Geology of Greater New Orleans

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GEOLOGY OF GREATER NEW ORLEANS

ITS RELATIONSHIP TO LAND SUBSIDENCE AND FLOOD~NO

J.O. Snowden W.C. Ward J.R.J. Studlick

With a GEOLOGIC WALKING TOUR OF DOWNTOWN NEW ORLEANS by L. E. Rieg GEOLOGY OF GREATER NEW ORLEANS:

Its Relationship to Land Subsidence and Flooding

J. O. Snowden W. C. Ward J. R. J. Studlick

With a Geologic Walking Tour of Downtown New Orleans by L. E. Rieg

PUBLISHED BY"

THE NEW ORLEANS GEOLOGICAL SOCIETY, INC P. O. Box 52171 New Orleans, LA 70152

Dan E. Hudson, President Duncan Goldthwaite, Vice. Pres. William W. Craig, Secretary George Hasseltine, Treasurer

James A. Seglund, President-Elect

Geology of Greater New Orleans: Its Relationship to Land Subsidence and Flooding

Introduction ...... 1

Geologic Setting ...... 3

Recent Geologic History of New Orleans ...... 3

Introduction ...... 3

Barrier-Island Sand Trend ...... S

Deltaic Sedimentation - General ...... S

Development of Deltaic Peat ...... 13

The Subsidence Problem ...... 14

Causes of Subsidence ...... 14

Relationship of Subsidence to Sediment Type ...... 14

General History of Subsidence Problems in New Orieans ...... 1S

Differential Subsidence ...... 1S

Hazards and Damage Due to Differential Subsidence ...... 1S

Kenner, Louisiana - A Case History ...... 17

Early Condition - Prior to 1924 ...... 18

Beginning Development - 1924 to 1949 ...... 18

Development after 1949 ...... 18

Subsidence 1924 to 1935 ...... 18

Subsidence 1935 to 1949 ...... 18

Subsidence after 1949 ...... 18

Time-Subsidence Curve - 1924 to 1978 ...... 19

How to Cope With Subsidence ...... 20

Keeping New Orleans Dry ...... 21

Introduction ...... 21

Mississippi River Flood Control ...... 21

Lake Pontchartrain Flood Control ...... 23

The Future ...... 23

Selected Bibliography ...... 24 Geological Walking Tour of Downtown New Orleans

Page

Building and Ornamental Stones ...... 27

Rock Types ...... 27

Igneous Rocks ...... 27

Sedimentary Rocks ...... 27

Metamorphic Rocks ...... 27

Artificial Stone ...... 27

Varieties of Rocks at Keyed Locations ...... 29

Building and Bridge Foundations in New Orleans ...... 30

Customhouse ...... : ...... 30

International Trade Mart ...... 31

Pontalba Apartment Buildings ...... 31

Greater New Orleans Mississippi River Bridge ...... 31

Dumaine Street Levee ...... 31 GEOLOGY OF GREATER NEW ORLEANS: ITS RELATIONSHIP TO LAND SUBSIDENCE AND FLOODING J. O. Snowden I , W. C. Ward i, and J. R. J. Studlick 2

Introduction remains today among the twenty largest metropolitan areas in the United States (Lewis, 1976). Until the early 1900's the city was restricted to the If the founders of New Orleans had known in relatively narrow levees of the Mississippi. This 1718 what geologists know today, they might have situation changed abruptly when inventor-engineer selected another site for the city. The soft and ea- Baldwin Wood designed a heavy-duty pump that sily compacted sediment under the swampy, made it possible to quickly raise huge volumes of marshy plain of the Mississippi River Delta does water a short vertical distance. Drainage canals not provide the ideal foundation for a metropolis. were dredged through the cut-over cypress swamps Growth of this large city in such an unlikely setting north of the city, and Mr.Wood's pumps were used has inevitably brought on chronic environmental to drain the land. Artifical levees were constructed problems directly related to the local geology. This to protect the newly drained land from flood- paper describes the geology-related problems ing. By 1920 developers were building on subsidence and flooding. Most of the information this land, much of it near or slightly below sea presented here is compiled from works published level. It was soon discovered that conventional by the US Army Corps of Engineers, the houses could be built successfully in the drained Department of Earth Sciences at the University of swamp lands without the use of pile-supported New Orleans, the US Soil Conservation Service, foundations. Construction continued until shortly and the Exxon Company. Inasmuch as this paper after World War II, by which time most of the old is written for all interested New Orleans-area cypress swamp had been reclaimed and developed. residents, the language used is non-technical. The remaining undeveloped "land" in Orleans The city of New Orleans was founded as the and adjacent Jefferson Parishes was the brackish- southernmost port of the Mississippi River where water marsh along the southern shore of Lake Pon- goods could be transferred to and from river boats tchartrain. This marshland was drained by the serving the vast interior Mississippi Valley. The same type of canal-and-pump system which had original city was built entirely on the "natural been used earlier in the cypress swamp. However, levee", the ridge of silty sediment that borders the area is underlain by as much as 15 feet of each side of the river. At New Orleans the highest marsh-grass peat, which provides a poor substrate part of the natural levee is about 15 feet above sea for construction. Subsidence of the land surface level; therefore, the levee provided a relatively dry became a major problem in the newly drained and firm foundation for building and, when areas because the underlying organic-rich sedi- augmented by a low artificial levee system, a ment was easily compressed. Today large parts of measure of protection from flooding. The city was the New Orleans metropolitan area must still cope isolated from "mainland" Louisiana by cypress with the damage caused by sinking land. swamps and grassy marshes on the east and west and by Lake Pontchartrain on the north. Thus for Land subsidence has accentuated another major nearly two hundred years New Orleans was an environmental problem in New Orleans -- "island city" accessible only by the river, the flooding. The age-old fight to keep New Orleans coastal water routes, and the shell roads dry is made increasingly difficult as parts of the (frequently washed-out) built on the natural levees. city continue to sink farther below the level of The strategic location of the city, however, more surrounding waters. The danger of flooding in the than offset the natural difficulties, and New New Orleans area varies from place to place, in a Orleans grew rapidly. Census figures show that pattern largely related to the local geology. almost from its beginning, New Orleans was one of the largest cities in North America. By 1835, it To provide the reader with a special insight into was virtually in a tie with Philadelphia for second New Orleans' continuing battle with Nature, we place in population among American cities, and it will present a brief account of the geologic history 1. Department of Earth Sciences, University of New Orleans 2. Shell Oil Company, New Orleans i~!i :III ~ iilI~!~

