Scientists differ on potential for river to save coast
Editor’s note: For nearly twenty years I’ve repeated a stock glib answer to the question of what can realistically be done to save and sustain much of Louisiana’s threatened coast. That answer has been that we “simply” need to reconnect the river to the delta, a concept that has become almost a truism, based on faith that “turning the river loose” would allow Mother Nature to reverse years of mismanagement. I no longer repeat that mantra, at least not without considerable hedging.
A fundamental premise of the ecosystem restoration component of Louisiana’s coastal protection and restoration program is that releasing most of the flow of the lower Mississippi River upstream from head-of-passes could offset projected landloss in the Pontchartrain and Barataria basins throughout the twenty-first century. Landloss in the Terrebonne basin would require more input from the Atchafalaya River.
What other solutions are out there? To be perfectly frank, there is no plan B. In my opinion, pumping dredged sediments through pipelines will be an important restoration tool in the near term (at least in critical local areas) but the Louisiana coast and other deltas around the world cannot be sustained on technology and brute force that depends on fossil fuel.
Inundation of the emergent landscape can be envisioned as the relentless progressive expansion of the tidal prism, the volume of water that regularly moves into and out of the coast, driven by astronomical and meteorological tides. The tidal prism grows larger each year by a volume precisely equal to the volume of mineral and organic sediment lost as the landscape sinks below sea level (Figure 1).
Neutralizing this process to achieve no net land loss would obviously require adding new mineral sediments and/or organic matter from net plant production at the same rate that relative sea level rise occurs and the volume of the tidal prism expands. This is not a trivial challenge.
In June 2009 LaCoastPost reported on a new study in Nature Geoscience on the capability of the Mississippi river to sustain its delta. The research was carried out by coastal geologists and Mississippi delta experts Mike Blum and Harry Roberts. Their results carry extremely sobering implications for Louisiana’s coastal protection and restoration program.
Blum and Roberts created a mass balance sediment budget for the deltaic plain. Their goal was to estimate the volume of sediment that would be required each year to offset estimated projected land loss, as extrapolated from historical data and different projections of accelerated sea level rise. Figure 2 illustrates their projection of the coast by the year 2090, under a no action scenario. The following is a quote from their study summary:
Sustaining existing delta surface area would require 18–24 billion tons of sediment, which is significantly more than can be drawn from the Mississippi River in its current state.* We conclude that significant drowning is inevitable, even if sediment loads are restored, because sea level is now rising at least three times faster than during delta-plain construction.
The annual input* of 18-24 million billion tons of sediment estimated by Blum and Roberts is 3.6 to 4.8 3,600-4,800 Superdome equivalents (SDEs). Dams on the Missouri River now trap half the sediment load formerly carried by the river. Thus these experts concluded that the triple whammy of delta subsidence, increasing sea level rise and a drastic reduction in riverborne sediments make the “salvation” of south Louisiana problematic at best and dubious at worst.
In striking contrast, on October 20 Eos (the journal of the American Geophysical Union) published the results of a separate study on a closely related topic. This research was also carried out by two highly credible geologists, David Mohrig and Wonsuck Kim, at the University of Texas (UT)-Austin. The latter effort was sponsored and summarized by the National Science Foundation (NSF) and also described by Science News.
Mohrig and Kim used a contrasting methodology and reached a far different and considerably more sanguine conclusion than did Blum and Roberts. Rather than constructing a mass balance budget for the entire delta, they used a numerical model to simulate twin east and west bank river diversions in Plaquemines Parish in the vicinity of the proposed Myrtle Grove diversion project, 93 mi south of New Orleans (Figure 3).
Their simulation model was calibrated using historical data on emergent land accretion of the Wax Lake Delta in Atchafalaya Bay, which formed as a result of the construction of the Wax Lake Outlet, a navigation channel dredged in 1942 off of the Atchafalaya River (Figure 4).
These scientists used their model to simulate the accretion of emergent landscape in the form of subdelta lobes into Barataria Bay on the west and Breton Sound (California Bay) on the east (Figure 3). The model was run under different estimates of diversion volumes (up to 45% of total river flow), suspended mineral sediment loads in the river and up to 4 mm/year sea level rise. They concluded that this volume of flow could offset up to 45% of the projected land loss through the year 2110.
I’m curious and intrigued that the publication of these two important back-to-back studies on the crucial question re saving the coast, and studies with such contrasting results, has not generated serious public discussion. Both projections were run under similar conservative premises re rates of subsidence, sea level rise and river sediments.
So which study is more realistic?
Len Bahr (email@example.com)
*While writing this piece I flashed on the fact that, according to this reference, 159 million tons of suspended sediment (32 SDEs) exit the river mouth and fall off the Continental Shelf into deep water each year. This volume of mineral sediments would seem more than adequate to overcome the sediment deficit described by Blum and Roberts. Am I missing something here?
*As shown by the struck through words above I made a serious error in misquoting the sediment deficit reported by Blum and Roberts. As pointed out by David Muth, I used the figure 18-24 million tons per year, while the quote uses the figure 18-24 billion tons (no time period expressed).