Geologic Conditions Beneath the New Orleans Area J. David Rogers, Ph.D., P.E., P.G. Karl F. Hasselmann Chair in Geological Engineering and Associate Director Natural Hazards Mitigation Institute University of Missouri-Rolla • Areal distribution of abandoned channels and distributaries of the Mississippi River. The Metairie Ridge distributary channel (highlighted in red) lies between two different depositional provinces in the center of New Orleans. • Major depositional lobes identified in lower Mississippi Delta around New Orleans. • North-south geologic cross section through the central • Gulf of Mexico Coastal Plain, along the Mississippi River Embayment. Note the axis of the Gulf Coast Geosyncline beneath Houma, LA, southwest of New Orleans. In this area the Quaternary age deposits reach a thickness of 3600 ft. • Transverse cross section in a west to east line, across • the Mississippi River Delta a few miles south of New Orleans, cutting across the southern shore of Lake Borgne. New Orleans is located on a relatively thin deltaic plain towards the eastern side of the delta’s depositional center, which underlies the Atchafalaya Basin, west of New Orleans. • Pleistocene geologic map of the New Orleans area. The yellow stippled bands are the principal distributory channels of the lower Mississippi during the late Pleistocene, while the present channel is shown in light blue. The Pine Island Beach Trend is shown in the ochre dotted pattern. Depth contours on the upper Pleistocene age horizons are also shown. • Contours of the • entrenched surface of the Wisconsin glacial age deposits underlying New Orleans. Note the well developed channel leading southward, towards what used to be the oceanic shoreline. This channel reaches a maximum depth of 150 feet below sea level. • Areal distribution and depth to top of formation isopleths for the Pine Island Beach Trend beneath lower New Orleans. • Block diagram of the geology underlying New Orleans. The principal feature dividing New Orleans is the Metairie distributary channel, shown here, which extends to a depth of 50 feet below MGL and separates geologic regimes on either side. Note the underlying faults, beneath Lake Pontchartrain. • Block diagram illustrating relationships between subaerial and subaqueous deltaic environments in relation to a single distributary lobe. The Lakeview and Gentilly neighborhoods of New Orleans are underlain by interdistributary sediments, overlain by peaty soils lain down by fresh marshes and cypress swamps. • Sedimentary sequence caused by overlapping cycles of deltaic deposition, • along a trend normal to that portrayed in the previous figure. As long as the distributary channel receives sediment the river mouth progrades seaward. The Lakeview and Gentilly neighborhoods lie on a deltaic plain with marsh and swamp deposits delta front deposits closer to Metairie-Gentilly Ridge, the nearest distributary channel. • Portion of the 1849 flood map showing the mapped demarcation between brackish and fresh water marshes along Lake Pontchartrain. This delineation is shown on many of the historic maps, dating back to 1749. 1816 flood map of New Orleans showing areal distribution of cypress swamps north of the old French Quarter. These extended most of the distance to the Lake Pontchartrain shore. • Distribution and apparent thickness of surficial peat deposits in vicinity of New Orleans. • Geologic map of the greater New Orleans area. The sandy materials shown in yellow are natural levees, green areas denote old cypress swamps and brown areas are historic marshlands. The stippled zone indicates the urbanized portions of New Orleans. • Geologic cross section along south shore of Lake Pontchartrain in the Lakeside, Gentilly, and Ninth Ward neighborhoods, where the 17th Street, London Avenue, and IHNC levees failed during Hurricane Katrina on Aug 29, 2005. Notice the apparent settlement that has occurred since the city survey of 1895 (blue line), and the correlation between settlement and non-beach sediment thickness. • Wood and other organic debris was commonly sampled in • exploratory borings carried out after Hurricane Katrina