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2 of the Mississippi River Delta complex. After the seaward by the deltaic accumulation is a complex geologic foundation has been laid, we will discuss of stream channels, levees, swamps, marshes, and the effects of subsidence and the fight against lakes, the whole of which is called the "delta flooding. It will be noted that some parts of plain". Figure 1 shows a portion of the Greater New Orleans are more hazardous from a Mississippi River Delta Plain in the vicinity of New geologic point of view than other parts. Orleans. The Mississippi River Delta region of southern Louisiana is quite young, geologically Geologic Setting speaking, and the deltaic sediments are still soft and unconsolidated. From the beginning, the way of life in New Orleans was greatly influenced by the underlying Recent Geologic History of New Orleans geology. The location of early settlements, the style of buildings, the routes of streets and Introduction highways, the drainage systems, and even the patterns of ethnic populations in older parts of the During the last Ice Age, the area which is now city, all are reflections of the geology of the Missis- southern Louisiana stood a few hundred feet above sippi River Delta. sea level. About 10,000-15,000 years ago the great glaciers begain to melt, and, consequently, sea- The Delta is constructed of billions of tons of level rose. Gulf waters flooded the New Orleans mud and sand that were eroded from the interior area about 5,000-6,000 years ago, and the of our continent, transported southward by the Mississippi River began to build its delta in the Mississippi River, and dumped where the river area southeast of Lafayette, Louisiana (Fig. 2). In entered the sea. The flat, low-lying land area built the last few thousand years the river has switched

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c~ courses several times, and deltas have accumula- portion of the delta surface (Figure 7a) is ted at various sites from the vicinity of Franklin, composed of stream channels called distributaries Louisiana, to south of Biloxi, Mississippi. (Fig. 2). which are flanked by low natural levees. Between the distributaries are troughs which hold As the river continued to dump sediment at its near-sea-level marshes and bodies of shallow mouth, land masses grew progressively farther water. Channels of the principal distributaries seaward. But the great weight of the deltaic extend for some distance across the gently sloping sediment also caused sinking of the earth's crust offshore surface of the delta to the inner margin of beneath the delta complex. When the river the steeper delta front where the distributary- changed course upstream and abandoned a deltaic mouth bars are situated. The offshore channels lobe, that portion of the delta plain continued to are bordered by submarine levees which rise subside, becoming progressively more inundated slightly above the offshore extensions of the inter- by the Gulf. distributary troughs.

Barrier-Island Sand Trend In the process of lengthening its course, the river occupies a succession of distributaries, each 5,000-6,000 years ago, before the beginning of of which is favorably aligned to receive increasing extensive deltaic sedimentation in the vicinity of flow from upstream (Figure 7b). The favored New Orleans, a series of northeast-southwest-tren- distributary gradually widens and deepens to ding sand deposits extended from the Mississippi become the main stream (Figure 7c). Its natural coast well into the New Orleans metropolitan area levees increase in height and width, and marshland (Figures 3 & 4). These are barrier-island, bar, and develops in the troughs adjacent to the shoal sands that were drifted westward by distributary. Levees along the main channel are longshore currents, as shown in Figure 5. Saucier built largely during floodstage. Along the distal (1963) called these sands the "Pine Island beach ends of the distributaries, however, levee trend." Although this sand trend was buried by construction is facilitated by crevasses (Figure 7a), younger Mississippi Delta sediments, it is now in which breach the low levees and permit water and many places only a few feet below the surface, and sediment to be discharged into adjacent troughs thus strongly influences subsurface engineering during intermediate river stages as well as during properties. Figure 6 is a map showing the depth floodstage. Abnormally wide sections of the levee to the top of the buried barrier sands. and of adjacent mudflats and marshes are created by crevassing, and some of the crevasses continue Deltaic Sedimentation - General to remain open and serve as minor distributaries while the levees increase in height. Crevasses also Deposition of the St. Bernard lobe (Fig. 2) of the occur along the main stream during floodstages Mississippi Delta began in the New Orleans area (Figure 7c) and permit tongues of sediment to approximately 4700-4500 years ago (Kolb and extend into the swamps and marshes for others, 1975). It is important to understand the considerable distances beyond the normal toe of stages of deposition and the resulting sediment the levee. types, because these sediments now comprise the land surface and shallow subsurface of Greater Distributaries with less-favorable alignment are New Orleans. Most of the following discussion of abandoned during the course-lengthening process, deltaic sedimentation was taken from Fisk's report and their channels are filled with muddy sediment. on recent Mississippi River sedimentation (Fisk, The marsh lands below New Orleans are veined 1960). with abandoned distributaries associated with the development of the Mississippi's present course. Each of the pre-modern Mississippi River courses was initiated by an upstream diversion The continual migration of various environments similar to the one presently affecting the of deposition produces a body of sediment that is Mississippi as the Atchafalaya River enlarges highly complex. The sediment type under the (Fisk, 1952). Stream capture was a gradual delta plain varies from place to place and from process involving increasing flow through a depth to depth. Most of the sediments are fine diversional arm which offered a shorter route to grained throughout the region, reflecting the type the Gulf. After capture was effected, each new of sediment load transported by the Mississippi course lengthened seaward by building a while the deltaic plain was being built. shallow-water delta and extending it gulfward. Approximately 75 percent of the present-day load Successive stages in course lengthening are of the river is silt and clay, and the remainder is shown diagrammatically on Figure 7. The onshore fine sand (Fisk and others, 1954). Sands are

5 LEGEND J

O ~ MISSISSIPPI RIVER POINT BAR L O C ABANDONED DISTRIBUTARIES E NIZ RELICT BEACHES E / i /~~L P o n tchor / troin / / I /

FIGURE 4 - Major geologic features of metropolitan New Orleans. Numbered contour lines represent the depth below mean sea level of the buried late Pleistocene land surface. These contours also show the land surface as it was 5000-6000 years ago. Other features shown are: the buried islands discussed in text; abandoned St. Bernard Delta distributary channel deposits; and point bar deposits of the modern Mississippi River. The dashed line in Lake Pontchartrain is a major east-west trending fault that was active during late Pleistocene time (adapted from Kolb, Smith and Silva, 1975).

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FIGURE 8 - Progressivestages of peat accumulation during deltaic development (from Fisk, 1960).

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12 deposited in bars at the mouth of the distributaries adjacent trough results in a progressive change and in thin sheets spread by marine currents at the from swamps to brackish-marine and saline delta front. Natural-levee deposits are wedges of marshes(Figure 8d). The entire area eventually silty clay that reach a maximum thickness of 30 becomes a saline marsh cut by tidal streams and feet at the margins of main channels and thin away holding extensive lakes and bays (Figure 8e). from the channels. Fringing the natural levees are Finally, enlargement of the water bodies organic-rich muds that were deposited on mudflats obliterates the marshes, and peat accumulation and in marshes and swamps. ceases.

