Saving the delta is a pipe dream sans sediment diversion projects.
by Len Bahr, PhD*
Climate change and delta deterioration
Last week the concentration of atmospheric CO2 reached 400 ppm, more than at any time in human history. This is the legacy of a century of accelerating combustion of fossil carbon, which has triggered dramatic changes in a climate pattern that had been extraordinarily stable for eleven millennia. The world ocean is warming, expanding and becoming more acidic, with very serious coastal consequences. Unfortunately, high levels of atmospheric CO2 will persist for decades, even were global emissions to drop precipitously.
While atmospheric CO2 has been rising annually (see Fig. 1) and anthropogenic climate change (ACC) has become apparent the world’s great deltas have been declining, exacerbated by sea level rise and growing competition for the river water that formerly sustained these dynamic landforms. America’s Delta is currently experiencing an annual sediment deficit of about 1 billion tons, caused by flood levees, river dams and channel dredging. This is equivalent to the loss of about 200 Superdome Equivalents (SDEs) every year and as a result our delta is disintegrating from within.
If you wanna get dirt, you gotta divert!
Most geophysicists, sedimentary geologists and river scientists familiar with the largest delta in North America agree that the ONLY feasible way to significantly reduce this sediment deficit and to restore the delta would be to divert massive pulses of river water and suspended sediments into the heart of the delta during exceptionally high river flood stages, when the water resembles cappuccino. River reintroduction projects will take decades to be effective, however, partly because the lower river carries only half of its historical sediment load.
As pointed out by Bob Marshall last week in TheLensNola.org, however, projections of the current delta building capacity of the river are based on conjecture and obsolete data from the 60s. Supposedly, the seminal study of the feasibility of restoring the delta with mud and sand from the Mississippi River is a work in progress. Marshall’s article describes this $25 million research effort, called the ‘Mississippi River Hydrodynamic and Delta Management Study,’ as a state of the art effort, with participation by the world’s experts on the river’s current hydrodynamics and sediment budget. On the other hand, IMHO the study, a cooperative effort with the state, the Corps of Engineers and the U.S. Geological Survey, is seriously flawed by its inexplicable omission of the Atchafalaya River (see Fig. 2).
Reconnecting the river to its delta on a massive scale will be problematic for many reasons, including widespread habitat conversion throughout the Mississippi drainage basin during the past century that has been accompanied by declines in water quality. Most of the huge watershed that formerly included extensive bottomland hardwood floodplain has been drained and converted to row crops (especially corn) to which lots of fertilizer (especially nitrogen) is applied, much of which runs off into the tributaries and heads for the gulf.
Flood levees south of Cairo confine the lower river to its channel and funnel the nitrogen-rich water and suspended sediments offshore, bypassing the drying and dying deltaic wetlands. This results in nearshore algal blooms, high primary production and excess biological oxygen demand, culminating in a large hypoxic zone each summer.
Among their other attributes, coastal wetlands are effective at assimilating and removing excess nitrogen in estuarine waters using three mechanisms: (1) bacterial denitrification; (2) incorporation into plant tissue via photosynthesis; and (3) nitrogen sequestration and burial in coastal sediments. The latter process is especially important in a deltaic setting where the surface sediments sink continuously.
Thus it has been argued that diverting river water into the Mississippi’s deltaic wetlands would capture excess nitrogen before the water flows offshore, which would shrink the hypoxic zone. Unfortunately, however, the nitrogen load carried by the river may exceed the assimilation capacity of the deteriorating wetlands, causing inshore as well as nearshore eutrophication.
Several researchers, including LSU Boyd Professor Gene Turner, USGS scientist Chris Swarzenski and Massachusetts scientists John Teal and Linda Deegan have warned that injecting river water into the Mississippi delta could do more harm than good. They argue that adding nitrogen-rich river water to deltaic marshes with organic soils can stimulate aboveground marsh production, while reducing root production and the soil binding capacity for which coastal wetlands are known.
To address this concern the National Oceanographic and Atmospheric Administration (NOAA) sponsored a workshop in 2011 titled: “Response of Louisiana Marsh Soils and Vegetation to Diversions.” This exercise culminated in a position paper that is summarized here, with the take home message that sediment-poor freshwater diversion projects will not build significant landscape but that pulsed sediment-rich diversions can be effective for that purpose. The inshore nitrogen pollution issue was not resolved and it remains unsettled.
Diversion critics point to the controversial Caernarvon freshwater diversion project built in 1992 as an example of how not to design and operate a river diversion project. In contrast, pro-diversion scientists point to the Wax Lake and Atchafalaya subdeltas that have been growing steadily since the 1970s, under uncontrolled annual flooding by river water and sediments.
Vocal diversion critics
In an April 10 post in TheLensNOLA.org Bob Marshall described the organization of a vocal non-technical group of opponents of river diversion projects. This group includes disgruntled oyster growers and commercial and recreational fishers, primarily from St. Bernard and Plaquemines Parishes, who don’t want to give up their oyster leases or be forced to travel further south to catch fish.
On April 17 Mark Schleifstein reported in NOLA.com that Garret Graves, the governor’s coastal advisor, enlisted the help of three coastal scientists to refute the anti-diversion arguments. These authorities included Robert Twilley, an expert on eco-engineering, wetland soils and biophysiology; James Cowan, a coastal fisheries expert and Denise Reed, a specialist on coastal sediment dynamics and marsh hydrology.
Diversion opponents seem to have been energized by the imminent collection of fines against BP that may provide near term funding to move forward with the diversion projects in the Louisiana Comprehensive Master Plan for a Sustainable Coast, approved in 2012. The rise of organized opposition has stiffened the backbone of the state to move forward on sediment diversions.