throughout the city. This core contains wood from the old cypress marsh that was recovered near the 17th Street Canal breach. Organic materials are decaying throughout the city wherever the water table has been lowered, causing the land surface to subside. • Overlay of 1872 map by Valery Sulakowski on the WPA-LA (1937) map, showing the 1872 shoreline and sloughs (in blue) along Lake Pontchartrain. Although subdivided, only a limited number of structures had been built in this area prior to 1946. The position of the 2005 breach along the east side of the 17th Street Canal is indicated by the red arrow. • Aerial photo of the 17th Street Canal breach site before the failure of August 29, 2005. The red lines indicate the positions of the geologic sections. • West-to-east geologic cross section through the 17th Street Canal • failure approximately 60 feet north of the northern curb of Spencer Avenue, close to the yellow school bus. A detailed sketch of the basal rupture surface is sketched above right. The slip surface was about one inch thick with a high moisture content (watery ooze). A zone of brecciation 3 to 4 inches thick was above this. Numerous pieces of cypress wood, up to 2 inches diameter, were sheared off along the basal rupture surface. • West-to-east geologic cross section through the 17th Street Canal • failure approximately 140 feet north of the northern curb of Spencer Avenue. Large quantities of bivalve shells were extruded by high water pressure along the advancing toe thrusts. Note the slight back rotation of the distal thrust sheet. • Bivalve shells ejected by high pore pressures emanating from toe thrusts on landside of failed levee at the 17 Street Canal (detail view at upper left). These came from a distinctive horizon at a depth of 2 to 5 feet below the pre-failure grade. • Stratigraphic interpretations and cross-canal correlations in vicinity of the • 17th Street Canal breach on August 29, 2005. The swamp much appeared to be thinning northerly, as does the underlying Pine Island Beach Trend. The lacustrine clays appear to thicken southward, as shown. The approximate positions of the flood walls (light blue) and canal bottom (dashed green) are indicated, based on information provided by the Corps of Engineers (IPET, 2006). SOURCES OF GROUND SETTLEMENT • The causes of historic settlement are a contentious issue in coastal Louisiana • There appear to be many different causes, summarized in the following slides • Topographic map with one foot contours prepared under the direction of New Orleans City Engineer L.W. Brown in 1895. This map was prepared using the Cairo Datum, which is 21.26 feet above Mean Gulf Level • Map • showing relative elevation change between 1895 and 1999/2002, taken from URS (2006). The net subsidence was between 2 and 10+ feet, depending on location. • Block diagram illustrating various types of subaqueous sediment instabilities in the Mississippi River Delta, taken from Coleman (1988). • Geologic cross section through the Gulf Coast Salt Dome Basin, taken from Adams (1997). This shows the retrogressive character of young lystric normal faults cutting coastal Louisiana, from north to south. The faults foot in a basement-saltdecollement surface of middle Cretaceous age (> 100 Ma). •This plot illustrates the en-echelon belts of growth faults forming more or less parallel to the depressed coastline. The Baton Rouge Fault Zone (shown in orange) has emerged over the past 50 years, north and west of Lake Pontchartrain. • Lowering the water table increases the effective stress on underlying sediments and hastens rapid biochemical oxidation of organic materials, causing settlement of these surficial soils. soils • The upper photo shows gross near-surface settlement of homes in the Lakeview neighborhood, close to the 17th Street Canal breach. Most of the homes were constructed from 1956-75 and are founded on wood piles about 30 feet deep. • The lower photo shows protrusion of a bricklined manhole on Spencer Avenue, suggestive of at least 12 inches of near surface settlement during the same interim. • Record of historic settlement in the town of Kenner, which is