An Evaluation of the Potential Effects of Fault-Driven Subsidence on the Proposed

The Morganza-to-the-Gulf Levee and The Terrebonne Sediment Pipeline

By Chris McLindon chris_mclindon@att.net

Proposed Sediment Pipeline

Marsh Creation Areas

Proposed Morganza-to-the-Gulf Levee

This map of the area outlined in yellow on the title page shows the proposed Morganza-to-the-Gulf Levee and the proposed sediment pipeline and marsh creation projects. The Advocate reported on June 17, 2014 that a report released by the U.S. Army Corps of Engineers revised the cost estimate for the levee project upward to $12.9 billion, a dramatic increase from the $887 million estimate when the project was authorized by Congress in 2007. Congress renewed approval of the project this year, but has not approved full funding. Terrebonne Parish has been working on the footprint of the project, and recently completed five miles of levees between floodgates on the Houma Navigation Canal and Bayou Grand Caillou. The parish is also pushing a proposal for a 30 to 36 inch pipeline to transport sediment to three identified marsh creation areas which lie just outside of the proposed levee. The initial cost estimate for this project is $1 billion. 1

Couvillion, B.R.,et.al, 2011, Land area change in coastal Louisiana from 1932

to 2010: U.S. Geological Survey Scientific Investigations Map 3164, scale

1:265,000, 12 p. pamphlet.

Both of these projects were conceived and designed in response to the magnitude of wetlands loss that has occurred since 1932 in a belt across central Terrebonne. Land loss shown on this map as color-coded patches was measured by the U.S.G.S. by comparing aerial photography from 1932 with satellite imagery from 2010. Comparison of this map with the modern satellite image on the next page shows that the colored patches indicating land loss by year on this map are open bodies of water on the satellite image. The designation of land loss on this map indicates that these open bodies of water were marsh in 1932, and that the marsh was converted to open water in the succeeding 78 years. The underlying premise of these projects is that the marsh provided a buffer to storm surge flooding, and that construction of the levee and the creation of new marsh in the open water areas is necessary to reduce the risk of flooding to the residents of areas to the north.

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The open bodies of water that have been formed by the loss of freshwater marsh are obvious in the comparison of this satellite imagery with the U.S.G.S. Land Loss Map on the previous page. It is also apparent that these areas have a fairly distinctive northern boundary that runs WSW to ENE across the area. The marshes south of this boundary have converted to open water. The marshes to the north have remained relatively unchanged since 1932, and show only minor amounts of land loss. This area has been the subject of much academic study over the past few decades. Most of that study has focused on the areas of land loss, but little attention has been paid to the more stable areas to the north, or to any attempt to explain the boundary. This red square outlines the area of one study that did examine a cause of land loss, and it offers a good explanation for both the concentration of land loss in the distinctive belt and the boundary that separates it from marshes to the north.

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Marsh Creation Areas

Kuecher, G.J., et.al., 2001, Evidence for Active Growth Faulting in the Terrebonne Delta Plain, South Louisiana: Implications for Wetland Loss and the Vertical Migration of Petroleum., Environmental Geosciences, v. 8, p. 7794

This is a map from a study on wetlands loss with the same red box outlined on the previous page. The authors, who included Dr. Harry Roberts, head of the Coastal Studies Institute at L.S.U., determined that the movement of geologic faults that cross the surface of the marsh were likely to be a principal cause of wetlands loss. These faults cut deep into the subsurface, and they are the same faults that are responsible for trapping the oil and gas in fields in the area. It is the vertical movement of the faults that causes the marsh surface to subside below sea level. In reality the marsh is not lost to erosion, it sinks below the surface and is drowned by water that fills in the subsided depression. Oil and gas geologists know that it is this same process of subsidence by faulting that has carried the oil and gas reservoirs, which were all once deposits of the Mississippi River at the surface, to the great depths at which they are found today. Subsidence due to faulting has been active in this area for millions of years.

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Highlighting the traces of the faults mapped by Dr. Roberts and his team shows the explanation for the boundary that separates the bodies of open water from stable marsh. The belt of open water that was once marsh lies along the southern or downthrown edge of the faults which is being subsided more rapidly than the upthrown areas to the north. It is the vertical movement of the faults that has resulted in the subsidence of the marsh on the downthrown sides of the faults causing them to sink below the surface and convert to open water. As will be seen on a subsequent diagram, studies have shown that the faults actually have a slightly rotational movement that slopes the marsh surface in toward the fault as it subsides. It is this rotation of the marsh surface that causes salt water to flow inward into the interior freshwater marsh. Saltwater moves into the subsided areas by gravity flow, and it is the juxtaposition of these areas of open salty water adjacent to the cypress swamps that causes them to die off.

