Eos, Vol. 90, No. 42, 20 October 2009
VOLUME 90
NUMBER 42
20 OCTOBER 2009
EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION
Is It Feasible to Build New Land
in the Mississippi River Delta?
PAGES 373–374
What if the Mississippi River levees were
cut below New Orleans? What if much of the
water and sediment were allowed to flow
out and build new deltas? Could deltaic land
loss be reversed, and indeed restored?
Using a conservative sediment supply rate
and a range of rates of sea level rise and
subsidence, a physically based model of deltaic river sedimentation [Kim et al., 2009]
predicts that approximately 700–1200 square
kilometers of new land (exposed surface
and in- channel freshwater habitat) could be
built over a century (Figure 1).
the past several decades. Freshwater habitats and storm surge protection are diminishing as the shoreline moves rapidly toward
New Orleans. Four years before Hurricane
Katrina, Fischetti [2001, p. 80] noted, “At this
rate, New Orleans will be exposed to the
open sea by 2090.”
Is the Mississippi delta a lost cause? Former secretary of the interior Bruce Babbitt argued, “Most of the Mississippi Delta,
some 10,000 square miles, lies less than
three feet above sea level. Beset by land
subsidence and rising sea levels, much of
this vast area will inexorably sink beneath
the waters by the end of this century” [Babbitt, 2007]. He stated that “Congress should
PAGES 373–384
suspend all coastal funding until the Corps
and Louisiana prepare a comprehensive and
realistic land-use plan for the entire delta,
applying modern science and fiscal discipline to determine what can and cannot be
salvaged.”
Arguments have been presented for opening levees to create engineered avulsions
for coastal restoration [e.g., Coastal Protection and Restoration Authority of Louisiana (CPRA), 2007]. Objections, however,
are numerous. First, dams over the Mississippi basin have so reduced sediment supply that material available for land building may be insufficient. Also, present- day
subsidence rates in the Mississippi delta
may be high enough to inhibit land building. Moreover, sea level rise associated with
global warming may cause land-rebuilding
schemes to fail, and direct sediment supply
from the Mississippi River to the delta may
be comparatively minor compared with that
Sinking Into the Sea
The Hurricane Katrina disaster of August
2005 highlighted a problem recognized for
decades: The Mississippi River delta is sinking into the sea [e.g., Fischetti, 2001]. In natural systems, large, fine-grained deltas subside due to sediment compaction, faulting,
and other effects. Subsidence is counteracted by over-bank sediment deposition and
avulsion into low areas. The result is a delta
in which subsidence and sedimentation balance over time.
Below the U.S. Army Corps of Engineers
Old River Control Structures in northern
Louisiana, engineered levees on the Mississippi River prevent over-bank deposition and
sudden changes in the course of the river
(avulsion). The sediment that would balance subsidence on the delta top is instead
delivered to the river’s mouth, which abuts
the continental shelf-slope break. Thus, the
sediment vital to maintaining freshwater deltaic wetlands drains uselessly into the Gulf
of Mexico. Additionally, natural subsidence
driven by compaction and deformation of
salt layers (salt-withdrawal tectonics) has
been exacerbated by hydrocarbon extraction [see Morton et al., 2005, Figure 25].
The result is a drowning delta. Morton
et al. [2005] indicate a land loss rate of
about 44 square kilometers per year over
BY W. KIM, D. MOHRIG, R. TWILLEY, C. PAOLA,
AND G. PARKER
Figure 1.View of the delta of the lower Mississippi River below New Orleans, schematizing predictions of the new land (delta surface) that could be built over 100 years starting from 2010.
Two diversions are considered: Barataria Bay and Breton Sound. The calculation is based on a
“base case” scenario: a subsidence rate of 5 millimeters per year and sea level rise rate of 2 millimeters per year. The inset shows results for a “best case,” subsidence of 1 millimeter per year and
sea level rise of 0 millimeters per year, and a “worst case,” with corresponding values of 10 and
4 millimeters per year. For the sake of clarity, land losses in the part of the deltaic wetlands not
subject to diversion are not estimated or shown. Image courtesy of NASA World Wind.