characterized • by 6.5 to 8 feet of surficial peaty soils. The steps in these curves were triggered by groundwater withdrawal for industrial use and urban development. This area was covered by dense cypress swamps prior to development. • Structural geologic framework of the lower Mississippi River Delta. Growth faults (solid black lines) perturb the coastal deltaic plain, as do salt domes (shown as dots). This study did not uncover evidence of growth faults materially affecting any of the levee failures from Hurricane Katrina, although such possibility exists. Mechanisms of Ground Settlement -1 • Elastic deformation of Mississippi Delta from silt deposition (isostasy) • Tectonic compaction caused by formation of pressure ridges and folding • Subsidence on seaward side of lystric growth faults • Drainage of old swamp and marsh deposits increasing stress on underlying clays • Biochemical oxidation of peaty soils Mechanisms of Ground Settlement -2 • Consolidation of compressible soils caused by surcharging with fill • Surcharging by structural improvements • Reduced groundwater recharge because of increase in impermeable surfaces • Extraction of oil, gas, and water casing pressure depletion • Solutioning of salt domes and seaward migration of low density materials (salt and shale) Coastal land loss in Louisiana is also exacerbated by sea level rise, which is currently averaging about 12 inches per century. CONCLUSIONS-SETTLEMENT and LAND LOSS • Coastal Louisiana is losing between 25 and 118 • • • square miles of land per year The approximate rate of subsidence is about 4.2 ft per century, while sea level rise is about 1 ft per century The soft, compressible materials underlying much of New Orleans provide poor foundations, which are sensitive to drainage and loading. Ongoing ground settlement and sea level rise have combined to create a tedious situation, whereby flood control infrastructure must continually be improved and elevated to provide a consistent level of protection. CONCLUSIONS-SITE CHARACTERIZATION • Most of greater New Orleans is underlain by • • • several layers of cypress swamp deposits These materials are highly permeable and compressible, with a dry density lower than water These materials commonly contain numerous weak horizons, often containing thin lenses of flocculated clay; all of which were found to exhibit strain softening This strain softening causes a significant loss of shear strength; sometimes diminishing to near zero. CONCLUSIONS – COST BENEFIT RATIOS • If the Corps of Engineers had authorized about $10 million more for detailed subsurface exploration and insitu soil testing beneath the flood wall additions erected in the last 25 years; • They might have prevented $100 billion in flood damages • This would have netted a cost-benefit ratio of 100,000 to 1 Sea-Level Rise and Subsidence: Implications for Flooding in New Orleans, Louisiana By Virginia R. Burkett1, David B. Zilkoski2, and David A. Hart3 Abstract Global sea-level rise is projected to acceler­ ate two- to four-fold during the next century, increasing storm surge and shoreline retreat along low-lying, unconsolidated coastal margins. The Mississippi River Deltaic Plain in southeastern Louisiana is particularly vulnerable to erosion and inundation due to the rapid deterioration of coastal barriers combined with relatively high rates of land subsidence. Land-surface altitude data collected in the leveed areas of the New Orleans metropoli­ tan region during five survey epochs between 1951 and 1995 indicated mean annual subsidence of 5 millimeters per year. Preliminary results of other studies detecting the regional movement of the north-central Gulf Coast indicate that the rate may be as much as 1 centimeter per year. Con­ sidering the rate of subsidence and the mid-range estimate of sea-level rise during the next 100 years (480 millimeters), the areas of New Orleans and vicinity that are presently 1.5 to 3 meters below mean sea level will likely be 2.5 to 4.0 meters or more below mean sea level by 2100. Subsidence of the land surface in the New Orleans region is also attributed to the drainage and oxidation of organic