Development of Deltaic Peat Wave action and flooding associated with the enlargement of the coastal water bodies may Peat and organic muck accumulations are destroy peat accumulations, or it may bury them widespread in several sections of the coastal with marine silts and sands. For example, peat Louisiana lowlands (Dodson, 1942). Data now underlies a thin cover of sandy marine sediment in available from a large number of borings provide the northern part of Chandeleur Sound (Kolb, details concerning local distribution of these 1958). Some of the sands derived from destruction deposits in the deltaic plain. These peats range in of the deltaic plain by marine processes are swept thickness from a few inches to more than 20 feet, to the gulf shore where they are incorporated in depending on the duration of organic accumulation delta-margin islands. Typical of these is Grand and on the amount of local subsidence. Peat is Isle southwest of New Orleans, where sand more largely confined to interdistributary troughs in the than 30 feet thick rests upon peat-bearing marsh various deltas or to levee-flank depressions deposits (Fisk, 1955). marginal to main-course levees. Peats are very thin on the modern delta, but thicker in The distribution and thickness of peat in the interdistributary troughs of abandoned deltas, New Orleans area, as mapped from several where continued subsidence allowed marshes to hundred borings is shown on Figure 9. Cross flourish for long periods. Generally, the thickest sections (Figure 10) show typical stratigraphic rela- peat deposits are in the levee-flank depressions tionships and indicate ages of the peat as along the active and abandoned minor river determined by radiocarbon analyses. Radiocarbon channels. age dates indicate that the peat began to develop approximately 3,000 years ago. In areas relatively The diagrams of Figure 8 show stages in the undisturbed by levee sedimentation, the rate of development of a typical peat deposit; they indicate accumulation averages one foot per 300 years. the changing character of vegetation during levee Changes in density with depth in continous peat enlargement and after abandonment of the sections suggest a decrease in porosity on the distributaries. order of one percent for each foot of burial. By use of radiocarbon dates the rate of compaction is Fresh-water plants are first to appear on determined to be approximately one foot each mudflats in the delta (Figure 8a). Peat begins to 1,200 years. form from the remains of cattails, sedges, and grasses in slightly brackish marshes no more than Peat which accumulated under the changing 18 inches above sea level (O'Neil, 1949). Marshes conditions shown diagrammatically on Figure 8 develop over broad areas within the interdistribut- reaches a thickness of 16 feet in the Bayou ary troughs during enlargement of the levees Bienvenue interdistributary trough between the (Figure 8b). In the central part of the trough, in adandoned Bayou Metairie-Sauvage distributary of areas removed from river sedimentation, peat may the Mississippi and the present channel of the develop entirely from marsh vegetation as the river (Fig. 10, A-A'). The spore-pollen content of trough subsides. Along the margins of a subsiding the peat provides a record of the change in trough, the organic accumulations reflect a vegetation along the Metairie-Sauvage distributary progressive change in vegetation accompanying as it enlarged, while it was being abandoned, and levee enlargement, from fresh-water marshes later as the present brackish marshes developed. through cypress-gum swamps to brackish and saline The interfingering of organic and inorganic marshes. Swamps developing in levee-flank sediment, shown on section A-A' of Figure 10, depressions shift toward the center of an indicates that peat accumulated while the interdistributary trough while a distributary en- Mississippi River was actively enlarging its larges and its levees widen (Figure 8c). After a channels. The peat is split by a wedge of silty-clay distributary is abandoned and river sedimentation natural-levee and crevasse deposits nearly 4 miles ceases, continued subsidence of the levees and wide and by a thin layer of organic-rich silty clay.

13 The local effects of subsidence, resulting from occurs as a result of three principal causes (see compaction of the peaty sediment by the also ASTM, 1965): accumulating mass of the overlying natural-levee deposits, are seen in the thickened levee section (1) Primary consolidation is the reduction in and the downwarping of underlying peat. volume of a soil mass caused by the application of Subsidence after the crevassing, which extended a sustained load to the mass and due principally to the natural levee eastward toward Bayou a squeezing out of water from the void spaces of Bienvenue, permitted the abnormally wide section the mass. of the levee to be buried by swamp and brackish-marsh peats. (2) Secondary compression is the reduction in volume of a soil mass caused by the application of Thick peat deposits southeast of New Orleans in a sustained load to the mass and due to the the English Turn bend of the Mississippi River adjustment of the internal structure of the soil (cross section B-B' of Figure 10) accumulated in an mass after the water is squeezed out. area of more active subsidence. Here the peat interfingers with silty and sandy levee-crevasse (3)Oxidation of organic matter results in the deposits and organic-rich clayey marsh sediment. reduction in volume of a soil mass as chemical These deposits rest upon an interdistributary- reactions occur which cause the organic matter to trough filling of typical silty-clay crevasse deposits, decompose into its mineral constituents. with lenticular silty sands which accumulated while Unnamed Bayou was flowing as a distributary of When the level of the groundwater (water table) the Mississippi River. The sandy channel fill of is lowered, the material above the new water table Unnamed Bayou, its associated natural-levee is no longer buoyed up by the subsurface waters. deposits, produce a gas in the peats. The sandy Therefore, an increased load is placed upon all nature of the nearsurface sediment in the vicinity material below the new water-table elevation. of Unnamed Bayou permitted little compaction, Deep strata, both organic and inorganic, then and the amount of subsidence is relatively small as undergo primary consolidation and secondary compared with that north and east of the river. compression over a period of years. Additional Thin peats east of Unnamed Bayou are split by compaction and subsidence are caused by the organic-rich crevasse deposits laid down during interaction of oxidation and secondary compression early stages of the enlargement of the present in the material above the new water table. Mississippi. Whether the volume change is due to primary consolidation, secondary compression, or oxidation The Subsidence Problem of organic matter, the total amount of subsidence is directly dependent upon the level to which the Subsidence, the relative lowering of the land water table is lowered by drainage. surface with respect to sea level, is a natural consequence of deltaic sedimentation in the New Relationship of Subsidence to Sediment Type Orleans area. Besides this, drainage and development in the city also have caused the When a part of a delta is drained for urban surface to subside. The amount and rate of development, such as in metropolitan New sinking relate to the complex geology of the delta. Orleans, subsidence may be generally accelerated, and different rates among the deltaic sediment Saucier (1963) calculated the average rate of types are very apparent: general subsidence in the New Orleans area to have been 0.39 feet per century for the past 4,400 (1) The natural levee-crevasse silts and sands are years. This figure is based on radiocarbon dates of affected the least. As these deposits formed the peat deposits and does not include the estimated high ground (up to 15 feet above sea level), most rate of sea-level rise during this p~riod. On a were not completely water saturated at the time of smaller scale, the process is acting on individual development. Further, as these coarser sediments landforms at different rates. For example, natural have a grain-support internal structure, they are levees and barrier sands, due to their higher bulk only slightly affected by dewatering of pore spaces. density, may actually subside faster than The same is true of the barrier-island sands. surrounding clay and organic sediments. (2) Backswamp and interdistributary-trough clay Causes of Subsidence deposits, which underlie much of the cypress swamp (Fig. 8) in the New Orleans area are According to Terzaghi (1943), land subsidence subject to shrinkage upon drying, as the internal

