causes and effect of relative sea level rise in coastal louisiana
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The Causes and Effects of Relative Sea
Level Rise in Coastal Louisiana
By Chris McLindon
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Sea Level
Conceptually sea level is a fixed entity. It is a surface of uniform distance from the center of the earth
that we consider to have an elevation value of zero. In reality it is anything but fixed, in space or in time.
The level of the ocean relative to that conceptual entity, called the geoid, varies across the surface of
the earth. NOAA has recently found that the rate as which sea level is changing also varies across theglobe. This image of the globe produced by the Virginia Institute of Marine Science shows the rate of
sea level rise in mm/year. In the strongest red areas sea level is rising up to 10 mm/yr. In the strongest
blue areas it is falling at the about same rate.
The creation of this image was made possible by data provided by the NOAA Laboratory for Satellite
Altimetry. The authors of this report clearly state:
Newtechnologies such as sea surface range measurements from earth-orbiting satellites now
provide a global assessment of absolute sea level (ASL) trends relative to the center of a
reference ellipsoid rather than fixed points on the earthssurface to which relative sea level
(RSL) measurements refer. New methodologies have also been applied to derive spatial averagesof ASL trends over large regions with greater accuracy. Notwithstanding these advances, there is
still no substitute for an accurate time series of water level measurements obtained locally,
preferably one spanning several decades, when assessing RSL trends that will affect a specific
community or township in the coming decades. RSL trends will determine local inundation risk
whether due to vertical land movement (emergence or subsidence) or the ASL trend found as the
sum of RSL trend and land movement when both are measured positive upward. In Chesapeake
Bay, RSL trends are consistently positive (rising) while land movement is negative (subsiding).
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The measurement of a time series of water levmeasurements obtained locally spanning sever
decades comes from the several tidal gauge statio
that are positioned around the coast. In simple
terms tidal gauges measure the elevation of surface
the ocean at a point on the earth at a given tim
Data from any of these gauges plotted over the perio
of several decades reveals a distinctive characterist
a sloped line indicating an apparent change in th
elevation of the ocean at that point over time.
turns out that the slope of this due to a change in se
level, and the data recorded by the tidal gauge can b
used to estimate the relative sea level rise that h
taken place at that point on the earth during the tim
span over which the data was recorded.
A comparison of the graphs of four representati
tidal gauges from around the coast of th
southeastern U.S. reveals something else the slop
of the historical elevation data differs for every gaug
Each tidal gauge around the world records its ow
measurement of relative sea level rise. The NationClimate Change Assessment map of vulnerability
sea level rise reflects these variations in relative se
level rise. Areas of the Atlantic Coast in Virginia a
North Carolina are experiencing rates of relative ri
that are more than twice that of Pensacola. The tid
gauge at Grand Isle Louisiana has recorded a rate
relative sea level rise that approaches five times th
rate of Pensacola.
National Climate Change Assessment
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1932
500 mm = 19.6
total relative sea
level rise since 1932
sea level rise
subsidence
The seminal work of Harry Roberts, head of the Coastal Studies Institute at L.S.U., published in 2011
with coauthor Mike Blum provided an excellent graphic representation of the reason behind the
variation in historical tidal gauge curves from one area to another. As was indicated by the authors of
the VIMS study, the relative sea level rise recorded by tidal gauges is affected by both the absolute risein global sea level and by the vertical movement of the surface of the earth caused by subsidence at the
location of the tidal gauge. The total relative sea level rise measured at any one point is the result of the
combination of both effects. Blum and Roberts illustrated this by graphically combining the curves for
the Grand Isle tidal gauge with the Pensacola tidal gauge and the Intergovernmental Panel on Climate
Change (IPCC) global sea level rise curve published in 2007 (the gray line plotted as Global Mean Sea
Level (GMSL)). It turns out that Pensacola is situated on a very stable ridge that extends southward from
the axis of the Appalachian Mountains, and it experiences effectively no subsidence. Correspondingly,
the relative sea level rise measured by its tidal gauge closely approximates the IPCC global curve. The
sea level rise at Pensacola is entirely due to absolute sea level rise. The much steeper slope of the tidal
gauge curve from Grand Isle is due to the additional effects of subsidence and the difference between
the slope of the Grand Isle curve and the global sea level curve is in fact a measure of subsidence that isoccurring at Grand Isle. This graph shows that over 500 mm, or nearly 20 inches, of total relative sea
level rise has occurred at Grand Isle since 1932.