Eos, Vol. 90, No. 42, 20 October 2009
supplied from offshore by hurricanes [Turner
et al., 2006].
To date, however, arguments neither for
nor against controlled avulsions have been
supported by quantitative models predicting evolution of the deltaic landscape as
a function of sediment supply, subsidence
and sea level rise rates, delta topographybathymetry, and other key factors. To gain
new insight, scientists are using quantitative
sediment transport and delta-building models to explore the feasibility of building new
land in the Mississippi delta.
Fixing a Drowning Delta
The subsidence problems on the Mississippi delta can be analyzed in terms of simple mass-balance considerations. Building
on this, a team of scientists from institutions
throughout the Mississippi basin have developed a numerical model that allows largescale simulation of the evolution of a radially symmetric delta with a top and sloped
front advancing into standing water. This
model is a natural extension of earlier models [e.g., Parker et al., 1998; Kostic and Parker, 2003; Parker et al., 2006, 2008].
The model was applied to two potential
engineered avulsions of the lower Mississippi: one into Barataria Bay and the other
into Breton Sound [CPRA, 2007] (see Figure 1 for locations), with 45% of flood discharges of sediment and water evenly distributed between the two. The “base case”
used current best estimates: sediment supply
to the lower Mississippi of 126 megatons per
year, a subsidence rate of 5 millimeters per
year, and sea level rise rate of 2 millimeters
per year (for input parameters and justifications, see the online supplement to this Eos
issue (http://www.agu.org/eos _elec/)). Figure 1 shows the model’s prediction for the
area of new land created on the delta top
(emergent land plus freshwater channels).
This base case scenario yields 918 square
kilometers of new area created in the 100
years after commencement.
There is already an “engineered avulsion”
of the Mississippi, i.e., the Old River Control
Structures. These structures allow substantial
flows of water and sediment into the Atchafalaya River, while preventing complete capture of the Mississippi River discharge. The
Atchafalaya River is actively building new
land in the Atchafalaya delta, and the subsidiary Wax Lake delta; the latter started building seaward in about 1980 (see Roberts et al.
[2003] for a detailed study). Figure 2 shows
the approximately 100 square kilometers of
new land built in the Wax Lake delta by 2005,
along with the shoreline position predicted
by the land-building model after 30 years of
delta evolution since 1980. The calculations
assume a subsidence rate of 5 millimeters
per year, a sea level rise rate of 2 millimeters per year, and sediment supply varying
from 25 to 38 megatons per year. The Wax
Lake delta thus not only counters the argument that the riverine sediment supply is
insufficient to build land against the current
Figure 2.View of the Wax Lake delta schematizing the planform in 2005, along with a hindcast of
shoreline position. The hindcasting uses a subsidence rate of 5 millimeters per year, sea level rise
of 2 millimeters per year, and sediment supplies of 25 and 38 megatons per year. Image courtesy
of J. Barras, U. S. Geological Survey.
subsidence and sea level rise but also verifies application of the land-building model
to the lower Mississippi River.
Debate persists about model input parameters, in particular about subsidence and
sea level rise [e.g., González and Törnqvist,
2006]. Calculations were thus performed
with subsidence rates from 1 to 10 millimeters per year and sea level rise rates from
0 to 4 millimeters per year. As noted before,
Figure 1 shows the predicted new land on
the delta top after 100 years for the “base
case” (a subsidence rate of 5 millimeters per
year and a sea level rise rate of 2 millimeters per year). By contrast, the “best case”
scenario uses a subsidence rate of 1 millimeter per year and no sea level rise, and the
“worst case” scenario uses a subsidence rate
of 10 millimeters per year and a sea level
rise rate of 4 millimeters per year. The predicted growth of new delta surface area for
these three scenarios is illustrated in the
Figure 1 inset. The area of land created in
a century ranges from 701 square kilometers
(worst case), through 918 square kilometers (base case), to 1217 square kilometers
(best case). So even in the worst case, considerable land area could be created over a
century.