14 structure of these clays is partly water-supported. necessary to add up to 3 feet of flU to level and .However, the low permeability of these clays elevate building sites. This is usually done on a usually prevents them from drying to more than a lot-b3;:-lot basis, and a variety of fill materials have few feet below their exposed surface, and, been used, ranging from broken concrete and therefore, subsidence is often minimal. Where the asphalt to topsoil. Sand dredged from the organic matter in these clays is more abundant Mississippi River or other nearby river channels is their subsidence potential is increased because the the prevalent fill material for residential sites at organic matter increases permeability and allows the present Figure 11 is a cross section of the deeper drying. Buried logs and stumps in these slab-and-piling foundation, which has been used deposits may provide pathways for moisture loss, for residential construction in the reclaimed and they decompose when exposed to air, thus marshland since the late 1950's. causing irregular hummocky subsidence. Differential Subsidence (3) Peat deposits in the marsh area (Fig. 8) are highly permeable and have by far the greatest P~bably the greatest single problem has not potential for subsidence when drained. Undrained been the general areal subsidence but the peat is typically 85% water, 10% organic matter, difference in subsidence between houses on pile and 5% mineral matter by weight. Thus, drying of foundations and the surrounding ground surface. the peat immediately causes it to collapse to a When houses or buildings are constructed using small percentage of its original volume. the slab-on-pilings technique (Fig. 11), the foundation is stabilized, but the area surrounding General History of Subsidence the building continues to subside, thus producing Problems in New Orleans differential subsidence. Many homeowners fill their yards with 12 to 24 cubic yards of soil each Prior to the mid-1950's, most of the construction year to compensate for this differential subsidence. in metropolitan New Orleans had been on the natural levees of the present Mississippi River and Figure 12 shows a representative cross section of its former distributary. A few Subdivisions ha~ a house foundation on piles and the surrounding also spread into the drained cypress swamp. Most area after differential subsidence has occured. A residential construction was on raised-floor gap may occur under the house slab, but generally foundations, supported by masonry pillars. the material immediately surrounding the piles Sometimes wooden pilings were driven to support adheres to the piles so that the gap beneath the the foundation pillars, but more often they were slab is much less than the total differential not. Most of these homes are still standing, subsidence. Because the material near the pile although irregular subsidence requires periodic foundation is actually supported to a certain extent foundation levelling in some neighborhoods. by piles, the effect is usally one of greater surface subsidence farther from the house. The most im- An unfortunate coincidence was the widespread portant factor is the magnitude of the differential change to the concrete-slab foundation system by subsidence between the house slab and the residential contractors at the time of urban surrounding area. development of the marshlands in the New Orleans Hazards and Damage Due to Differential area. Some of the early construction of the Subsidence reclaimed marshland proved disastrous. The soft spongy peat failed to support heavy concrete slabs, Major effects of subsidence have been which simply sank into it, occasionally tilting and widespread damage to sewer-, water-, and breaking in the process. It was soon discovered, natural-gas lines, and to" streets, driveways, and however, that if enough wooden pilings were sidewalks, as well as to structures. Recent case driven 30 to 40 feet through the peat into the clay studies have revealed tilting of houses over filled below, the friction on these pilings would support canals, negative skin friction on piles, cracked the slab. In February of 1979, some 25 years after slabs, and other types of structural distress. The the initial development of marshlands, the general difficulties are too numerous and the Jefferson Parish Council passed an ordinance complete ramification of subsidence damage is too requiring residential contractors to use pilings in lengthy to present in this paper; only the worst the thick-peat areas of the Parish. hazard caused by differential subsidence will be discussed. As had been the case in the cypress swamps reclaimed earlier, the drained land surface of the As troublesome as subsidence-caused mainten- marsh is so low and so hummocky that it is ance problems are, the greatest hazard in the

15 J.,,d ' : ' 6."'4" ". "'"':. ~'."-' .4~L ". :,~:"; i~°~ L , • ,~ , °o o o ,o °,~ ° ""• o %° ,. ;;C~..:;:.'.;::.,:t..~';' '" " :..'~, II MoteriolS,~/

m Peot Q,t LI,. ~'~ ~--p i I i ~-~ O ¢1" I O F

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FIGURE 11 - Section of typical slab.on.piling construction before differential subsidence has begun (from snowden, Simmons, Traughber, and Stephens, 1977).

Concrete Slob7

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FIGURE 12 - Section of slab-on-piling construction after differential subsidence (from Snowden, Simmons, Traughber, and Stephens, 1977).

16 marshland peat area is from natural-gas superimposed for reference. All the explosions are explosions. Gas and other utility lines are buried believed to have been caused by subsidence-re- in the peat. The stress created by differential lated gas line ruptures. However, preliminary subsidence is sometimes enough to rupture gas work by Petterson (1976) suggests that the lines, releasing gas into the highly permeable oxidation of the drained peat may also generate drained peat. If the fill layer is less permeable methane gas in potentially explosive quantities than the peat, the gas may migrate some distance, under a sealed concrete slab. eventually accumulating under a concrete slab foundation. Since 1972, five homes have been Kenner, Louisiana - A Case History destroyed by natural-gas explosions. Figure 13 is a map published by Louisiana Gas Services The last major residential development in Company showing measured differential subsi- Jefferson Parish is in the city of Kenner. It is dence rates and recent explosion sites in Jefferson Kenner where the thickest peat is found and the Parish. Lines indicating peat thickness are greatest subsidence problems are encountered.

L ,4 K E O N T C H A R T R A / N

l/ WEST ESPLANADE AVE

NEW ORLEANS INTERNATIONAL AIRI I

f , s / M s

EXPLOSIONS, 1972 - 1976 I. Thien, 3905 Henlcon PI., April 14, 197:) 2. Mortlnez, 3405 Horing Rd., May I, 1974 3. Reggio, 1705 Airline Pork, Oct. 26,1974 4. Little, 3501 Henicon PI., Sept. I, 1975 5. Corona, 3114 Williom$ Blvd.,Oct. 3,1976

LEGEND Me,so.Eo [] 0"- 6" [] 12"-18" SUBSIOENCE [] 6,,-,2,, [] ,8,,-2,,,

VII WL i1 Mill

FIGURE 13 - Measured differential subsidence in Jefferson Parish, Louisiana related to peat thickness. Blank area in western part of map is undeveloped or recently developed. Subsidence data are from Louisiana Gas Services Company (from Snowden, Simmons, Traughber, and Stephens, 1977).