The relative sea level rise due to subsidence is obviously the more impactful element making south
Louisiana vulnerable to the effects of sea level rise. An understanding of subsidence in south Louisiana,
and how it impacts the land surface requires an understanding of mechanisms of subsidence that are
controlled by the geology that is going on below the surface. To properly understand south Louisiana
one must look at it from a bottomupperspective.
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Predicted present-day vertical motions in mm/yr from delta & ocean loads
Ivins, et al (2006)
Grand Isle, LA
The 2006 publication by Erik Ivins of the C.I.T. Jet Propulsion Laboratory and Roy Dokka of the L.S.U. Center
for GeoInformatics produced the first predictive map for the general rates of subsidence caused by a
response to the sedimentary load emplaced by the Mississippi River during the outwashing of the melting
ice pack that followed the end of the last ice age. The values of subsidence are measured at Continuous
Global Positioning System (CGPS) stations indicated by the red diamonds. The red star indicates the
location of the Grand Isle tidal gauge, which recorded the effects of this subsidence over most of the
twentieth century. This map represents a generalized pattern related to the loading. More detailed
investigations have revealed dramatic variations of subsidence rates across the area. The study of tidal
gauge data in the area just west of Grand Isle by Shea Penland of U.N.O in 1990 estimated rates of relativesea level rise due to subsidence of up to 23 mm/yr. Dixon used GPS data in 2006 and measured similar
rates of subsidence just south and east of the city of New Orleans.
Relative Sea Level Rise in
mm/yr from Tidal Gauge Data
Penland, 1990
Grand Isle
Subsidence in mm/yr measured by GPS
Dixon 2006
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The same area in which Penland had used tidal gauge data to measure relative sea level rise due to
subsidence was later studied in 2001 by a team lead by Harry Roberts. They used geological mapping
techniques that included seismic data to map the traces of faults that extend across the marsh surface.
While many of the most well known faults, like the San Andreas, are known for their lateral or strikeslip movement along the surface, faults in south Louisiana are characterized by mostly vertical
movement. These listric faults are part of a family of geomorphic features that also includes
landslides. The vertical movement of these features occurs by slippage along a slide surface, and is
expressed by the formation of an escarpment. The escarpment of the faults that cross the marshes is
much more subtle than the one formed by the landslide that took a devastating toll in Washington State
in the spring of 2014, but it is expressed nonetheless.
Fault Slide surface
Fault Escarpment
Washington State landslide
Kuecher, et al (2001)
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Fault traces expressed at
the marsh surface
Rotation of the marsh surface by faultingGagliano et al, 2003
Fault
Displaying the traces of the faults mapped by the Roberts team on satellite imagery of the marshsurface shows how the escarpments of these listric faults are expressed. Each fault clearly defines the
boundary of a set of open bodies of water that line up along it. Sherwood Gagliano, the CEO of the
environmental research company Coastal Environments Inc., showed in his 2003 study that these
bodies of water, that he called faultbayswere formed by rotation of the marsh surface associated
with the downward movement of the faults along their slide planes. The vertical movement of the
marsh surface along the fault traces caused the marsh ecosystem to drown in saltwater as it continually
lost elevation due to subsidence. It is the sloping marsh surface caused by the rotational effects of the
faults that allowed for the intrusion of salt water from the Gulf of Mexico into the interior marshes.
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8 per decade
In the top image the measurements of subsidence derived from tidal gauge data by Penland are overlain
on the surface fault traces crossing the marsh. The values subsidence in mm/yr have been converted to
inches per decade, and they clearly reveal the relationship between the faults and the areas of
increased subsidence. The lower image is the U.S.G.S. Land Area Change (or landloss)Map is shown
with the fault traces over lain. A comparison of these two images leads to the inescapable conclusion
that the hot spots of wetlands loss indicated by the colored patches on the U.S.G.S. map are cause by
the subsidence of the marsh surface by the vertical movement of faults.
