Portions of the Mississippi River’s main ship navigation channel, including stretches between Southwest Pass and the river’s mouth and between New Orleans and Baton Rouge, would be deepened to 50 feet under a plan and environmental assessment recently released by the Army Corps of Engineers. That’s three to five feet deeper than present.
The project would allow access to the ports of Plaquemines, New Orleansand South Louisiana by the new, larger “Panamax” ocean-going vessels that were built with much more cargo capacity to take advantage of the expansion and deepening of the Panama Canal. Other U.S. ports on the Gulf, East and West coasts either already have completed similar navigation channel deepening projects or are trying to get them approved.
The increased depth will reduce the need to load some ships with less cargo weight than their size allows, or to unload cargo from ships before they enter the river’s mouth. Officials also hope that the deeper dredging will increase the intervals between required maintenance dredging.
The deepening project would have an initial construction cost of $88.9 million, of which the Louisiana Department of Transportation and Development would have to pay $22.2 million. The state’s share includes $2.5 million to acquire land and easements, move underground pipelines or other utilities and dispose of any hazardous materials found during construction.
The corps estimates that maintaining and operating the deeper channel will cost about $21.6 million a year, all would be paid by the federal government, and the net annual benefits to the U.S. economy at $96.8 million. In a study recommending deepening, the corps said it would result in a national economic benefits to cost ratio of about 5.47 to 1.
The southernmost part of the project is between river mile 14 above Head of Passes near Venice and mile 22 below Head of Passes, at the river’s mouth. The portion of the river below Head of Passes is Southwest Pass.
The navigation channel in that area would be officially deepened from the present 48 feet below its lowest low-water level — during lowest tidal levels — to 50 feet, but in reality it would be 54 feet in the beginning. That’s because the dredging also would include 2 feet of “advanced maintenance” and 2 feet of allowed “overdepth”, both aimed at increasing the time between required maintenance dredging.
The deepening project could take as long as four years to complete and produce about 18 million cubic yards of sediment. The sediment could be sent away “beneficial use” such as creating wetlands, under federal rules that require the corps to limit the amount of added cost to transport sediment away from the channel.
Wetlands could benefit
The corps expects to create 1,462 1/2 acres, or more than 2 1/4 square miles — of new wetlands in the federal Delta National Wildlife Refuge and the state Pass A Loutre Wildlife Management Area, both on the east side of the river near Head of Passes. The corps now has access to 143,264 acres in the area for disposal of material dredged from the southern part of the river to meet the present 48-foot depth, and it expects to add 24,054 acres for the new dredging project.
The corps study cites the wetland creation program as a major benefit of the project to offset its other environmental effects.
“The creation of marsh would provide an increase in fish and wildlife habitat including nesting habitat for water fowl and nursery habitat for fish,” the report said. “Consumptive recreation use would likely increase as a result of an increase in quality and quantity of fish and wildlife habitat. Bird watching opportunities are also expected to increase because of improved habitat for neo-tropical migratory songbirds.”
But dredging has its downsides. The study says increased saltwater intrusion caused by the deeper channel and by relative sea level rise will likely result in a loss of more than 833 1/2 acres of the new wetlands over 50 years, based on loss rates for the area between 1932 and 2010.
“However, it is anticipated that the proposed project would not result in overall adverse direct or secondary impacts to the aquatic environment and human environment in or near the project area,” the report said.
Louisiana could pay more money to move dredged sediment moved farther away, to areas identified by the state as more in need of wetland restoration. But there are no current plans to do so, said Bren Haase, head of the state Coastal Protection and Restoration Authority’s coastal master plan planning team.
Congress has authorized and apporopriated $100 million for beneficial use dredging, as part of the 2004 Louisiana Coastal Area Ecosystem Restoration Study. That money could be used to extend the distance of pumping the dredged material. But state officials have refused to use that money for areas in lowermost Plaquemines Parish because they think the money can be better matched with other dredging elsewhere, where the material can be put to better use in protecting inland populated areas.