17 The history of subsidence in Kenner is ments began in earnest about 1949 and were representative of what has occured in much of the completed by 1953. This provided the first drained and developed marshland. Examination of opportunity for major housing construction in the this subsidence history should provide a guideline northern section of Kenner. The first residential for establishing the proper waiting time between development in northern Kenner, begun in drainage and construction in order to mitigate 1953-54, had unpaved shell roads. The subsidence-related damage. The following case development continued and accelerated with the history of land subsidence was reported by construction of major thoroughfares such as Traughber, Snowden and Simmons (1978). Interstate Highway 10 and Veterans Highway. Im- provements to existing streets and construction of Early condition - Prior to 1924 new ones brought the development to its present stage. Originally the areas of lower elevation in Kenner were marsh and swamp, similar to those now Subsidence 1924 to 1935 existing in St. Charles Parish to the west. Inasmuch as the level of Lake Pontchartrain As previously stated, the elevation of the original adjacent to the Kenner area is nearly one foot marshland in 1924 was +1 foot MSL. A 1935 above mean sea leuel (MSL), it can be concluded survey shows that subsidence had lowered the that the original marshland was at this same elevation of the area near the lakeshore, generally elevation. The upper layers were generally very north of present-day 42nd Street, between sea level light and, in many cases, tended to float. The and q-1 foot MSL (United States Geological dry-unit weight of this material could be as little as Survey, 1938). The 1935 survey shows the 10 pounds per cubic foot. Drainage of this type of subsidence had caused an interior basin to land results in large amounts of immediate form. This low-lying area, generally between sea subsidence as the unstable upper material level and -1 foot MSL, acted as a temporary compresses. holding reservoir during periods of heavy rain. The area to the south of the interior basin Beginning Development - 1924 to 1949 increased in elevation toward the river.

Initial drainage pumping started in the mid 1920's, with the old New Orleans-Hammond Subsidence 1935 to 1949 Highway (State Highway 33) acting as a protective embankment along the shore Of Lake Pontchar- In 1948 the low lands in Jefferson Parish were train. By 1946 the crest of this load embankment still swampy and supported heavy growths of wild had been reduced from an elevation of +5 feet cane, sawgrass, and other tropical and semi-tropical MSL to approximately +3 feet MSL due to wash or vegetation. Pumping was still beneficial, primarily subsidence. The four pumping stations in only to those areas of Kenner nearest the river. Jefferson Parish were poorly maintained and Pumping Station #3 remained abandoned, and the Pumping Station #3 was taken out of operation in other pumping stations were poorly maintained; 1932 because of a break in the foundation. The only 5 of the original 8 pumps were still operating. pumping primarily benefited those areas of Kenner The New Orleans-Hammond Highway embankment closer to the river, and the northern areas had deteriorated or subsided so that it was remained swampy throughout the period of 1924 to inadequate for protection against even normal high 1949. tides in Lake Pontchartrain. Some measure of protection against flooding was provided by Development after 1949 maximum subsidence areas, which served as catch basins for rain water that slowly drained by On September 19, 1947, a hurricane struck the pumping. These conditions and the fact that the Lake Pontchartrain area, causing sustained area near the lake was still at an elevation between flooding in the lower-lying areas of Jefferson sea level and +1 foot MSL (U.S. Army, Corps of Parish. The old New Orleans-Hammond Highway Engineers, 1948) indicate that no substantial areal embankment was breached and overtopped. The subsidence occured between the years of 1935 and flood damage which occured was the impetus for 1949. construction of a new levee-protection system along the lakeshore and the Jefferson Parish-St. Charles Parish line. Improvements in the pumping Subsidence after 1949 capacity of the drainage system also were implemented. The levee and drainage improve- A survey made in 1970 by the Corps of

18 Engineers shows the same basin-like topography YEAR that existed in 1935. The interior area was on the 1924 1934 1944 1954 1964 1974 1984 1994 2004 2014 order of 1 to 2 feet lower than the strip of high ground near the lake. However, comparison of the I0 1970 survey and the 1935 survey shows a general overall subsidence of approximately 4 feet in the 2O part of Kenner north of Interstate 10 highway.

Since there apparently was no appreciable lowering 3O Estimated Subsidence If No Further Improvements of the water table or subsidence during the period Had Occurred at These 1935-49, it has to be concluded that approximately 4O i mes 4 feet of subsidence occurred in that area during the 21-year period between 1949 and 1970. This 5O has been confirmed by field measurements of areal subsidence near houses that were built at different 6O i--!.... i times. 7O If NO Further 1978 Lowering of \ Because of better drainage, the areas along the ~ Ground...... Water er southernmost part of the interior basin experienced 8O Appro×i~ate-~" ~ urs Subsidence j Estimated ~ .... even more subsidence. Comparison of data from History I Future I If Further I Subsidence | Lowering of 1935 to 1970 indicates that there was approximate- 9O t Ground " ly 5 feet of subsidence between those dates along / Water the southern boundary of the interior basin. This IO0 A Initial Pumping C Improved Surface Drainage is in an area with thick layers of peat (5 to 6 feet) and Levee Protection overlain by thin layers of clay. There are small 13 Minimal Uniform Pumping DIncreased Development and local areas with subsidence probably as much as 7 Subsurface Drainage feet near canals where the drainage drawdown was greatest. FIGURE 14 - Approximate subsidence history and estimated future subsidence for Kenner, Louisiana, north of Interstate 10. Normalized for peat thickness of 8 feet (from Traughber, Snowden, and Simmons, 1978). Time-Subsidence Curve - 1924 to 1978