soils, aquifer-system compaction related to ground-water withdrawals, natural compaction and dewatering of surficial sed­ iments, and tectonic activity (geosynclinal downwarping and movement along growth faults). The problem is aggravated owing to flood-protection measures and disruption of natural drainageways that reduce sediment deposition in the New Orleans area. 1 U.S. Geological Survey, Lafayette, La. National Oceanic and Atmospheric Administration, National Geodetic Survey, Silver Spring, Md. 3 University of Wisconsin, Madison, Wis. 2 Accelerated sea-level rise, the present altitude of the city, and high rates of land subsid­ ence portend serious losses in property in the New Orleans area unless flood-control levees and pumping stations are upgraded. The restoration and maintenance of barrier islands and wetlands that flank New Orleans to the south and east are other adaptations that have the potential to reduce the loss of life and property due to flooding. Accu­ rate monitoring of subsidence is needed to provide calibration data for modeling and predicting sub­ sidence in coastal Louisiana, as well as for support for constructing and maintaining infrastructure and levees. GPS technology is being tested in the New Orleans region as a means for more frequent, less expensive subsidence monitoring. INTRODUCTION Accelerated sea-level rise is regarded as one of the most costly and most certain consequences of global warming. If sea-level rise increases at rates projected by the United Nation’s Intergovernmental Panel on Cli­ mate Change (2001) during the next century, many of the world’s low-lying coastal zones and river deltas could be inundated. Several of the world’s most heavily populated coastal cities are particularly vulnerable to inundation due to human interactions with deltaic pro­ cesses. Such is the case in the New Orleans metropoli­ tan area, where more than 1 million people are protected from river floods and storm surge by levees and pump­ ing stations, and where the land is gradually sinking at rates that exceed 20th century sea-level rise. GEOMORPHOLOGIC SETTING Most of the present landmass of southeast Louisi­ ana was formed by deltaic processes of the Mississippi River. Over the past 7,000 years, during a period of relatively small fluctuations in sea level, the river deposited massive volumes of sediment in five deltaic Abstract 63 Subsidence Observations Based on Traditional Geodetic Techniques, and Numerical Models complexes that now lie in various stages of abandon­ ment (, 1967). The Chandeleur Island chain that lies to the southeast of the city of New Orleans is an ero­ sional feature of one of these ancient deltas. A com­ bination of levees, diversion structures, and reduced suspended sediment discharge have essentially halted the aggradation of the Mississippi River delta in south­ east Louisiana. Levees constructed along the banks of the Missis­ sippi River from Cairo, Ill., to Venice, La., (about 30 km south of New Orleans) prevent the flooding of the adja­ cent land by sediment-laden river water, halting the dep­ ositional processes that naturally maintained the altitude of the land surface in southeast Louisiana above sea level. Three large diversion structures constructed upriver near Simmesport, La., now route up to one-third of the water and sediment load from the Mississippi River westward into the Atchafalaya River to protect New Orleans, Baton Rouge, and many other cities in southeast Louisiana from flooding. The volume of sed­ iment delivered by the Mississippi River to Louisiana has been reduced by almost one-half since 1950 by the construction of reservoirs on the major tributaries of the Mississippi River (Meade, 1995). Most of the land surface of the New Orleans Met­ ropolitan Statistical Area (MSA), a region that includes all or parts of seven parishes, is sinking or “subsiding” relative to mean sea level. Subsidence of the land sur­ face in the New Orleans region is also attributed to the drainage and oxidation of organic soils (Earle, 1975), aquifer-system compaction related to ground-water withdrawals (Kazmann, 1988), natural compaction and dewatering