Causes and Effect of Relative Sea Level Rise in Coastal Louisiana

8/11/2019 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

[email protected]


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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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Causes and Effect of Relative Sea Level Rise in Coastal Louisiana