Field measurements were taken to determine the The new levee and improved drainage system differential subsidence which has occured at caused the subsidence rate to increase in 1949. residences built at various times since about 1949. Although some remedial measures were taken to These measurements and information on subside- improve the flood protection system after the nce for the period 1924-49 were used to determine hurricane of 1947, it was not until 1949 or 1950 the approximate subsidence history for the area of that significant construction began on the levee Kenner north of Interstate 10 Highway (Fig. 14). system. This levee was essentially finished by The curve represents the subsidence history for a 1952 or 1953, and the first residential development location with an average peat thickness of 8 feet. north of Interstate Highway 10 was occupied in Octorber 1954. The real-estate developments at that time had shell roads and side ditches for The time period from 1924 to 1935 (Fig. 14) surface drainage. represents the initial drainage of the area after construction of the Hammond Highway embank- Increased subsidence rates occurred during the ment. During this time the water table was period of 1949-59 (Fig. 14). As the new pumps lowered. At the end of this period the water table and the new levee system improved the drainage, was still approximately at the ground surface, but there was substantial lowering of the water table. the elevation of the ground surface was 1.5 to 2 The rate of subsidence slowed toward the end of feet lower than it was in 1924. The period between the 1950's as the water table again held fairly 1935 and 1949 was a time of minimal and relatively constant during a period of slower development. static drainage levels (Fig. 14). During this period The subsidence rate would have continued to the embankment and pumps were often in a state decrease as estimated by the middle dashed line in of disrepair, with Pumping Station #3 completely Figure 14 if no further improvements in drainage out of operation. Little subsidence occurred during had been made. this time period, and if no further improvements in drainage or levee protection had occurred, the In 1959 or 1960 construction in the area north of settling would have followed a curve something Interstate Highway 10 accelerated with a dramatic like the upper dashed line in Figure 14. increase in the number of paved streets. The roof

19 areas and paved areas greatly reduced the open The barrier-island sediments (Fig. 15) also offer ground available for absorption of rainwater so that potentially good building sites. Earle (1975) the volume of surface runoff increased. This reported that homes built in Eastern New Orleans caused both general lowering of the water table over barrier-island sands buried 10 feet deep or and increased demands on the pumping system. less (the boundaries used on Figure 15) had very The majority of the subsidence due to this few subsidence-related problems. Although the increased development probably occured between barrier island sands do not form ridges of higher 1959 and the mid 1960's. Since the mid 1960's the elevation, their subsidence potential may be almost subsidence rate has slowed, and it is now between as low as that of the natural levees. Additional 0.5 and 1.0 inch per year. If no further changes in research is currently underway at the University of the water-table level occurs, the rate of subsidence New Orleans to investigate the possible relation- should continue to decrease. For this condition, ship of land subsidence to the location and depth the estimated future subsidence is shown by the of burial of the barrier-island sands. lower dashed line extending beyond the year 1978 in Figure 14. However, if there is further lowering Earle (1975) also found that a narrow strip of of the water table, there will be another period of land along the shore of Lake Pontchartrain in rapid subsidence, as indicated by the steeper eastern New Orleans had very low subsidence dashed-line curve. Therefore, any new drainage potential and a somewhat higher elevation than the projects in the area must be carefully designed to surrounding land. This land, which Earle called avoid further lowering of the water table. the "lake fringe", is apparently composed of sand, silt and shell material reworked by wave action. It is very firm and about equal to the natural levee How To Cope With Subsidence deposits in subsidence potential.

Within the New Orleans metropolitan area, the The remaining land in metropolitan New Orleans problem of land subsidence ranges from severe to is chiefly underlain by fine sediment and organic minimal. The prospective homeowner's best matter that collected in backswamp areas of the defense against subsidence is to locate within the Mississippi River, or in interdistributary marshes. areas with the least potential for subsidence. In These wetland sediments all have undergone some the past, information on subsidence potential has degree of subsidence, and the problems that have not been readily available, and, remarkably, resulted vary considerably from area to area. The present property prices seem to be little affected thickness of near-surface peat (Fig. 9) and the by subsidence, except in neighborhoods where it is drainage history are major factors, as indicated by severe and obvious. the Kenner case history.

Figure 15 (in back cover pocket), is a general The geologic map (Figure 15) can be used as a geologic map of greater New Orleans. As general guide only. There are variations within all discussed earlier, the entire region is underlain by of these depositional units that may drastically deltaic and alluvial deposits of the Mississippi affect subsidence potential. River. The differences between these depositional units are subtle and in an urban setting may be The U.S. Soil Conservation Service recently very difficult to recognize without surface completed a detailed soil mapping project of both sampling. There is abundant evidence, however, the east and west banks of Jefferson Parish that subsidence potential is directly related to property. Both of these soil maps, with accompany- sediment type throughout the region. ing booklets, are available free of charge from the U.S. Soil Conservation Service (see Selected Biblio- Natural-levee sediments, both of the present graphy for address). Earle's (1975) study of land Mississippi River and of the abandoned St. subsidence in eastern New Orleans is available for Bernard delta distributary channels, are sands and inspection at all university libraries in New Orleans. silts with small amounts of organic matter. These sediments are relatively firm and well drained and The most important thing a prospective buyer form ridges of the highest elevations within the can do, however, is to carefully inspect the metropolitan area. Most of the older neighbor- property for evidence of subsidence. Driveways, hoods of the city of New Orleans are located on sidewalks, entrance steps, and other structures not these natural levees. The subsidence potential is supported by pilings are good indicators of generally low, and in some cases it has been subsidence. If these have been recently replaced, possible to build on these sediments without using it could be an indication of subsidence. Ask the pilings. neighbors about their subsidence problems.

20 ..~.-----.~,-...... ~-~ t e I [ P 0 N T~ CHAR"Y T RAI

(~z ~ ~L ~T I I

FIGURE 15 GEOLOGIC STREET MAP METROPOLITAN NEW ORLEANS

] NATURAL LEVEE DEPOSITS OF THE MODERN AND ANCIENT MISSISSIPPI RIVER

] LAKE FRINGE DEPOSITS

] ARTIFICIAL FILL (DREDGED FROM LAKE PONTCHARTRAIN)

] FINE GRAINED AND ORGANIC ALLUVIAL AND DELTAIC DEPOSITS• ,./'DEPTH TO BURIED BARRIER ISLAND SANDS-ONLY ~10 DEPTHS OF IO FEET OR LESS ARE SHOWN•

/ /

); / ' __

'/ [ ~"

,'/ .

