o 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 February 1980 Copyright 1980 by New Orleans Geological Society. TABLE OF CONTENTS 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 metropoli- tan 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~!~ /} ~/~i!!!!iii!!!ill ~iiiiii!iiiii3iiiiiiiii~i!iiii!!iiii!ii!i!i!!iiiii:i~i! ii!~ii - 'iiii~"'~/~ !' !!~ ii 7 ii i Q ~i~ii4: 3 ~: :13!i31~ ~ :!~S m 0 o Q B. ~i':~ii~ ¸ ~ii!~ii31iil ~.~ ~ 0 t~ 2 of the Mississippi River Delta complex. After