As we have continued our tour of the Mermentau Watershed, there are common themes which have arisen. The communities of Lake Arthur and Grand Chenier, LA are perhaps the most vulnerable in the lower watershed region, as their geographic locations place them directly in, or hydraulically connected to, the lower Mermentau Basin (which includes White Lake and Grand Lake). There is a belief, right or wrong, that the US Army Corps of Engineer’s operation of the Gulf Intercoastal Waterway (GIWW) has raised the flood threat to the Lower Mermentau Basin, and Lake Arthur in particular. Upstream stakeholders in places such as Acadia & Evangeline Parish, and specifically Church Point have voiced support of providing relief in the Lower Mermentau, as they understand that once downstream relief is provided, those improvements can continue upstream. Everyone we have visited with agrees that relief must start downstream. While adding upsatream capacity must also be part of any flood relief solution, In this update we will briefly discuss history of the lower Basin and potential improvements.
Lower Basin Historic Hydrology
As the locals there know, and others such as myself have come to learn, the current infrastructure surrounding the Lower Mermentau Basin have cutoff the natural flood outfall through the coastal marshes south of Lake Arthur. Historically, during large flood events, stormwater from the Mermentau River would overflow its banks, pass through the White/Grand Lakes region, then sheet flow across the marshes and pass through the smaller bayous between Grand Chenier and Pecan Island, LA out to the Gulf of Mexico. This hydrologic regime no longer exists today.
The Coming of Commerce
The first major commerce-driven infrastructure change in the Lower Basin was the dredging of the Old Intracoastal Waterway (now Gulf Intracoastal Waterway, or GIWW) in 1912 which connected Franklin, LA to the Mermentau River. Construction of this leg of the GIWW anticipated freight traffic related to rice and sugar production, oil, lumber and salt mining (Alperin, January 1983). LA Highway 82 was constructed over decades, and the final portion which connected Pecan Island in Vermilion Parish and Grand Chenier in Cameron Parish, was completed in 1958. These coastal, inter-tidal marshes south of LA 82 were then starved of the necessary, intermittent freshwater supply crucial to their survival, while the marshes north of LA 82 was then converted into a mostly freshwater basin. This freshwater basin is protected from saltwater by a series of levees. Rain events in the upland watershed are passed through the lower basin via lock structures on the Mermentau River (Catfish Locks) and GIWW (Black Bayou, Schooner Bayou & Leland Bowman Locks). The inherent flaw in this drainage concept is that the operation of these locks is subjected to the coastal tides. Any knowledgeable Lake Arthur, LA resident will tell you they have heard too many times that the Catfish Locks cannot be opened to let upland stormwater out, due to the coastal tide being higher than upstream-side. If the locks were to be opened under this scenario, damaging saltwater would pour into the lower Mermentau Basin, rather than stormwater flowing out.
Upland changes such as drainage improvements to facilitate residential and agricultural development have been ongoing for the last century. Natural upland marshes were drained for agricultural production and residential developments have increased the speed and volume of runoff. Agricultural production has crept ever further south, into the historically inter-tidal brackish areas, which are now kept fresh by the saltwater protection system described above. This all leads to economic prosperity but inevitably damaging results to the Lower Mermentau Basin. For the portions north of LA 82, we have cutoff its natural drainage relief while increasing the freshwater supply. For the portion south of LA 82 which are unprotected, the opposite has occurred: freshwater supplies have been cutoff so that marshes which evolved for brackish environments experience only saltwater exposure.
Though the discussion of coastal land loss has been ongoing for decades, I have attempted to highlight a few of the major congressional initiatives which have proven to be major factors in today’s discussions.
