Louisiana Coastal Area 6 Project Profile: Convey Atchafalaya River Water to Northern Terrebonne Marshes

By Angelina Freeman (Environmental Defense Fund), Natalie Snider (Coalition to Restore Coastal Louisiana) and John Lopez (Lake Pontchartrain Basin Foundation)

As we discussed in a previous post, four of our organizations recently reviewed and provided comments on the six near-term Louisiana Coastal Area 6 project reports and final environmental impact statements.

In contrast to the robustness of the Medium Diversion at White Ditch project, the narrowing of ambition in the design of the Convey Atchafalaya River Water to Northern Terrebonne Marshes/Multipurpose Operation of the Houma Navigation Lock is striking and suggests that as currently configured, it may not achieve the objectives of the project.

As stated in the Draft Feasibility Report:

“The purpose of the project is to reverse the current trend of marsh degradation in the project area resulting from subsidence, erosion, saltwater intrusion, and lack of sediment and nutrient deposition.  The project proposes to accomplish this by utilizing fresh water, sediments, and nutrients from the Atchafalaya River and the Gulf Intracoastal Waterway (GIWW).”

The tentatively selected plan (TSP) will reduce land loss rates by a mere 10 percent over the 50-year project period, and this benefit will be lost if there is intermediate or high relative sea level rise.  Additionally, none of the alternatives would prevent marsh collapse at the high relative sea level rise (RSLR) rate.

Although none of the alternatives meet the full objectives of the project, there are some benefits to be realized from this project.  The northern and central Terrebonne Basin is in dire need of additional fresh water and sediment inputs.  But the TSP does not make any change to the Avoca Island levee, which is one of the root causes of land loss in this area. 

The groups therefore urge further consideration of a gate diversion structure in a new Avoca Island levee opening; a structure that would return the hydrology of this part of the coast more to the distribution of flows that existed prior to construction of the levee.  Only a design that taps into the head of the Atchafalaya River above Morgan City will result in significant expansion to the east of the area currently benefited.