Building New Land
Building substantial amounts of new land
in the Mississippi delta is indeed feasible.
Land building by controlled avulsions is not
an alternative for short-term levee strengthening to protect coastal communities. Nonetheless, the exigency of strengthening levees
should not overshadow the fact that meaningful land building requires a societal commitment over the scale of a century. Society
has, however, already made this commitment through rebuilding the city of New
Orleans and maintaining the Old River Control Structures.
Further, calculations show that despite
this new land, enough flow into the main
channel of the Mississippi River is left to
maintain navigation (see the online supplement). The diversion structures themselves
would need to be hard- engineered to control the water and sediment supply to the
new deltas. They would need to be sufficiently deep to ensure the diversion of sand
as well as mud, as the deposition of sand is
necessary for substantial land building. Fortunately, the cost of such structures is by no
means insurmountable, as has been shown
in the case of the Old River Control Structures. Downstream of these structures, however, new land and habitat can be formed
by natural processes with minimal human
intervention.
The calculations presented above are
based on a diversion of only a fraction of
the flood flow in the Mississippi River below
New Orleans. Results also do not include the
expected inflation of deposits by the gradual
formation of organic soil. To capture the full
potential for land building, calculations can
be extended to include (1) all of the sediment currently estimated to be delivered to
the lower Mississippi and the Atchafalaya
rivers (~210 megatons per year), and (2) an
inflation of deposit height such that 20% of
a column consists of organics. Using the
base case values for rates of subsidence and
sea level rise, approximately 2740 square
kilometers could be built by the year 2100.
This number represents about one fourth of
the land loss estimated by Blum and Roberts [2009] in the absence of countermeasures, but about half of that estimated using
the projected land loss rate of Barras et al.
[2003] of 62 square kilometers per year prorated over 100 years.
While it is clearly not feasible to restore the
entire delta (the focus of the analysis of Blum
and Roberts [2009]), even the minimum land
areas regained through methods described
Eos, Vol. 90, No. 42, 20 October 2009
here would represent dramatic improvements
over the “do nothing” situation in which
nearly all of the delta will be lost.
Acknowledgments
This paper is a contribution of the
National Center for Earth- surface Dynamics,
a Science and Technology Center funded by
the U.S. National Science Foundation (EAR0120914).
References
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letter to the editor, Wash. Post, p. A22, 18 May.
Barras, J., et al. (2003), Historic and predicted
coastal Louisiana land changes: 1978–2050,
Natl. Wetlands Res. Cent., U.S. Geol. Surv., Baton
Rouge, La.
Blum, M. D., and H. H. Roberts (2009), Drowning of the
Mississippi delta due to insufficient sediment supply
and global sea-level rise, Nat. Geosci., 2, 488–491.
Coastal Protection and Restoration Authority of
Louisiana (CPRA) (2007), Integrated ecosystem
restoration and hurricane protection: Louisiana’s
comprehensive master plan for a sustainable
coast, Baton Rouge.
Fischetti, M. (2001), Drowning New Orleans, Sci.
Am., Oct., 77–85.
González, J. L., and T. E. Törnqvist (2006), Coastal
Louisiana in crisis: Subsidence or sea level rise?,
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Kim, W., A. Dai, T. Muto, and G. Parker (2009),
Delta progradation driven by an advancing sediment source: Coupled theory and
experiment describing the evolution of elongated deltas, Water Resour. Res., 45, W06428,
doi:10.1029/2008WR007382.
Kostic, S., and G. Parker (2003), Progradational
sand-mud deltas in lakes and reservoirs: Part 1.