? / . / ./

k ••U•E •••

t When one is considering buying property in a become stalled near the city. To minimize flooding, neighborhood where the homes are all new, it is man has taken a tip from Nature by (1) building very important to consult available maps to artificial levees to allow the river and lake to rise determine the general underlying sedimentary higher and (2) creating artificial crevasses to unit. In addition, considering the size of release floodwaters into flood basins. investment that a home represents today, it would be wise to take subsurface borings to confirm the Mississippi River Flood Control sediment type. There are a number of firms in the area that will do this at a reasonable price. Bienville, the founder of New Orleans, directed his engineer Tour to construct levees to protect the If these fairly simple precautions are taken, it is city from flooding. The Vieux Carre - the original probably possible to avoid most subsidence city was laid out as a rectangle along the problems altogether. However, if you already own riverbank and protected by the first artificial levees property with subsidence problems, there are on the river, each only three feet high. Each several things you can do to cope with them, short square of the city was surrounded by a drainage of selling the property and moving to a ditch which became filled with water when it non-subsiding neighborhood. If the house is rained, creating "little islands" Residents still supported by adequate pilings, it will not subside refer to these blocks as "ilets." even where subsidence is severe, but it may be necessary to support driveways, garages, patios, As the city spread upriver, construction was and walkways on pilings, or periodically replace begun to control the Mississsippi. By 1735, levees them. Flexible connections on utility lines may be were built on both sides of the river from 30 miles installed to prevent subsidence-related breaks in above the city to 12 miles below. By 1812, water, sewer, and gas pipes. When fill is used to constructed levees stretched upstream to Baton compensate for subsidence, make certain that it is Rouge. sandy material, such as river sand or "spillway" sand. This material, will be permeable enough to Navigation was also of concern, because allow flammable gas to pass through rather than transfer of goods from the river levee to Lake trapping it. When fill is delivered, make sure the Pontchartrain was difficult. Governor Carondelet delivery truck does not run over any utility lines, built a canal from the back of the natural levee to as it may break them. If subsidence has created a Bayou St. John. At the canal's end, a turning gap between the underside of the foundation slab basin was built (now Basin Street). In 1832, the and the ground surface (as in Figure 12), do not New Basin Canal connected the lake directly with seal up this space with fill. Studies have shown the levee back. Pontchartrain Boulevard and that sealed-slab spaces make ideal collection Interstate Highway 10 now follow this abandoned chambers of flammable gases. canal. Other canals were built, not only for navigation, but to drain excess water from the Now that the relationship between lowering of natural levees into the backswamp. Most of these the water table and subsidence is understood, canals were open sewage pits; they have since homeowners should be concerned about new been covered. drainage projects that will lower the regional water tables. From the Kenner study, we can see that City growth continued at a rapid pace, but by this triggers new episodes of subsidence in organic 1900, space had become limited. As the city sediments. expanded toward Lake Pontchartrain, more drainage canals were dug and excess water was pumped to Lakes Pontchartrain and Borgne. Keeping New Orleans Dry Levees were constructed to protect the newly: drained lands. However, this action began a Introduction vicious cycle. Drainage caused land subsidence and lowered the ground elevation; thus, the area New Orleans has developed and prospered in an was more flood-prone and levees had to be built inhospitable physical setting only through the higher. More levees were built to encircle the city resourcefulness and technology of man. Although in a continuous line. man has tried to make New Orleans safe from the surrounding waters, no one can guarantee that After the Orleans Levee Board was established there will be no flooding. Even with the best of in 1920, a 51/2 mile-long seawall was built about controls, localized flooding has occurred, especially half a mile out from the old shoreline of during long periods of intense rain when fronts Lake Pontchartrain. The area back of the

21 MORGANZA FIOOOWAY

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FIGURE 16 - Lower Mississippi Valley, showing locations of the Bonnet Carre and Morganza Spillways. seawall was filled with lake-bottom deposits. The heights. More than 2,200 miles of levees were result of this effort was the Lakefront Development built to control floods. These modern levees are - more than 2,000 acres of extra land at 5 to 10 feet higher and broader earthen embankments (10 above lake level, the highest part of the city except times as wide at the base as they are high). for the levees along the river. Berms, additional earth structures, were added on the inner levee sides to add mass for stability and seepage control. In addition, channels were With improved technology and suburban sprawl, stabilized by building revetments on the outside more swamps were drained, and by the 1950's the bends of the river to reduce erosion. city began to grow eastward and westward. Much of the development within the city could not have More important to the existence of New Orleans occurred without the continuing control 6f the was the building of floodways which allow the Mississippi River. After a major flood in 1927, passage of excess flow during flood stages. At massive federal funding was focused on the lower Bonnet Carre (30 miles north of New Orleans; Fig. river from Cairo, Illinois, to the mouth, with the 16) and at Morganza (at the Mississippi state line; goal of encouraging the river to develop and Fig. 16) the Army Corps of Engineers constructed maintain a channel which was navigable and which floodways to guarantee flood control for cities would contain flood waters. The initial effort was a farther down the river. These flood gates not only shortening of the river by cutting through narrow prevent breaks in the levee, but also can be necks of land between river loops. This resulted in intentionally opened to keep flood waters from a faster flow of flood water and lower flood reaching New Orleans.

22 Since 1932, the Bonnet Carre Spillway has been water toward the southern shore. Lakefront levees opened to Lake Pontchartrain five times. The would then be topped by waters and flooding operation of the spillway may have short-term would result. adverse environmental effects caused by the sudden influx of river sediments into Lake Hurricanes occur every two or three years in the Pontchartrain and by mixing of flood waters with area, but, fortunately, no hurricane has yet taken the brackish and salty lake waters. However, some such a critical path to the city. Hurricane Betsy in longer-term effects are compensating. For 1965 and Camille in 1969 caused extensive flooding example, some fish populations are expanding due although the storm center was more than fifty to the increased nutrient supply, and sediment miles away. In response to this threat, an trapped in the spillway is used as a source of ambitious hurricane protection project was begun landfill material. in 1967. Three control complexes were planned at points of tidal access into the lake: (1) Rigolets in The Morganza Floodway (Fig. 16) is capable of the easternmost portion of Lake Pontchartrain, (2) flooding a 20-mile-wide path through the Chef Menteur Pass in the southeastern part of the Atchafalaya Basin to the Gulf and has been used lake, and (3) Seabrook, a navigation lock at the only once (1963) since it was built in 1954. When Industrial Canal entrance. Control structures at opened, it saved New Orleans but extensively these sites will include a series of gates, levees, damaged many settlements, including Morgan dams, and locks. A network of levees and City. floodwalls will encircle the Industrial Canal area, Further flood protection at Simmesport in the New Orleans East, and Chalmette. Along the south shore of the lake, levees will protect St. West Atchafalaya Floodway was designed to carry Charles and Jefferson Parishes, while Mandeville floodwaters not handled by the two floodways on the north shore will be protected by seawalls. constructed earlier. In addition, construction of In addition, an evacuation plan has been developed locks at Old River (Fig. 16) about eighty miles to further safeguard residents of this area. above Baton Rouge prevented the Atchafalaya River from capturing the flow of the Mississippi The Future and provided further means of controlling floods.

