as the EPA, to place this material would be very difficult if not impossible due to the high environmental standards.
Placing the material offshore comes with its own challenges. To do that it would cost approximately five times the current dredging cost and would dramatically impact the Corps regional and national funding budget.
“That would cause humongous negative regional impacts … commerce would cease the way we understand it in Savannah Harbor,” added O’Kane.
Those negative impacts are constantly mitigated in the Savannah region through the District’s efforts of innovative dredged material placement and stewardship of the DMCA’s dike systems in Savannah.
Through this stewardship the Corps ensures the DMCAs continue to be a vital piece of the whole puzzle that keeps Savannah, and the nation’s, economic engine running.