Long distance pipeline would carry sediment from Atchafalaya River to rebuild marshes
Terrebonne Parish is ready to pitch a billion-dollar sediment pipeline concept for inclusion in the state’s coastal restoration plans.
According to a recent report in the Houma Daily Courier, parish officials see the project as an alternative to a proposed sediment diversion from the Atchafalaya River into western Terrebonne as the state develops the next iteration of the coastal masterplan.
These days Dularge, Chauvin, Montegut and Pointe-aux-Chenes cling to their bayou ridges as the basins in between have largely faded into open water, a welcoming conduit for storm surge.
Transporting sediment across the long distance between ideal sources in the Mississippi and Atchafalaya rivers to these marshes is a particularly expensive challenge. But it’s one that must be met forthe future habitation of Houma and the longterm viability of billions of dollars in planned levee construction across southern Terrebonne, parish officials contend.
The concept mimics in some ways a pipeline being built by the state to carry sediment from the Mississippi west into the Barataria basin but eclipses it in scale in both miles of pipe and cost.
Two coastal engineering firms recently explored potential sources of marsh building dirt in the Atchafalaya River and the feasibility of piping that sediment first into fading marsh west of Dularge and eventually east of Lake Boudreaux.
“The pipeline does seem to be constructible. There are challenges: right of way issues, crossing other pipelines and of course cost. But we don’t think that any of those issues are deal breakers,” said Terrebonne Parish Coastal Restoration Director Nic Matherne.
The concept calls for dredges working night and day to pull sand from the bottom of the Atchafalaya south of Morgan City. That sand would be flung through a 30- to 36-inch pipe for up to 50 miles across westernTerrebonne into the parish’s problem points outside planned Morganza-to-the-Gulf levee footprint.
In the most robust iterations of the idea, the pipeline would pump some 5 million cubic yards of sand annually for 10 years, building some 30,000 acres of marsh at four target locations.
The price tag for building 7,000 to 8,000 acres at each of the four sites is estimated to top $3 billion, though Matherne stresses the parish would more realistically hope for the state to build downsized versions of the concept, which could cost less than $1 billion.
Engineers hired by the parish centered in on Crew Boat Cut in the Atchafalaya as a possible source of sediment as the dirt has the right weight for building marsh and is projected be replenished at a rate quick enough to mine the area for the project lifespan, said Maarten Kluijver of Baton Rouge firm Moffatt & Nichol.
The area is also near other U.S. Army Corps of Engineers dredging for navigation purposes, the materials of which could hypothetically also find its way into the pipeline, Kluijver said.
Matherne said the parish has an advantage in cooperation from pipeline company Kinder Morgan, which has leant its support for running the pipeline along the right of way occupied by its Tennessee Gas pipeline, a 24-inch, high-pressure natural gas pipeline that travels close to the sediment sources and runs through southern Terrebonne.
Marshes near Raccourci Bay west of lower Dularge is the first hypothetical target in the parish’s preliminary pitch. Matherne said this area has had several other federally funded, smaller restoration projects, so a large marsh creation project there would help solidify those gains and provide protection to the western edge of Bayou Dularge.
The next area would be marshes just south of Falgout Canal, which have faded into some 5,000 acres of open water between the lower Dularge levees and the Houma Navigation Canal. That open water will eventually be lapping against the southern reaches of the Morganza-to-the-Gulf Levee. It’s also pressed against lower Dularge, the most densely populated area of Terrebonne left out of the Morganza system.
The cost of running the pipeline to these two points is estimated at $800 million, which is pricey compared to smaller projects. Matherne argued the marsh created would cost between $100,000 and $125,000 an acre, which is less expensive per acre than other projects included in the state’s coastal masterplan.
But this leaves out the two other points that are the rim of Terrebonne Bay between Bayou Terrebonne and Bayou Point-aux-Chenes and further north in Wonder Lake near Isle de Jean Charles.
“Between the barrier islands and the rim of Terrebonne Bay you have 12 to 15 miles of open water in some spots. So rebuilding our barrier islands is important. But 12 to 15 miles is a lot of open water for surge to kick back up. So we need the second line of defense on the north rim of Terrebonne Bay,” Matherne said.
Running the pipe to these marshes would cost more than $3 billion, several times more expensive than any marsh creation concept in the region.
The high price tag is indicative of the difficulty of building marsh to protect these populated areas.
“With limited funds we may feel it is more important to possibly construct a portion of the Bay Raccourci project and a portion of the Falgout Canal project,” Matherne said. “Realistically maybe this feasibility study shows us the most intelligent way to rebuild eastern Terrebonne is to utilize (sediment from) Terrebonne Bay. It is not the most ideal situation, but we also don’t have unlimited funding.”
While significantly less expensive, pulling sediment from inside the system also has its drawbacks and has previously been frowned upon by the state restoration money gatekeepers.
“One engineer describes only taking sediment from inside the same system is just rearranging the deck chairs on the Titanic. You are still sinking,” Matherne said.
He noted the state has indicated some willingness to explore using more local material for marsh creation in such extreme circumstances.
The state is accepting project concepts to be considered in the next version of the masterplan for 2017 and beyond.
EDITOR’S NOTE: Houma Daily Courier Staff Writer Xerxes Wilson filed this report.