Lake Pontchartrain Flood Control Substantial efforts have been made to control flooding in the New Orleans area, but the mighty Lake Pontchartrain presents the threat of Mississippi River is difficult, perhaps ultimately hurricane-induced flooding. If a hurricane were to impossible, to control, and the effects of a approach New Orleans from the Gulf along a critical-path hurricane are unknown. The survival certain critical path east of the city, winds would and growth of New Orleans require an unending blow from the south and/or east resulting in high struggle to modify and contain natural processes. tides in Lake Borgne and Lake Pontchartrain. What progress is made in this direction depends, Once the eye of the hurricane moved past the city, in large part, on a knowledge of the geology of the winds would become northerly, pushing the lake Mississippi River Delta. •

23 SELECTED BIBLIOGRAPHY

American Society of Testing Materials, 1965, Gagliano, S.M. and J.L. VanBeek, 1970, Geologic Standard definitions of terms and symbols and geomorphic aspects of deltaic processes, relating to soil mechanics: ASTM Standards, Mississippi Delta Systems: Center for Part II. Wetlands Resources, Louisiana State Univer- sity, Baton Rouge, Report 1. Coleman, J.M. and S.M. Gagliano, 1964, Cyclic Gould, H.R. and J.P. Morgan, 1962, Coastal sedimentation in the Mississippi deltaic plain: Louisiana swamps and marshland: in Geology Gulf Coast Assoc. Geol. Societies, Transac- tions, v 14, p. 67-80. of the Gulf Coast and Central Texas, published by Houston Geological Society for the 1962 Dodson, W.R., 1942, Observations and studies on annual meeting of the Geological Society of the peat deposits of Louisiana: Louisiana State America, p. 287-341. University Bulletin 343, 27 p. Kolb, C.R., 1958, Geological investigation of the Mississippi River-Gulf outlet channel: Vicks- Earle, D., 1975, Land subsidence problems and burg, Miss., U.S. Army Corps of Engineers maintenance costs to homeowners in east Misc. Paper 3-259, 22 p. New Orleans, Louisiana: Occasional Paper No. 1, School of Environmental Design, Kolb, C.R., F.L. Smith, and R.C. Silva, 1975, Dept. of Landscape Architecture, Louisiana Pleistocene sediments of the New Orleans- State University, Baton Rouge, 12 p. Lake Pontchartrain Area: Vicksburg, Miss., U.S. Army Corps of Engineers Exp. Sta. Tech. Fisk, H.N., 1952, Geologic investigation of the Report S-75-6, p. 49. Atchafalaya Basin and the problem of Kolb, C.R. and J.R. VanLopik, 1958, Geology of Mississippi River diversion: U.S. Army Corps the Mississippi River deltaic plain, southeast- of Engineers, Vicksburg, Miss., 145 p. ern Louisiana: Vicksburg, Miss., U.S. Army Corps of Engineers Exp. Sta., Tech. Report Fisk, H.N., 1955, Sand Facies of recent 3-483, p. 120. Mississippi delta deposits: 4th World Petroleum Congress Proceedings, Sec. 1, LeBlanc, R.J., 1973, Significant studies of modern p 377-398. and ancient deltaic sediments: Gulf Coast Assoc. Geol. Societies Transactions, v. 23, Fisk, H.N., 1960, Recent Mississippi River p. 18-21. sedimentation and peat accumulation: in, Ernest Van Aelst, editor, Congres pour Lewis, P.F., 1976, New Orleans, the making of an l'avancement des etudes de Stratigraphie et de urban landscape: Cambridge, Mass., Ballinger Geologie du Carbonifere, 4th, Heerlen, 1958: 115 p. Compte Rendu, v.I, p. 187-199. O'Neil, T., 1949, The muskrat in the Louisiana Fisk, H.N. and E. McFarlan, Jr., 1955, Late coastal marshes: Louisiana Dept. Wildlife and Quaternary deltaic deposits of the Mississippi Fisheries, 152 p. River: in Poldervaart, A., editor, The Crust of the Earth, Geol. Soc. America Spec. Paper 62 Otvos, E.G., 1978, New Orleans-South Hancock p. 279-302. Holocene barrier trends and origins of Lake Pontchartrain: Gulf Coast Assoc. Geol. Fisk, H.N., E. McFarlan, Jr., C.R. Kolb, and L.F. Societies Transactions, v. 28, p. 337-355. Wilbert, 1954, Sedimentary framework of th~ modern Mississippi delta: Jour. Sed. Petrology Petterson, R.C., 1976, Unpublished report to v. 25, p. 76-99. Louisiana Gas Services Company.

Frazier, D.E., 1967, Recent deltaic deposits of the Saucier, R.T., 1963, Recent geomorphic history of Mississippi River: Their development and the Pontchartrain basin: Louisiana State chronology: Gulf Coast Assoc. Geol. Societies University Studies, Coastal Studies Series, No. Transactions, v. 17, p. 287-315. 9, 114p.

24 Snowden, J.O., Wm. B. Simmons, E.B. Traughber United States Army Corps of Engineers, 1970, 1979, Subsidence of Marshland peat in the Unpublished survey data, New Orleans New Orleans area, Louisiana: in J.W. Day, District, Flood Plain Planning Branch. Jr., D.D. Culley, Jr., R.E. Turner and A.J. Mumphrey, Jr., editors, Proceedings, Third United States Geological Survey, 1938, Indian Coastal and Estuary Management Symposium, Beach Quadrangle Map, Jefferson Parish, Louisiana State Univ. Division of Continuing Louisiana. Education, Baton Rouge, p. 273-292. United States Soil Conservation Service, 1970, Soil Snowden, J.O., Wm. B. Simmons, E.B. Traughber Survey of portions of Jefferson, Orleans, and and R.W. Stephens, 1977, Differential St. Bernard parishes, Louisiana: U.S. Dept. of subsidence of marshland peat as a geologic Agriculture, Soil Conservation Service, 3445 N. hazard in the Greater New Orleans Area, Causeway Blvd., Metairie, Louisiana 70002, p. Louisiana: Gulf Coast Assoc. Geol. Societies 125. Transactions, v. 27, p. 169-179. United States Soil Conservation Service, 1977, Soil Terzaghi, K., 1943, Theoretical soil mechanics: survey of the east bank of Jefferson Parish, New York, Wiley, p. 510. Louisiana: U.S. Dept. of Agriculture, Soil Conservation Service, 3445 N. Causeway Traughber, E.B., J.O. Snowden, and Wm. B. Blvd., Metairie, Louisiana 70002, p. 79. Simmons, 1978, Differential subsidence on reclaimed marshland peat in metropolitan New United States Soil Conservation Service, 1978, Soil Orleans, Louisiana: Proceedings, Engineering survey of the west bank of Jefferson Parish, Foundations Conference-Evaluation and Predi- Louisiana: U.S. Dept of Agriculture, Soil ction of Subsidence, American Society of Civil Conservation Service, 3445 N. Causeway Engineers, New York, p. 479-499. Blvd., Matairie, Louisiana 70002, p.125.

United States Army Corps of Engineers, 1948, 15 Wagner, F.W. and E.J. Durabb, 1976, The sinking April Review Report, Lake Pontchartrain, city: Environment, v. 18, No. 4, p.32-39. Louisiana.