effect of river diversions on the ports of the lower mississippi river

8/13/2019 Effect of River Diversions on the Ports of the Lower Mississippi River

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EFFECT OF RIVER DIVERSIONS ON THE

PORTS OF THE LOWER MISSISSIPPI

An evaluation of the historical relationship between natural and artificial

diversions of the Mississippi River and shoaling of the river channel

Channel Shoaling


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Although New Orleans was founded in 1718, the city did not begin its role as a significant port for

international commerce until 1880. The prior year Captain James Eads had successfully constructed

jetties at the mouth of Southwest Pass that breached the shoaling mouth bars and allowed ships to

enter the river directly from the Gulf of Mexico year round. John Barry documented the success of the

jetties in his novel :

In 1875, when Eads began work on the jetties, 6,857 tons of goods were shipped from St. Louis through New

Orleans to Europe. In 1880, the year after he finished, 453,681 tons were shipped by the same route. New

Orleans rose from the ninth-largest port in the United States to the second-largest, trailing only New York. (In

1995, by volume of cargo greater New Orleans ranked as the worlds largest port.) Rising Tide, 1997

In 1880 the newly formed Mississippi River Commission submitted a report to the congress that gave an

assessment of the river, and its vital role in transportation. One of the principal issues addressed in

this report was shoaling of the river channel and its effect on navigation. The report drew a very clear

relationship between openings in the banks of the river (either as natural crevasses or manmade

diversions) and shoaling in the channel:

"Shoals are found in the river immediately below crevasses, which is difficult to refer to any other cause that theloss of current velocity which takes place below the crevasse. As a portion of the volume of the river is drawn off

by the crevasse when it is first made, it is impossible that the current below the crevasse can then be as rapid as it

was before the occurrence. Being less rapid, it unable to sustain the whole quantity of matter held in suspension

by the more rapid current above the outlet, and consequently its surplus sediment falls to the bottom below the

crevasse." Mississippi River Commission Preliminary Report, 1880

The same report specifically addressed a proposal to create a diversion of the river that would flow into

Lake Borgne. The intention of the proposed diversion was to provide an outlet for flood waters to

lessen the effects of flooding in the main river channel. The commission recommended against the

diversion of the basis that it, like other diversions of the river, would create shoaling in the river that

would obstruct navigation:

"Should an outlet be made to connect the river with Lake Borgne, results similar to those which have occurred at

Cubitt's Gap and The Jump must be confidently anticipated. Below Cubitt's Gap it is an undisputed fact that the

former depth of the river has been largely reduced since the gap occurred. Below The Jump it is not so definitely

known what the shoaling has been, as charts of previous soundings are not now available in any exist, but the

Light-House Board has placed two buoys a short distance below The Jump, to warn vessels away from the shoals

that exist there. It is not possible to make the proposed outlet into Lake Borgne without creating a shoal in the

river below it "Mississippi River Commission Preliminary Report, 1880

It is apparent that shoaling around the natural diversions of the river remained and issue throughout the

nineteenth and twentieth century. This Notice to Mariners was published by the Hydrographic Office

under the authority of the Secretary of the Navy:

A recent surveyof the lower Mississippi River made by the U.S. Army Engineers shows that a shoal on which the

steamer J.I. Luckenbach grounded on December 18, 1907, has entirely disappeared, and that there is now a depth

of 8 fathoms at mean low water at that point. The removal of this shoal was probably largely due to the effect of

sills placed at Cubits Gap and Pass a Loutre to divert the flow of water into Southwest Pass The Master, Mate

and Pilot, June 1908

Measurements of river flow by the Corps of Engineers in the spring of 1989 showed that the percentage

of the rivers flow entering Cubits Gap had increased from 10% to 20% of the total flow of the river that


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passed Venice just upstream. In the years prior to this measurement a crevasse in the banks of the

river just inside the Gap had formed, creating a natural diversion called Brandts Pass. It is clear that

the increase in the portion of the rivers flow entering Cubits Gap is directly related to the formation of

the Brandts Pass Crevasse, which allowedriver water to flow into the adjacent marsh. The Brandts

Pass Crevasse Splay is the deposit of sediments that resulted from this natural diversion of the river, and

is often pointed to as an example of the ability of sediment diversions to build new land. A 2010 study

of the crevasse by the U.S. Geological Survey National Wetlands Research Center found that about 1200

acres of new marsh were created by the splay. The Corps of Engineers reported in 1990 that shoaling

of the main channel of the Mississippi River just downstream of Cubits Gap had occurred during the

time that the Brandts Pass crevasse was open, and that this shoaling had resulted in the grounding of

the Soviet vessel the Marshal Koniev, severely disrupting navigationfor a period of weeks. The Corps

report recommended a dredging program to alleviate the effects of the shoaling.

In the same year that the Corps released this report the passage of the Coastal Wetlands Planning,

Protection and Restoration Act (CWPPRA) provided federal funding for projects that were designed to

restore the marshes of south Louisiana. Principal among those projects were freshwater diversions

that were designed to reconnect the river to the marsh. The stated objectives of projects such as the

Caenarvon and the Davis Pond Diversions were to divert freshwater were to provide freshwater andnutrients to the marsh to counter the effects of saltwater intrusion and to restore marsh growth.

Twenty four years later there are many that question the effectiveness of these diversions to maintain

the health of the marsh and to offset the rate of wetlands loss in south Louisiana.

