Port of Morgan City Tackles Excessive Silt with Both Tradition and Innovation
Manson Construction Co. hopper dredge Newport worked back and forth along the bar channel in the Port of Morgan City, Louisiana, not removing sediment, but agitating it enough to meet the 1,200 milligrams per liter for density down to 18 feet, allowing safe navigation through the area.
The Port of Morgan City is the last stop, by way of the Atchafalaya River, for water streaming toward the Gulf of Mexico from both the Red River and a 30 percent stake of the Mississippi River, courtesy of the Old River Control Structure above Baton Rouge. Morgan City is constantly dealing with a buildup of rapidly ac-cumulating sediment in the river channel. In 2016, high flood water combined to bring even more sediment issues in that area. Manson won a contract for a demo project, and emergency dredging work that came from the flooding.
Morgan City lies at an important juncture of the east-west Gulf Intracoastal Waterway (GIWW) and the southernmost access point to the GIWW’s Port Allen Route, which joins the Mississippi River near Baton Rouge.
With both the Red River and Mississippi River experiencing record early season flows beginning around Christmas 2015 and carrying through this past spring, officials in the parishes near the mouth of the Atchafalaya were also engaged in a flood fight rarely seen in that time of year.
For the Port of Morgan City, besides dealing with high water and restricted waterways, channel depth soon became an issue as massive amounts of sediment dropped out of the water column both within the river channel and off-shore in the bar channel, which was already suffering from a buildup of “fluff,” a soupy, rapidly-accumulating silt that has stifled ship traffic at the port for close to a year.
The Port of Morgan City had set aside some of its waterway dredging budget from fiscal year 2015 to fund what port executive director Mac Wade describes as a “demo project” along the bar channel, the 10- to 12-mile offshore stretch of the port’s navigation channel, which is authorized to be 20 feet deep and 400 feet wide.
Using a cutterhead dredge—at a cost of about $100,000 a day, Wade said—the bar channel can be dredged down to its authorized depth easily enough. But immediately, the fluff begins to fill in at a rate of about a foot and a half per month. That rapid sedimentation rate caused the port to lose several import/export contracts in 2015 because of draft restrictions.
Wade’s demo project called for a hopper dredge to crisscross the bar channel. Instead of filling up the hopper then depositing the mate-rial elsewhere, the hopper dredge working the Morgan City bar channel would bypass the hop-per and merely spill the sediment overboard as it idled along—basically dredging by agitation.
“Will agitation work on the bar channel?” Wade said of the demo project. “That doesn’t cure all our other dredging issues as far as sand, but we believe agitation will work in the bar channel.”
The Corps approved the demo and released a request for proposal earlier this year. A con-tract was awarded to Manson Construction for the work. But then sediment from the early year flood settled in at mile 142 on the Atchafalaya River below Morgan City, bringing navigation at a main approach to the port to an abrupt stop.
“We had enough money in the original contract to bring [the bar channel] down to 20 feet. I was supposed to get 107 days of dredging,” Wade said. “But I had to take money off this project to move it to emergency dredging. I had to get the river open because it had been plugged because of the flood.”
Manson received the contract to remove the plug at mile 142. The cutterhead dredge Leonard J went to work there. In fact, Port of Morgan City officials received approval for de-positing silt from mile 142 nearer to the bank in an area never before used as a placement site. Now, a series of small islands peek out of the water. Within just a few weeks, vegetation was already growing on the islands and birds were nesting there.
After removing the plug at mile 142, the Leonard J moved upriver closer to the port and farther up toward Berwick Bay.
The emergency dredging, though, shaved the length of the demo project from 107 days to 67. Still, Wade was unfazed as he and Port of Morgan City officials eagerly surveyed the bar channel as the Manson hopper dredge Newport methodically worked the bar channel.
Wade divided the bar channel into four sections, from thickest to least thick. The goal was to drop the water density of 1,200 milligrams per liter down to 18 feet. Wade said 1,200 milli-grams per liter is the benchmark set by industry for safe navigation.
“We started in the worst section of the bar channel where the material was the thickest,” Wade said. “We basically went from nine and a half feet to 18 feet. Then we started the second section.”
While working the second section, the Newport would go back over the first section on every third pass. Once the first two sections were down to 18 feet, the dredge moved on to sections three and four and worked until the depth reached 18 feet there. With that accomplished, the plan for the dredge was to work all four sections at once to see if constant agitation would maintain channel depth.
“The preliminary survey results are showing that it has greatly reduced the density in the channel,” Wade said. “We’ve been able to get that 1,200 milligrams per liter down to 18 feet in the channel. It was at nine and a half feet, and we’ve been able to push it down to 18. Are we getting it all out of there? No, but that’s not the intent. It’s not depth, it’s density.”
If the port and the Corps agree that agitation is the key to maintaining channel depth in the bar channel, Wade said there are options avail-able for achieving that at a lower price than a cutterhead dredge—or even a hopper dredge.
“We’re going to look at doing a couple more demos in the very near future to try to see if we can reduce costs down more,” Wade said. “We know we have to be out there many days a month to keep [the waterway] navigable and reliable.”
Reliable access to the Port of Morgan City with a channel 20 feet deep is key for fabrication and shipbuilding operations in the area and for the port to secure import/export con-tracts like the ones the port enjoyed in 2015. If preliminary results are accurate, securing re-liable, deep, cost effective access shouldn’t be difficult, but it will require innovation.
“We have proven two things: agitation does work, and we can reduce the cost a bunch,” Wade said.Edit Module