of surficial sediments (Gosselink, 1984), and tectonic activity (geosynclinal downwarping and movement along growth faults) (Howell, 1960; Jones, 1975). SUBSIDENCE AND SEA-LEVEL TRENDS Observations of local subsidence in the New Orleans region were derived from precise leveling data collected by the National Geodetic Survey (NGS) dur­ ing 1951–55, 1964, 1984–85, 1990–91, and 1995. The subsidence network included a total of 341 benchmarks. Land-surface altitude data sets for each epoch (time period between surveys) were prepared using a mini­ mum constraint least squares adjustment tied to a benchmark in eastern Orleans Parish (Zilkoski and Reese, 1986). Files containing the location of benchmarks and the differences in adjusted heights 64 were converted into ArcView shapefiles and projected into Louisiana State Plane Coordinates, South Zone NAD83, in feet. The annual rate of subsidence at each benchmark was determined by dividing the differences in adjusted heights by the number of years between leveling. Benchmark locations were integrated with gener­ alized maps of soils and geology covering parts of Orleans Parish, which lies at the center of the New Orleans MSA. It is important to note that the soil and geology data sets were digitized from small-scale, paper, photocopied maps to test the initial concepts of using GIS to support development of a subsidence model (Hart and Zilkoski, 1994). The source of the geology map is “Geology of Greater New Orleans—Its Relationship to Land Subsidence and Flooding” by Snowden and others (1980), and the source of the soils map is the “Soil Survey of Orleans Parish, Louisiana” by the Soil Conservation Service (Trahan, 1989). Figure 1 shows subsidence rates for 165 bench­ marks that were consistently surveyed during the period from 1951 to 1995. Table 1 shows the number of bench­ marks surveyed, mean annual subsidence rate, and standard deviation for soils and geologic units for each of the four epochs identified above. The average rate of subsidence among soil types was between 4.0 and 6.0 mm/yr for all but the Aquents soil classification, which makes up about 13 percent of the land area in the Parish (Trahan, 1989). There appears to be a noticeable decrease through time in the mean subsidence rate for the Clovelly-Lafitte-Gentilly soil classification as com­ pared to the others. Also, the overall mean subsidence rate for all soil types increases from the 1951–64 epoch to the 1964–85 and 1985–91 epochs, and then apparent rebound is seen during the 1991–95 epoch. Precipitation was very heavy in the New Orleans region during 1991, which may be related to the apparent high rates of subsidence during 1985–91. Additional correlations may exist between land subsidence and other, more detailed and accurate soil and geology data sets, as well as other environmental factors that may have an effect on subsidence. These other environmental factors include drainage infrastructure, levee locations, drain­ age pumping-station operations, well locations and withdrawals, ground-water recharge, application of fill and overburden, land use, the history of human settlement and urban development, and the bulk and density of buildings. The 1951–95 altitude data also showed some interesting differences among survey epochs and Sea-Level Rise and Subsidence: Implications for Flooding in New Orleans, Louisiana o M i i ssipp ssi SUBSIDENCE AND SEA-LEVEL TRENDS Jefferson Parish Rive W090o 7.5' 0 Orleans Parish St. Bernard Parish Plaquemines Parish Lake Pontchartrain X X 6 13 X 19 Hydrography Streets Parish boundary Uplift 26 KILOMETERS Less than 5 mm/yr Greater than 5 mm/yr Subsidence 1951–95 EXPLANATION Figure 1. Mean annual local subsidence rates for five soil types and the four major geologic units in the New Orleans region. N30 W090 o15' r 65 66 Sea-Level Rise and Subsidence: Implications for Flooding in New Orleans, Louisiana 14 56 280 Alemmands, drained— Kenner, drained Aquents Total 264 Total 7 Lake fringe deposits 166 63 Alluvial soils Natural levee deposits 28 Artificial fill marks bench- ber of Num- 36 Harahan-Westwego Geologic unit 13 161 marks bench- ber of