All of the original river diversions constructed under the CWPPRA were freshwater diversions, meaning

that they only drew from the very top layer of the rivers water. The upper layers of the river water

carry only a minor portion of its sediment load in suspension. It was soon realized that in order to

offset the rate of wetlands loss deeper cut diversions that mimicked the natural crevasse splay

diversions would be necessary to create new emergent marsh. In 2003 the first of these, the West Bay

Sediment Diversion, was opened with a deep cut into the west bank of the river just upstream of Head

of Passes, and across the river from Cubits Gap. The West Bay Diversion produced many unexpectedresults in the early years of its operation. Instead of building new land, the current of the river leaving

the diversion actually eroded a portion of West Bay. There was no emergent marsh created in the first

few years of operation, and in 2010 a Corps of Engineers report found that the diversion was

responsible for shoaling of Pilottown Anchorage Area just downstream of the opening in the bank. On

September 8, 2009 the Coast Guard had reported that the oil tanker Eagle Tucson ran aground at mile

marker 3 on the Lower Mississippi, near Pilottown. Just like the effects of the natural crevasses at

Cubits Gap, Brandt Pass and The Jump, as reported in previous years by the Corps, the alteration of the

rivers current by the West Bay Diversion had resulted in shoaling downstream. The Diversion was

ordered closed in 2010 by a federal judge, and shortly thereafter an agreement was reached to allow it

to reopen if the Louisiana Coastal Restoration and Protect Authority (LCRPA), which manages it, paid for

the dredging of the Pilottown Anchorage.

The West Bay Sediment Diversion is still in operation. The LCRPA dumped a portion of the dredge

spoils in the mouth of the diversion to try to reduce the erosional effects of the current, and there has

since been the creation of a minor amount of emergent marsh. After thirteen years and over $50

million in costs the Diversion, which was to survey as a demonstration project for future sediment

diversions, can only be characterized as a failure. The area around the opening of the diversion,

however remains a challenge to river navigation. The Time-Picayune reported that on June 13, 2011

an 800-foot vessel, the Ratna Puja, which was exporting carbon black oil ran aground just above Cubits


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Gap. The tanker had a 42-foot draft. On June 12, 2013 750-foot bulk carrier Arouzu loaded with pig

iron grounded above Head of Passes.

Despite the failure of the West Bay project, sediment diversions are still considered a principal

component of the LCRPAs Master Plan for coastal restoration. The 2012 Master Plan includes

proposals for eight sediment diversions on the Lower Mississippi River. The first of those being moved

toward construction is the Mid-Barataria Sediment Diversion at Myrtle Grove. The just-released 2015

LCRPA annual budget provides for $22 million on planning the Mid-Barataria Diversion. Like the West

Bay Diversion, this project will be a deep cut into the west bank of the river that will divert a portion of

the rivers flow into the Barataria Basin. A 2012 study of the proposed diversion by the Water Institute

of the Gulf examined the likely outcomes of several different possible configurations of the diversion by

mathematical modeling. The critical criteria of the various configurations evaluated by the model was

the sediment to water ratio (SWR), or how much sediment was being diverted with the water that

moved through the opening in the bank. Scientists at the Water Institute determined that an SWR

greater than 1.0 was required to minimize shoaling in the river channel. Four of the five potential

configurations for the Mid-Barataria Sediment Diversion examined by this study yielded SWR values less

than 1.0, meaning that shoaling would be a concern. The LCRPA has obviously selected the one

configuration with a SWR greater than 1.0 for the next stage of planning, but mathematical models arerarely 100% accurate. What this study shows is that there is a considerable concern about the

potential for shoaling in the main channel of the Mississippi River due to the construction of the

diversion, and that only in the most perfect conditions can it be modeled not to induce shoaling.

The Mississippi River has been vital component to the economic growth of the United States throughout

its history. The combination of ports along its banks represents the largest port complex in the world.

Access of ships to these ports is the most essential component of their function, and shoaling of the

river is the most significant impediment to that access. Since the inception of ship traffic on the river,

there has been a very clear relationship between openings in the river banks as natural or artificial

diversions and the formation of shoals in the river channel. The West Bay Sediment Diversion, which

was built as an evaluation and demonstration project for potential future sediment diversions is only thelatest example of this relationship. The modeling of the proposed Mid-Barataria Sediment Diversion

also points to a serious concern about its potential to cause shoaling in the river channel. This concern

and the failure of the West Bay Project to achieve its goal to create new marsh to offset wetlands loss

bring into serious question the viability of these projects as components of the LCRPA Master Plan.

The future success of the ports of the Lower Mississippi River it likely to be threatened by the

construction of diversions of the Mississippi River.

by: Chris McLindon, January 2014

[email protected]

References

Cahoon, D.R., et al, Sediment infilling and wetland formation dynamics in an active crevasse splay of the

Mississippi River Delta, Geomorphology, 2011, v.131, 57-68

Hydrologic Office under Secretary of the Navy, 1908, Notice to Mariners, No. 20

Kolker, A.S., et.al., 2012, Depositional dynamics in a river diversion receiving basin: The case of the West
mailto:[email protected]:[email protected]:[email protected]


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Bay Mississippi River Diversion, Estuarine, Coastal and Shelf Science, v. 106, p. 1 -12

Little, C.D., 2010, Mississippi River Geomorphology & West Bay Diversion, 2nd Joint Federal Interagency

Conference, 12 p.

Meselhe, et.al., 2012, Numerical modeling of hydrodynamics and sediment transport in lower

Mississippi at a proposed delta building diversion, Journal of Hydrology, v,472-473, p. 340-354

The Mississippi River Commission Preliminary Report, 1880

U.S. Army C.O.E., Technical Report HL-90-20, 1990, Dredging Alternatives Study Cubits Gas, Lower

Mississippi River, V1 35 p

effect of river diversions on the ports of the lower mississippi river

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