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Fault traces expressed at surfaceDokka, 2011
Subsidence velocities in mm/yr
measured by GPSDixon, 2006
A very similar coincidence to the studies of Penland and Roberts that showed the relationship of faulting
and subsidence as measured by tidal gauge data occurred in the area in which Dixon measured
subsidence by GPS. The surface traces of faults mapped by Roy Dokka in the eastern part of the city of
New Orleans were published in a 2011 study. Dokka showed convincingly that the vertical movement of
the faults he mapped directly accounted for the subsidence that Dixon could measure with GPS data.
Theses subsidence velocities were as great as the 20 mm/yr, or 8 inches per decade, measured by tidal
gauges in Penlands study. These two study areas strongly indicate that while Dokka and Ivins
generalized map of predicted subsidence velocities captures the general shape of the area of coastal
Louisiana affected by relative sea level rise due to subsidence, the variation of rates of subsidence and
the concentration of areas of much higher subsidence within that shape are controlled by the vertical
movement of faults.
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Dixon Area of Maximum
Subsidence ~ 8 inches per
decade
Ecosystem Investment Partners
Marsh Creation Project Outline
Fort Procto
The relationship of the pattern o
faulting mapped by Dokka and th
subsidence velocities measureby Dixon also perfectly explain th
patterns of land loss that hav
been experienced across the are
and are illustrated on the U.S.G.S
Land Area Change Map. Th
limitation of GPS data is that
must be measured on dry land
This explains the scatter of point
across the wetlands areas outsid
of the protective levees of th
city. It is a striking reality that th
elevations of the surface inside a
outside of the levee ar
essentially the same, both ar
below sea level. The area
outside of the levees are natural
inundated by the salt water tha
flows into the subsided areas.
Fort Proctor stands as a testament to the effects of relative sea level rise due to subsidence over time. The for
was constructed 150 feet inland from the shore of Lake Borgne in 1865. Based on a comparison with othe
forts of the same era around the Gulf, it was almost certainly 5 feet above sea level. The fort is now in the Lak
and 4 feet below sea level. The subsidence rate that is necessary to have submerged Fort Proctor by 9 feet i
150 years is the same rate that was measured by Dixon using GPS in 2006. The significance of this rate o
subsidence, that is likely to be experienced within the red dashed outline, will be brought to bear on the mars
creation project being undertaken by Ecosystem Investment Partners , which was featured in the July 12, 201
article by John Schwartz in the New York Times. A simple calculation using the data at hand reveals that th
shallow mud flats created by this project will be below the surface in less than two decades.
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Surface Fault Traces
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Land Loss in Southeast Louisiana
A comparison of these three images clearly illustrates the relationship between subsidence, faulting, land loss
and the submergence of the Louisiana coastal wetlands. Dokka and Ivins map of the general pattern of
subsidence outlines a central area that is coincident with the thickest deposits of sediment carried into the area
during the outwash associated with the melting of the ice sheet on the North American Continent. This central
area, outlined in red dash encircle the entire area of the coast that has experienced land loss over the past 80
years, and the axis of this central area generally coincides with the axis of maximum land loss. Within the
central area of subsidence concentrated hot spots of local land loss are directly associated with the traces offaults crossing the marsh surface. Faults are the primary mechanisms of subsidence that allowed for the
accumulation of sediments delivered by the river. The downward movement of the faults along their slip
planes causes locally higher rates of subsidence along the surface traces of the faults. This movement acts to
submerge the marsh ecosystem below sea level and the area of maximum subsidence is converted to a body of
open water. The satellite image shows that the areas where land loss is occurring are areas that are following
the natural progression from marsh to open water that has accompanied the subsiding coastal plain throughout
its development.
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The L.S.U. Center for GeoInformatics created
this set of predictive maps on the left to
illustrate the effects of total relative sea level
rise due to the combination of absolute sea
level rise and relative sea level rise due to
subsidence. The areas in pink are areas that
are considered vulnerable to the effects of
sea level rise in the given year. Much of
those areas in the year 2010 are land areasthat are within protection levees, but below
sea level.
It is very important to note that these areas
are designated as vulnerable strictly on the
basis of the anticipated effects of sea level
rise and subsidence. This is not a measure of
land loss due to coastal erosion. Based on
the rates of subsidence and sea level rise that
can be measured today these areas will be
submerged below sea level.
The images on the top right show a similar
predictive model from the 2009 publication
by Blum and Roberts in which they stated:
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-
l i i
Blum & Roberts, 2009
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