On April 30 Todd Masson reported in NOLA.com that about 300 diversion opponents met to articulate their opposition to river diversions. According to the article, these diversion opponents have enlisted the formidable political support of St. Bernard and Plaquemines Parish Presidents Dave Peralta and Billy Nungesser, respectively.
On May 5 NOLA.com published a response to these critics, an op ed editorial by David Muth, Louisiana director of the National Wildlife Federation (NWF), as passionate a proponent of reconnecting the river to its delta as anyone I know. Clearly the proponents and opponents of river diversions are talking past each other.**
Mother Nature or diesel fuel?
When opponents of river diversions are asked about alternatives to sediment diversions to address the catastrophic coastal land loss that threatens southeast and central Louisiana, they respond that we should dredge and pump coastal sediments to re-create barrier beaches and marshes on a scale equivalent to the loss rate. One of the prominent advocates of this so-called ‘solution’ is Kerry St Pe’, executive director of the Barataria/Terrebonne National Estuarine Program (BTNEP) and a long time critic of large river diversion projects.
Pipeline conveyance of dredged sediments is an effective way to build landscape on a modest scale, measured in hundreds of acres and tens of millions in dollars…but not in hundreds of square miles and tens of billions of dollars. The Master Plan calls for $3.8 billion in sediment diversion projects and $20 billion in landscape re-creation, using dredged sediments conveyed by pipeline. The former is projected to eventually add over 300 sq mi., while the latter is projected to net 200 sq mi., at a much higher price tag and with no assurance that the recreated landscape would be sustainable in the long run. The high cost of dredging and pumping sediments is primarily the high cost of diesel fuel, which is likely to increase dramatically over the decades.
In January, 2011 Popular Mechanics carried an article on saving the delta using sediment diversions. The proof of concept rests in the Myrtle Grove medium scale sediment diversion project, yet to be constructed.
The only sediment diversion project currently in operation is known as West Bay, an uncontrolled opening in the river levee on the west bank above Head of Passes (see Fig. 3) that allows sediment-rich water to flow west out of the channel whenever the river exceeds a certain stage. This corps-designed project has been problematic since it was completed in 2002.
West Bay is building a subdelta but it has caused shoaling along the river channel that has required periodic dredging. Benjamin Alexander-Bloch reported on the West Bay project in NOLA.com on March 13, 2013.
Why not use nitrogen pollution as a political tool?
The river diversion controversy suggests an opportunity for the state to kill two birds with one stone, moving forward on the design of large and medium scale sediment diversion projects, while mounting a campaign upriver to lower excess nitrogen in Ol’ Man River, so as to eliminate the dead zone and resolve the concern of inshore eutrophication.
We should partner with the feds on the Mississippi River/Gulf of Mexico Watershed Nutrient Reduction Task Force to persuade the corn growing upriver states to get serious about nitrogen runoff. This could be accomplished with a financial incentive package for corn farmers, funded at least partly by BP. Their nitrogen pollution is stymieing our fundamental need to reconnect the lower river to its delta.
Mark Davis, who directs the Institute on Water Resources Law and Policy at Tulane University, recently addressed the clumsy attempt by the state to dismiss the nitrogen issue and a missed opportunity to make a stand against nutrient pollution in the river:
Two studies, one in New England on fertilizer pollution and one in Louisiana on freshwater diversions, are questioning the basic premise at the heart of the Coastal Master Plan – putting the river back into the marsh. These studies show that the nutrient pollution in river water could actually weaken salt marshes, an important but not historically dominant feature of Louisiana’s coastal ecosystem. The concern is that the abundant nutrients in Mississippi River water will spark unsustainable above-ground growth of marsh grasses, the root system will not be able to keep up, and the imbalance will lead to failure of the marsh vegetation. The diversion (we prefer the term “river reintroduction” ) study was commissioned by the Coastal Protection and Restoration Authority, but then dismissed by CPRA head Garret Graves as being unresponsive to the state’s needs. Without refereeing what the study does or doesn’t say or whether it was what the state ordered or not, we do note that the aim of the ongoing federal/state Hypoxia Task force is to actually reduce the nutrient levels in rivers feeding the coast so they are fit to use. At the least the studies seem to reinforce the importance of that work.
Final thoughts: where have the geologists been?
As important as wetlands are, their function is limited by the sediment platform on which they grow, a platform that formed over 6,000 years of net deposition of Holocene mineral sediments from the Mississippi River. Since its very inception in 1989 Louisiana’s coastal restoration program (later renamed protection and restoration) has been dominated by coastal wetland ecologists like me, folks who deal in relatively short term surface processes, not the long term geophysical and riverine processes that underlie the delta. In other words, the planning expertise has been dominated by those who deal primarily with surface processes on the visible veneer of the delta, not the riverine hydrodynamics and sedimentary processes that created the delta and the underlying tectonic processes and shallow and deep subsidence to which the delta ultimately responds.
It’s a pity that the folks who described in great detail the ontogeny of the largest delta in North America and who performed the forensic work on delta dysfunction were only recently invited to the planning table…after key restoration plans had already been drawn up. Better late than never, I suppose.
*Founding editor firstname.lastname@example.org
**On April 22 Richard Burgess reported in The Advocate that the corps has agreed to divert more river water into the Atchafalaya Basin this month. This is ironic in that the extra water was requested by crawfishers and other commercial and recreational fishers, philosophical cousins of the river diversion opponents in southeast Louisiana.