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The marsh ecosystem in this area is built on a substrate of sediment that was deposited by the Mississippi River system when its active channels were what are now the bayous that spread out like the fingers of a hand pointing southward. When the river channels were active, overbank flooding and crevasse splays built up the elevation of the natural levees on which cultural development has occurred. Flooding also continued to supply the marsh system with new sediment that allowed it to keep up with the rate of subsidence and maintain its elevation. As soon as the river flow was shut off by the damming of Bayou Lafourche at Donaldsonville in 1904, the sediment supply was cut off. The downward movement of the faults has been continuous through time and the marsh surface on the downthrown side of the faults has been subsiding since they were deposited, as long as there was sediment being delivered elevation was maintained, as soon as it was shut off, the effects of subsidence became apparent.

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The colored patches, that have been called hot spots of land loss across the coastal wetlands have an obvious causal relationship with the traces of the active faults. Once the supply of freshwater and sediment flowing into the network of bayous from the Mississippi River through Bayou Lafourche was cut off in 1904, the downward movement of the faults was no longer matched by the addition of new sediment at the marsh surface. Subsidence of the marsh surface due to faulting began to have an effect in the beginning of the twentieth century, but that effect could not be reliably measured until the advent of aerial photography in 1932, which is why land loss figures are quoted from that year. There are several ways that subsidence can be observed, but it turns out that getting very accurate measurements of subsidence values has been technologically challenging.

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1932

470 mm = 18.5 total relative sea

level rise since 1932

sea level rise

subsidence

One of the most reliable ways to observe the effects of subsidence over a period of decades, and to begin to assign it a numerical value is the historical record of tidal gauges. The daily recordings of tidal gauges are elevations of the high and low ranges. It is well established that the global rise in sea level can be seen in historical tidal gauge records all over the world. Graphing the of tidal range over a long enough period of time reveals a sloped line that is the relative rise in sea level. The slopes of these lines can be different for different areas, and it has been determined that the more steeply sloping lines on tidal gauge graphs are due to subsidence. The slope of the Grand Isle gauge is greater than the slope of the Pensacola gauge over the same period of time. It turns out the Pensacola is on a very stable ridge crossing the Florida Panhandle, and it is not effected by subsidence. The slope of the Pensacola graph is very close to the accepted value for global sea level rise. The Grand Isle gauge on the other hand slopes more steeply, and the difference in slope between these two gauges is due to subsidence at Grand Isle. In this way the tidal gauge records can be used to get a measurement of subsidence. In the case of Grand Isle it has experienced 470 mm or 18.5 inches of relative sea level rise since 1932, most of that being due to subsidence.

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Penland, S., et.al., Relative Sea Level Rise and Delta-Plain Development in the Terrebonne Parish Region, Louisiana Geological Survey, Coastal Geology Technical Report No. 4, 121 p.

A research group led by Dr. Shea Penland of U.N.O. examined the tidal gauge records for the stations in this area. The black dots show the locations and names of the gauges. The numbers are the estimated rates of subsidence for each gauge in millimeters per year. The colored lines are the contours of subsidence showing a pattern of high subsidence across the interior marshes, which diminishes toward the coastline. There have been significant advances in the technology of measuring subsidence in recent years including measuring changes in elevation, and therefore subsidence velocities, using GPS. It may turn out that the estimates of subsidence rates derived from tidal gauges will be corrected to some extent by advancing technology, but the relative pattern of change will not change. This map clearly shows a belt of relatively high subsidence across central Terrebonne and Lafourche Parishes.

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8 per decade

Overlaying and slightly adjusting the colored contours from Dr. Penlands map onto the fault trace map shows a very clear relationship that documents the subsidence of the coastal marshes due to the downward movement of active faults. It makes perfect sense that areas downthrown to the active faults would be experiencing the highest rates of subsidence, and that the effects of that subsidence would be evident in the land loss that has occurred in the area. On this map the rates of subsidence estimated from the tidal gauges has been converted to inches per decade. The cumulative effect of these values over several decades is obvious, and it is consistent with the depth of the open bodies of water in the highest subsidence areas. Some areas may have subsided by as much as 4 feet since 1932.

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A closer examination of a portion of this area reveals more details of the processes by which subsidence by fault movement has caused the wetlands loss measured since 1932. The area within the red outline is crossed by the surface traces of several faults and there is good documentation of the patterns and expressions of fault movement at the surface over time. As has been done on a more regional scale, this can be seen by comparing recent satellite imagery with the U.S.G.S. Land Loss Map. The following pages will also introduce comparative historical aerial photography to illustrate the patterns of change. Finally the implications of this evaluation for the proposed levee and sediment pipeline projects will be considered.

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DULAC

Lake Boudreaux

CHAVIN

MONTEGUT

The pattern of faulting across the marsh surface can be seen more clearly in this detailed view, which is within the red outline on the previous page. Most of the faults clearly demark the northern boundary of open bodies of water that have been formed by subsidence along the faults. These bodies of open water also lie between the natural levee ridges of the former distributary channels of the Mississippi River system from a time when this was the active delta of the river.