The corps report said some possible environmental effects of dredging, including noise, will be avoided by requiring contractors to limit operations when some endangered species such as manatees might be in the area, or by delaying dredging or other operations in the case of nearby bird rookeries.
The lower river project must also take into account the need for the corps to build an underwater berm downriver of Plaquemines Parish water intakes during very low river periods. The berms would be designed to block salt water from entering Plaquemines’ intakes and those of upriver water systems.
The wedge of salt water in the river is present throughout the year in Southwest Pass. But it moves upriver when there’s not enough freshwater carried by the river to keep the wedge from moving north. The corps report says present research indicates there won’t be an increase in the number of times the corps must build the underwater sill, which now averages every 10 years.
The portion of the river between mile 14 above Head of Passes and the Bonnet Carre Spillway in St. Charles Parish already is deeper than 50 feet, much deeper in some parts. In the Port of New Orleans, for example, the bottom of the navigation channel can be as deep as 200 feet below the normal lowest surface of the water.
However, the Port of New Orleans has requested that access ways between wharves on the river’s east bank and the channel be deepened to 50 feet, to take advantage of the new depth at the mouth of the river. While that’s not included in the current corps plan, language approving that proposal is included in a water projects bill under final consideration in the Senate.
Kenner-Baton Rouge dredging
The second area that is part of the corps-recommended deepening project stretches from just south of the Port of Baton Rouge to Kenner. This includes the Port of South Louisiana.
The project calls for deepening three “river crossings,” the straight reaches of the river between river bends, with the sediment deposited in deeper water areas just downstream.
The crossings are:
- Fairview, between river miles 111 and 117 adjacent to Luling
- Belmont, between river miles 151 and 156 and adjacent to Oak Alley in St. James Parish
- Rich Bend, between river miles 155 and 160 near Convent in St. James Parish.
The corps estimates that about 616,600 cubic yards of sediment will be dredged from these three crossings over two years. Once the work is finished, the average annual maintenance within those crossings would increase by about 3.1 million cubic yards, again with the material disposed just downriver of each in deeper water.
State officials agree with the corps that this disposal method makes sense. That’s because the cost of either moving the material by barge downstream or by trucks to other locations to rebuild wetlands would be cost prohibitive.
In choosing its recommended plan, the corps rejected a no-action plan, which would have kept some existing depths and increased others to 48 feet. It also rejected plans that would have included another nine river crossings north of the ones in the preferred, to let deeper-draft ships to reach the Port of Baton Rouge.
The current corps proposal would be the third change in the river’s navigation channel depths since Congress in 1985 authorized the agency to deepen the channel to 55 feet between the river’s mouth and Baton Rouge in 1985. The first phase completed in December 1987 deepened the river from 40 feet to 45 feet between the Gulf of Mexico and Donaldsonville, at river mile 181. The second phase, completed in December 1994, deepened the river from 40 feet to 45 feet between Donaldsonville and Baton Rouge.
The state transportation department, as the local sponsor, limited the corps’ options for this third phase to those including the 50-foot depth. Deeper dredging was not considered by the state because it would have significantly added to the project’s cost, and for other considerations. But it could be reconsidered in the future.
Public review, input
A copy of the full report, including 10 appendices, is available at the corps’ web site.
The public has through Jan. 17 to comment on the plan by:
- Calling 504.862.2517
- Emailing MRSCdmin@usace.army.mil
- Mailing U.S. Army Corps of Engineers, CEMVN-PD, Attn: Steve Roberts, 7400 Leake Ave., New Orleans 70118.
Questions about the project may be submitted to Steve Roberts of the corps’ environmental compliance branch by e-mail at email@example.com or by telephone at 504.862.2517 or facsimile at 504.862.2088.
A decision by the corps’ New Orleans District commander on the plan is expected by March, followed by the release of a feasiblity design for the plan in September and a “director’s report” by the chief of the corps. The director’s report is to be submitted to Congress in March 2018.
A public hearing on the dredging plan is scheduled Wednesday (Dec. 14) 10 a.m. to noon in the corps’ District Assembly Room, 7400 Leake Ave., New Orleans.