Louisiana Coastal Protection & Restoration Authority Hosts Webinar On Sediment Diversions In Mississippi Delta
Officials from Louisiana’s Coastal Protection & Restoration Authority (CPRA), the agency tasked with overseeing efforts to breathe new life into the state’s vanishing coastline, hosted a webinar October 21 focused on sediment diversion projects along the Mississippi River delta. The webinar is part of a series by CPRA focusing on “engineering with nature,” or aligning natural processes and engineering to reconnect the Mississippi River to its delta.
The presentation focused on several small crevasses and one large sediment diversion along the river. A crevasse is defined as a cut in either a natural or artificial levee.
“Crevasses are a natural process,” said the CPRA’s Bryan Gossman, who hosted the webinar. “They’re part of the delta cycle.”
Gossman displayed an aerial view of a natural crevasse that first occurred in 1978. By 2008, sediment traveling through the crevasse had built a remarkable amount of new land. He then displayed an artificial crevasse along the Mississippi River.
“The only difference is, instead of waiting for the river to break through its banks, we help the process along by digging a crevasse channel,” said Gossman, who later added, “The river’s doing the work. Engineering principles are applied to make the process more efficient to create better results.”
The benefits are twofold, Gossman said.
“We talk a lot about building land, and building land is very important,” Gossman said. “But we’re not just building land. We’re building habitat, enhancing habitat for fish and wildlife.”
Gossman first outlined a crevasse project initiated in 1993, called MR-01 or the Small Sediment Diversions project. The project consisted of eight new and five redredged crevasses in the Birdfoot Delta near the mouth of the Mississippi River, with water and sediment flowing into both the Delta National Wildlife Refuge and the Pass-A-Loutre Wildlife Management Area. Gossman said the project was only monitored for three years. At the end of the three years, though, the project had created 237 acres of new land.
A second artificial crevasse project, called MR-06 or the Channel Armor Gap Crevasse, was built near the mouth of the river at Main Pass in 1997. A previous project at that site involved a canal connecting the river to a bayou. However, because the flow rate was still so low, the state and the U.S. Army Corps of Engineers built a larger canal that went straight from the river to a pond to the east.
For this project, the team monitored it for 20 years, measuring flow, sediment and vegetation and conducting elevation surveys and land-water analyses. At the beginning of the project monitoring, water in the receiving bay was as deep as 3 to 4 feet. Roughly 20 years later, the mean receiving bay elevation had increased to a foot above the water line.
“By the end, we’ve got some spots in the receiving bay that are as high as 2 or 2-1/2 feet in elevation,” Gossman said.
Aerial views from 1998 and 2019 reveal marked growth in vegetation, both dense roseau cane and semi-aquatic, transitional vegetation.
“This is land that’s been hit by storms,” Gossman said. “It’s resilient. It’s still there.”
In all, about 360 acres were created in the area over the course of the monitoring period. The cost estimate ranged from $2,444 per acre to $4,888 per acre, Gossman said, which is far below the estimated $50,000-per-acre cost of building marsh with dredged material.
“When you’re in a situation where you can let the river move the sediment for you, there is a significant cost savings,” he said.
Lastly, Gossman overviewed the MR-09 or Delta Wide Crevasses project, a phased crevasse construction project initiated in 1999. Subsequent phases were built in 2005 and 2014, with a final phase to come next year.
“In each one of these phases, we’ve got new crevasses being created and existing crevasses being maintenance-dredged to keep the channels open,” Gossman said. “Total cost for phases one through four is $4.7 million.”
Crevasses in this project are scattered across the Birdfoot Delta, with one at Baptiste Collette.
Like in the other projects, MR-09 is measured over time for variables like elevation change and vegetation. Gossman displayed data from one of the crevasses, which happens to be near a station that’s part of Louisiana’s Coastwide Reference Monitoring System (CRMS). That CRMS station found an annual positive elevation change of 6.24 centimeters per year. Over 10 years, that translates to about 2 feet in elevation, Gossman said.
Gossman compared that growth at CRMS station 2627 to the average elevation change recorded across all CRMS sites (0.59 centimeters per year) and at CRMS stations on the Mississippi River (2.39 centimeters per year).
“Out of all of those 400 CRMS sites, our CRMS site right here has the highest rate of elevation gain in the whole system,” he said. “And that’s located within the developing splay of our constructed crevasses.”
For another crevasse in the project, CPRA identified a positive land area change of about 164 acres, or a growth rate of almost 11 acres per year between 2001 and 2016.
Across the entire MR-09 project, CPRA tallied 739 new acres from the crevasses. Cost per acre thus far is about $4,700. Gossman said an extension request is in place to extend the project through 2045, which would raise the project cost to $8.2 million.
Following Gossman’s discussion of the four crevasse projects, Theryn Henkel, coastal resources scientist with CPRA, looked at a much larger crevasse on the Mississippi River at West Bay. The project was first approved in 1992 and constructed in 2003 at a cost of $3.8 million. The West Bay sediment diversion is a partnership between CPRA and the Corps of Engineers. The initially planned flow rate of Mississippi River water into West Bay was 20,000 cubic feet per second (cfs).
For its first seven years, little to no benefit was measured from West Bay, such that the Corps of Engineers considered scrapping the project altogether. Around 2010, with support and input from local leaders from Plaquemines Parish, the Corps began using dredged material from the Mississippi River to build a series of Sediment Retention Enhancement Devices (SREDs) in the bay.
“These help slow down the flow and enhance sediment deposition,” Henkel said.
Aerial photography of the area suggests land building really took off in the years after the Corps built the SREDs, with land growth between 2013 and 2020 pronounced. Henkel also displayed cut-fill and depth analyses, with the area presenting a net gain of 26 million cubic yards of material between 2011 and 2020.
“I like to put things in Superdomes, a local unit,” Henkel said, referring to the home stadium of the New Orleans Saints. “That’s enough to fill the Superdome more than five and a half times, so it’s a huge volume of sediment that’s been deposited in this area.”
Henkel noted that’s a combination of sediment naturally deposited and beneficially used from the Corps’ maintenance dredging program.
Henkel noted the fluctuating flow rate through the diversion and varied depths of the main channel from the river. That channel, she said, was at its deepest in 2011 and 2015, likely due to high-water seasons, but has not shallowed and migrated a bit. Overall, Henkel said, West Bay, while still adding new land and vegetation, has not been as successful as originally predicted.
“West Bay is a very dynamic environment, and that delta splay organization is still going on,” she said.
As with MR-09, CRPS is suggesting the project life for West Bay be extended another 20 years to 2043.
Surveying the four projects in question, Henkel said crevasses in general prove to be a long-term, cost-effective way for building land.
“Vegetation colonizes really quickly and effectively once you get land above water,” she said.
Moving forward, Henkel argued that the small crevasses—beginning around 2,500 cfs.—CPRA and the Corps has already built can serve as “analogs” for the much larger sediment diversions—around 75,000 cfs.—that are part of the state of Louisiana’s coastal master plan.
“The data we’re taking to talk about elevation change over time, all that data can be used to help predict the performance of sediment diversions,” Henkel said.
The engineering for the two large diversions, one on the east bank and one on the west, will also seek to optimize location, angle of the channel and flow rate to capture more sediment.
CPRA’s Wednesday webinars, which span topics like barrier islands, the CRMS system, the process of formulating the state’s coastal master plan and more, are available online at coastal.la.gov/webinar-wednesdays.