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DULAC

Lake Boudreaux

CHAVIN

MONTEGUT

Dr. Sherwood Gagliano has been at the forefront of the study of effects of faulting on the marsh surface for over twenty years. This set of comparative aerial photographs from his 2003 study shows the distinctive pattern of subsidence due to faulting. The marsh surface was continuous across the area in 1953. By 2000 a distinct pattern of the formation of a body of open water with a sharp lateral boundary was obvious. The trace of this boundary is coincident with the trace of the Lake Hatch Fault mapped by Dr. Harry Roberts and his team in their 2001 study

Gagliano, S.M., et.al., 2003, Neotectonic framework of southeast Louisiana and applications to coastal restoration, Trans. G.C.A.G.S., v.

53, p. 262272

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DULAC

Lake Boudreaux

CHAVIN

MONTEGUT

A very similar pattern of wetlands loss can be seen in comparing aerial photographs from near Wonder Lake. In 1971 the lake was surrounded by relatively continuous marsh. By 1998 most of the surrounding area was beginning to convert to open water, but the northern boundary of this forming body of open water had a sharp delineation that was coincident with the trace of the Spur Fault as mapped in the Roberts study. The elevation of the surface of the marsh to the north of this fault, on the upthrown side, remained stable and has maintained its elevation until today. This is evident on this satellite image and on the land loss map on the next page. The aerial photographs also show a portion of the proposed levee as a yellow line. This will be discussed further.

Wonder Lake

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The colored patches on the U.S.G.S. Land Loss Map show the concentrated areas where wetlands loss has occurred. These patches reveal the same pattern of sharp linear edges that are coincident with the faults mapped in the area. There is also a strong correlation between where wetlands loss is occurring and the areas of high subsidence mapped by Penland has his group using historical tidal gauge data. It is the vertical movement of the faults that has subsided the marsh surface below sea level causing the subsided areas to be inundated by water, and appearing to create land loss.

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The two green lines show the location of the stylized cross section that describes the effect of fault movement on the marsh surface. This diagram is also taken from Gaglianos 2003 study, and it clearly illustrates how vertical movement of the faults both subsides and rotates the marsh surface creating open bodies of water that he called fault bays. The sloping marsh surface allows salt water to move into the subsided areas by gravity flow, and it is this rotational sloping of the surface that is the primary mechanism for saltwater intrusion into the marsh that has changed the hydrology. The marsh surface on the northern or upthrown sides of the faults are relatively unsubsided and they have shown virtually no land loss since 1932.

Rotation of the marsh surface by faulting

Fau

lt

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DULAC

Lake Boudreaux

CHAVIN

MONTEGUT

Proposed Morganza to Gulf Levee

Marsh Creation Area

Marsh Creation Area

The proposed levee and sediment pipeline projects can now be put into proper context. The areas of land loss that the marsh creation areas seek to restore have clearly been formed by natural subsidence due to the vertical movement of faults. These same faults have been actively subsiding in this area for millions of years, and it is only the constant re-introduction of sediment of the annual flooding cycle of an active river system that maintained the elevation of the marsh surface. Once that sediment supply was cut off, the marsh began to subside below the surface. The one-time emplacement of sediment by a pipeline project will be immediately subjected to the effects of subsidence. Given the rates of subsidence estimated by Penland, these marsh creation areas would very likely subside below the surface within twenty years of their creation. 17

The proponents of the Morganza-to-the-Gulf Levee and the sediment pipeline project undoubtedly have designed these projects with the best intentions for the people of Terrebonne and Lafourche Parishes in mind. These good intentions cannot, however, overcome the fundamental forces of nature. The rates of subsidence due to faulting in the area examined here make it virtually impossible to build and sustain either the levee, as it is currently proposed, or the marsh creation areas. These high subsidence areas are following the natural progression from marsh to open water that every historical delta system has followed as far back as geologic history records the Mississippi flowing into the Gulf of Mexico. It is useless to attempt to fight these natural forces. Properly using the science discussed here would suggest that flood protection infrastructure should be placed north of the major fault systems across the more stable natural levee systems. The money that might otherwise be spent trying to push back against the insurmountable forces of nature would be more properly invested in educating the public about the realities of subsidence in the coastal wetlands, and the inescapable conclusions that a reasonable projection of the measured rates of subsidence would draw. With the proper information in hand many people living in these high subsidence areas might choose to accept a buyout offer that would allow them to move to higher and more stable ground. The next two pages illustrate nearly all wetlands loss measured across the Louisiana coastal plain since 1932 is primarily due to natural subsidence caused by fault movement. Please feel free to contact me if you would like to discuss this further. Chris McLindon chris_mclindon@att.net

Commentary

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Land Loss in Southeast Louisiana

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