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channelized fluvial and sheet flow: I. Theory,
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delta, in River Flow 2006: Proceedings of the
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Lisbon, Portugal, 6–8 September 2006, edited
Seismic Imaging in Three Dimensions
on the East Pacific Rise
PAGES 374–375
The U.S. R/V Marcus G. Langseth (operated by the Lamont- Doherty Earth Observatory of Columbia University) sailed in
late June 2008 from Manzanillo, Mexico,
to the 9º50’N area of the East Pacific Rise
(EPR), a site of vigorous hydrothermal venting (Figure 1). The cruise, MGL0812, the
first research deployment of the Langseth’s
advanced three- dimensional (3-D) seismic imaging capability, had as its objective
obtaining high-resolution images of crustal
structure beneath the ridge crest and adjacent regions.
The benefits of 3-D seismic imaging had
been outlined in a U.S. National Science
Foundation (NSF)– sponsored workshop
in 2005 [Mutter and Moore, 2005]. Short
courses on techniques of 3-D survey planning were given at AGU Fall Meetings in
2007 and 2008. This brief report describes
experiences during the cruise, with the
objective of aiding future researchers in
planning cruises using Langseth’s unique
imaging capability for 3-D.
groups of hydrophones spaced 12.5 meters
apart. Paravanes (towed submerged planar
devices; see Figure S3 in the electronic supplement) separated the streamers to 150meter spacing so that the total spread separation between the two outermost streamers
was 450 meters. The air gun source comprised four linear arrays, each with nine
by R. M. L. Ferreira et al., pp. 3–11, Taylor and
Francis, London.
Parker, G., T. Muto, Y. Akamatsu, W. E. Dietrich,
and J. W. Lauer (2008), Unravelling the conundrum of river response to rising sea-level from
laboratory to field: Part I. Laboratory experiments, Sedimentology, 55(6), 1643–1655.
Roberts, H. H., et al. (2003), An embryonic major
delta lobe: A new generation of delta studies
in the Atchafalaya-Wax Lake delta system, Gulf
Coast Assoc. Geol. Soc. Trans., 53, 690–703.
Turner, R. E., et al. (2006), Wetland sedimentation from hurricanes Katrina and Rita, Science,
314(5798), 449–452.
Author Information
Wonsuck Kim and David Mohrig, Department of
Geological Sciences, University of Texas at Austin;
E-mail: [email protected]; Robert Twilley, Department of Oceanography and Coastal Sciences,
Louisiana State University, Baton Rouge; Chris Paola,
Department of Geology and Geophysics, University
of Minnesota, Minneapolis; and Gary Parker, Department of Civil and Environmental Engineering and
Department of Geology, University of Illinois, UrbanaChampaign
guns, for a total of 1650 cubic inches, towed
beneath a linear float that allows towing
depth to be held constant at 7.5 meters. The
air gun source was fired in a “flip-flop” manner, alternating between two port and two
starboard linear arrays so that each sail line
along which the vessel traveled acquired
eight common midpoint (CMP) profiles
spaced 37.5 meters apart with a 3300-cubicinch source and shot spacing of 37.5 meters.
Streamers and air gun arrays were navigated
with a combination of a Global Positioning
3-D Seismic Acquisition
To acquire 3-D data, researchers on
board Langseth deployed a system similar
to that used for industry acquisition (see Figure S1 in the electronic supplement to this
Eos issue (http://www.agu.org/eos _elec/)).
Four solid Thales/Sercel hydrophone array
“streamers”—with no fluid inside the
streamer jackets, thus improving streamer
signal- to-noise ratio—were deployed. Each
streamer was 6 kilometers long and contained 468 groups of hydrophones, with the
Fig. 1. (a) The complete line coverage from R/V Marcus G. Langseth cruise MGL0812. (b) A detail
from Figure 1a of the area of three-dimensional acquisition. The locations of hydrothermal vents
are indicated by stars. Survey sail lines were acquired in regular racetrack pattern loops. Extended line changes that can be seen outside the regular racetracks accommodated the maintenance
of air guns and other equipment.