SEDIMENT DIVERSIONS

The Mississippi River is the eighth largest river system in the world by discharge and drains over 40% of the contiguous US. The formation of the Mississippi River Delta began approximately 7,500 years ago at the end of the last glacial period, when sea level rise slowed enough to allow sediments to accumulate at a faster rate than sea level rise. All major current deltas around the world were formed during this era of Holocene sea level rise deceleration, but not all delta formation and maintenance is driven by the same forces.

 

Deltas can be river-, wave-, or tide-dominated. Due to the high river discharge and small tidal range, the Mississippi Delta is a river-dominated delta, producing the elongated shape resembling fingers of levees extending seaward. The large drainage area and high discharge supplies the northern Gulf of Mexico with the seventh highest sediment load in the world. As the Mississippi River fills with sediment, it tries to find an easier route to the Gulf. This has led to the river switching course roughly every 600 to 1,000 years since its formation.

 

Over the course of its history, the Mississippi has had six Holocene Delta complexes, excluding the current Atchafalaya and Wax Lake Deltas that take roughly 30% of the Mississippi River flow. Each delta complex is in various stages of compaction, subsidence, and building; natural deltas are dynamic systems. The delta switching, combined with the high sediment load, has resulted in the current wetland rich Louisiana coast, with 30,000 sq. km (11,500 sq. mi) of delta plain and 41% of the coastal wetland area of the United States.

A schematic of the basic principle of a sediment diversion. Source: CPRA
NASA Satellite Imagery of the Atchafalaya and Wax Lake Deltas, October 1, 1984 (left) and November 6, 2014 (right).

Before human intervention, the Mississippi River delivered large volumes of sediment and freshwater to Coastal Louisiana, carried from areas as far north as Canada. The Mississippi River Delta was built through the process of annual flooding as the river and its distributaries would overflow their banks and deposit sediment. For decades, scientists and engineers have suggested constructing sediment diversions to reconnect the Mississippi River to the wetlands and marshes it was once connected to before the leveeing of the Mississippi River. They have been included in innumerable coastal plans, beginning in 1984, and have been studied by the U.S. Army Corps of Engineers and the State of Louisiana.[1] Following the Deepwater Horizon spill in 2010, funds were dispersed through the National Fish and Wildlife Foundation (through the Gulf Environmental Benefit Fund), the Natural Resource Damage Assessment, and the RESTORE Act to put towards restoration and compensation for damages caused across the Gulf. These entities specified priorities for funding, including sediment and freshwater diversions.[2]

 

The goal of a sediment diversion is to move large amounts of sediment from the river into the surrounding wetlands by maximizing the volume of sediment transported by the least amount of water. The size of a diversion is generally referred to by the volume of water in cubic feet that flows through the diversion structure in a second. This measure is represented by the term cubic feet per second (cfs). As reference, the Mississippi River flows at 1,200,000 cfs during flood stage in New Orleans and the Atchafalaya averages a flow rate of 225,000 cfs. Current freshwater diversions, such as Caernarvon, flow at generally less than 10,000 cfs.

 

Conceptually speaking, the basic design of a sediment diversion includes an inlet structure (in the Mississippi River) where sediment and water can enter the structure, a conveyance structure made up of a series of gates, and an outlet structure which discharges the sediment and water into receiving wetlands.[3] The design also includes an interior drainage pump station and highway and railroad modifications. The final design of the two most prominent sediment diversions, the Mid-Barataria and Mid-Breton Diversions, has not yet been released yet.

 

The Mid-Barataria Sediment Diversion is currently being planned for a location at Mississippi River mile marker 60.7 on the west side of the river, near Myrtle Grove. The State is still considering how and when the diversion will be opened, but most modeling has been conducted using a maximum flow of 75,000 cfs when the river is at 1,250,000 cfs. This will likely be adjusted as other considerations such as flooding time for plants and fisheries, particularly oysters and shrimp, are taken into account. The Mid-Barataria Sediment Diversion is in the engineering and design phase with an estimated construction start date of 2020 and completion in 2025. In the near term (20 years), it is estimated to create 8,041 acres of land and in the long term (50 years), it is estimated to create 29,686 acres of land.

Conceptual design of a sediment diversion. Source: CPRA

The Mid-Breton Sediment Diversion is currently being planned for a location north of the Mid- Barataria Sediment Diversion near Wills Point on the east side of the river. The plan is to operate at 35,000 cfs when the river is above 1,000,000 cfs. This diversion is in the engineering and design phase. In the near term (20 years), it is estimated to create 5,066 acres of land and in the long term (50 years), it is estimated to create 15,831 acres of land.

 

Combined, these two diversions are predicted to have a net land gain of 45,517 acres over 50 years compared to a future without action. This land will grow gradually as sediment accumulates in the wetlands and open waters of the basins. During the first few years, there is expected land loss in the outfall areas as the landscape adjusts to higher volumes and flows of water. Ideally, the diversions will be opened over an extended period of time (5-10 years) to help create channels that can handle the excess water, reduce flooding risks, and allow plants, fish, and wildlife to adjust to new conditions. As the areas fill in with sediment, delta-like land will begin to form in each basin. The goal, once the diversions are built, is sustainable land growth for years to come as the wetlands of the deltaic plain are reconnected with the river.

 

The estimated project cost of the Mid-Barataria Sediment Diversion is $998 million and the estimated project cost of the Mid-Barataria Sediment Diversion is $479 million. The Louisiana Coastal Protection and Restoration Authority (CPRA) is the lead state agency who will be constructing these projects. The U.S. Army Corps of Engineers, New Orleans District will complete the Environmental Impact Statements (EIS) for these projects. Other federal and state agencies are involved in various capacities. A draft EIS is expected to be released in 2019 for the Mid-Barataria Sediment Diversion. CPRA would like to break ground on this project in 2020. The draft EIS for the Mid-Breton Sediment Diversion is expected in 2020 with construction beginning in 2022.

 

 

 

[1]      Louisiana Coastal Area: Feasibility Report on Freshwater Diversion to Barataria and Breton Sound Basins (Rep.). (1984). Retrieved       

       https://biotech.law.lsu.edu/blog/EIS-LCA_Fresh-Div-Barat-and-Breton-Sound-Basin_Feasibility-Study_Volume-1.pdf

[2]    http://www.nfwf.org/gulf/Pages/fundingpriorities.aspx

[3]       http://coastal.la.gov/wp-content/uploads/2018/03/State-of-the-Program_MBSD20180528.pdf

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