Dredging heart of port meeting
By ROBERT R. JONES III
The Morgan City Harbor and Terminal District continues to search for effective methods to keep the Atchafalaya Bar Channel clear of sediment to reduce the need of expensive cutter head dredging.
Historically, the Port of Morgan City tried to dredge to keep the channel clear over the years with maintenance dredging programs. But the process to remove sediment takes three to six months across the channel and ship fathometers that give the bottom depth of the channel to captains begin to pick up sediment again in about two weeks, though it is not consolidated (solid).
After six to eight weeks, the channel filling with sediment becomes a navigational issue, according to Water Resources Engineer Jonathan Hird of Baton Rouge-based Moffatt & Nichol.
At a district meeting Tuesday, Hird presented to the board on-going efforts between his firm, the U.S. Army Corps of Engineers and the Port of Morgan City to examine new methods to keep the channel clear of “fluff,” or light sediment, which settles to the bottom and eventually becomes mud.
He said there is no way to prevent the sediment from forming in the channel and no way to prevent the eventual need of traditional cutter head dredging, but there are several options to manage fluff and reduce the need for mechanical dredging.
“There is no silver bullet to keep silt off the bottom of channel, so we are trying to develop a methodology to manage it,” Hird said. “The port is the crossroads of the Atchafalaya River with strategic importance as an inland-protected port, and the state is focusing on managing the problem.”
There are two approaches to solving the problem — exploring three ways to keep sediment on the bottom suspended in the water column and the use of new fathometers to allow captains to see the true navigable depth of the channel.
Hird said currently, ships sound the bottom for depth with fathometers, read the top of the fluff and see that as the bottom of the channel, but the navigable depth may be several feet lower down. The readings give a false bottom, but new equipment would allow captains to see the true navigable depth.
“We are looking at the adoption of dual-frequency fathometers that could see both the top and bottom of the suspended fluff,” he said. “With these, ships can see they have navigability even if the top reading is 14 or 15 feet to maintain navigable 20-foot depth.”
But more importantly, the Port of Morgan City is exploring three ways to keep the channel bottom clearer for a longer period of time between cutter head dredging.
The district and Hird are planning to use a piece of equipment that could be quickly deployed to “knock the top off the shoal on a routine schedule.”
The first option explored was agitation dredging, which entailed dragging a huge I-beam across the bottom of a channel to agitate fluff and suspend it in the water column of the Atchafalaya Bar Channel.
Hird said it was the simplest, least expensive option, but did not work effectively due to a lack of downhill current in the waterway.
Now, the port is looking into water injection or side-cast dredging.
Water injection dredging involves using water or air pumped from a vessel down into the fluff to reduce density and suspend the material into the water. This method is expected to cost about $18,000 a day.
The final option is side-cast dredging, where fluff is pumped off the bottom of the center of the channel and sprayed to the sides of the channel. This option will cost about $30,000 a day.
But both are considerably cheaper than traditional cutter head dredging, which costs about $100,000 a day and are expected to be tested in spring 2013 when wind and tidal conditions are right to optimize success.
“If captains call concerned about depth, the (water injection or side-cast) equipment, which will be kept at the port, can be sent out quickly to remove the shoal (fluff),” Hird said. “This would be a supplemental tool to use between the dredging cycles every four to six weeks to maintain navigation through the channel.
“This won’t replace dredging cycles, but it will improve efficiency on the annual maintenance schedule and ships coming into the port will also know if their fathometers say 15 feet, they will know the last five feet are fluid mud and they can sail through it.”