In 1990, the US Congress passed the Coastal Wetlands Planning, Protection and Restoration Act (CWPPRA). Also known as the Breaux Act (due to former LA Congressman John Breaux’s involvement), CWPPRA’s mission is to “identify, prepare, and fund construction of coastal wetlands restoration projects.” (https://lacoast.gov/new/About/). As with all federal programs, funds are available for projects that align with the legislation’s mission, but funding requests must clearly meet that mission with little to no deviation. The Coastal Protection and Restoration Authority (CPRA) was formed in a December 2005 Louisiana Legislature special session to address recovery efforts following Hurricanes Katrina and Rita. During the Katrina/Rita aftermath, the Federal Government requested one state agency to consolidate coastal activities and account for federal relief funds (https://coastal.la.gov/about/structure/). The result was CPRA, and, in 2009, the LA Legislature created the Office of Coastal Protection & Restoration (OCPR) to act as the implementation and enforcement arm of CPRA, which is the state agency administering CWPPRA.
The Coastal Reference Monitoring System (CRMS) was established in 2006 to monitor coastal Louisiana and the agencies’ efforts to reduce coastal land loss. CRMS is a network of 392 vegetation gauges (58 of which lie in the lower Mermentau Basin) which are funded by CWPPRA and operated by CPRA officials.
On May 14th, I had the good fortune of attending a presentation by CPRA Biologist Leigh Anne Sharp, where she discussed and presented interpretations of the preceding 10 years’ worth of CRMS gauge data. The results were eye-opening to say the least. There is not space enough here to discuss all of the CRMS revelations, but one critical component bears mentioning. The increases in freshwater supply and the isolating infrastructure surrounding the lower Mermentau Basin appear to be the major risk factors to the health of the lower Mermentau Basin. Subsidence does not appear to be the major culprit in coastal land loss, as some research has posited (though it does have an impact); freshwater vegetation growth appears able to “keep up” with subsidence rates. The infrastructure discussed previously (LA 82, GIWW locks) are increasing the flooding times for the lower basin, which is effectively drowning the marsh. Marsh vegetation can extend itself to stay above these rising water levels, but this extension adds pore space to its root structure, which weakens it. The weakened vegetative mats are then more susceptible to wave action which breaks-up the shorelines and recedes the marsh line. Even in the portion of the lower Mermentau Basin above LA 82 which is not subjected to coastal waves, winds can create enough wave action to break-up and cause recession of the vegetation boundary of Grand/White Lakes.
Everyone agrees that preserving coastal marshes is a worthy effort, but how does all of this tie into the entire Mermentau River Watershed’s flood risk?
In the near center of the lower Mermentau Basin, and at the southern boundary of the Watershed, lies the Rockefeller Wildlife Refuge (RWR). For 100 years, the LA Dept. of Wildlife and Fisheries (LDWF) has conserved the wildlife and habitat of RWR’s 71,000 saltwater-protected acres through various management efforts. However, RWR lies south of LA 82, meaning it’s natural, intermittent freshwater supply has been cutoff, as described previously. Philip “Scooter” Trosclair, biologist and RWR Program Manager, has developed a conceptual plan to bring freshwater from the “flooded” Grand/White Lakes region to the freshwater-starved marshes of RWR. Mr Trosclair’s project has been in planning for approximately 5 years and proposes to create a network of drainage channels connecting the existing cross-drains under LA 82 to the RWR marshes. Then, using the pumping capacity which today exists on nearly 30,000 acres of the RWR’s managed lands, sorely needed freshwater, sediment and nutrients will be drawn into the RWR’s marsh management area. The pumping systems then can lift the water over the RWR’s protection levees and, most crucially, potential high tides. This would create the first known hydraulic connection of the Grand/White Lakes region to pumping capacity able to lift floodwater overs potential high tides, helping to counteract the problems which exist today at the existing saltwater locks.
This improvements project seems to tick all the boxes for agency missions (CWPPRA, LDWF, local Drainage Boards) and stakeholder concerns:
- It Helps to alleviate flood risk to lower basin residents.
- It provides freshwater, sediments and nutrients to the Rockefeller Marshes.
- It Will assist in lowering water levels and inundation periods for the lower basin north of LA 82.
- Perhaps most attractively, it utilizes existing infrastructure already in-place by connecting the Grand/White Lakes region and RWR pumping system via the cross-drains under LA 82.