Num- Clovelly-LafitteGentilly Sharkey-Commerce Soil type Num- -5.2 -5.6 -8.1 -3.9 -4.8 4.7 5.1 8.3 3.4 2.1 (mm) ence (mm/yr) dev. differ- 209 124 3 56 26 marks bench- ber of 7.2 .4 4.5 5.5 -13.6 -6.4 -6.8 3.5 (mm) dev. Std. 5.4 4.8 5.4 9.2 1.9 3.8 (mm) dev. Std. -5.7 -10.6 (mm/yr) ence differ- annual Average Std. annual Num- -6.5 -9.4 -8.1 -3.3 -2.2 Average 234 49 10 33 12 -6.4 (mm/yr) ence differ- 1964–85 4.4 3.9 4.3 4.3 1.7 130 marks bench- ber of 1951–64 -4.5 -3.9 -5.5 -3.3 -3.0 -5.0 4.7 (mm) ence (mm/yr) dev. differ- annual Average Std. Average annual 1964–85 1951–64 [mm/yr, millimeters per year; mm, millimeters] -8.5 -9.2 -8.4 17 66 334 280 158 6 86 30 marks bench- ber of Num- -9.3 58 -9.1 -9.2 -9.5 -8.6 -9.7 (mm/yr) ence differ- annual Average 1985–91 -1.8 -8.5 (mm/yr) ence differ- annual Average 17 176 marks bench- ber of Num- 1985–91 3.1 3.6 2.2 2.4 1.1 (mm) dev. Std. 3.6 3.2 2.7 5.4 1.8 2.4 (mm) dev. Std. 306 171 7 98 30 marks bench- ber of Num- 362 74 17 65 19 187 marks bench- ber of Num- 3.3 4.1 .5 2.5 2.2 (mm/yr) ence differ- annual Average 1991–95 2.7 .8 2.1 2.3 .6 3.9 (mm/yr) ence differ- annual Average 1991–95 3.8 2.4 2.7 5.6 1.8 (mm) dev. Std. 3.9 3.3 4.3 6.7 2.3 2.2 (mm) dev. Std. 132 73 3 36 20 marks bench- ber of Num- 148 33 4 16 8 87 marks bench- ber of Num- Table 1. Mean annual subsidence rates for five soil types and the four major geologic units in the New Orleans region, 1951–95 -5.2 -4.9 -9.1 -4.6 -6.5 (mm/yr) ence differ- annual Average 1951–95 -4.8 -5.9 -4.0 -4.0 -1.3 -4.8 (mm/yr) ence differ- annual Average 1951–95 Std. 2.8 3.0 .7 2.4 1.6 (mm) dev. Std. 2.9 2.6 2.4 3.2 .8 2.8 (mm) dev. Subsidence Observations Based on Traditional Geodetic Techniques, and Numerical Models geologic units (table 1). Mean annual subsidence in levee deposits, alluvial soils, artificial fill, and lake fringe deposits ranged from 4.6 to 9.1 mm/yr. It should be noted that one recent analysis of NGS Gulf Coast elevation data by Louisiana State University and NGS (Roy Dokka, Louisiana State University, oral commun., 2003) suggests that the absolute subsidence rates for the New Orleans region could be about 5 mm/yr or higher, but the relative differences would be the same. Relative differences in subsidence rates among the four survey epochs might be explained by a more thorough exami­ nation of rainfall data, ground-water extraction and recharge, land-use change, and other factors mentioned previously. Global sea level has risen about 120 m as a result of melting of large ice sheets since the last glacial maximum about 20,000 years ago (Fairbanks, 1989). The most rapid rise occurred during the late and post­ glacial periods followed by a period of relatively stable sea level during the past 6,000 years (Mimura and Harasawa, 2000). During the past 3,000 years, sea level rose at an average rate of about 0.1 to 0.2 mm/yr, but by the end of the 20th century the rate had increased to approximately 1.0 to 2.0 mm/yr or 100 to 200 mm per century (Gornitz, 1995; Intergovernmental Panel on Climate Change, 1996). The Intergovernmental Panel on Climate Change (2001) projects a two- to four-fold acceleration of sea-level rise over the next 100 years, with a central value of 480 mm. The rate of land subsidence in the New Orleans region (average 5 mm/yr) and the Intergovernmental Panel on Climate Change (2001) mid-range estimate of sea-level rise (480 mm) suggests a net 1.0-m decline in elevation during the next 100 years relative to present mean sea level (fig. 2). A storm surge from a Category 3 hurricane (estimated at 3 to 4 m without waves) (National Oceanic and Atmospheric Administration, 2002) at the end of this century, combined with mean global sea-level rise and land subsidence, would place storm surge at 4 to 5 m above the city’s present altitude. The effect of such a storm on flooding in the New Orleans MSA will depend upon the height and integrity of the regional levees and other flood-protection projects at that time. An additional factor to be considered when eval­ uating the future vulnerability of New Orleans to inun­ dation is the current altitude of the land surface. Much of the heavily populated area in Orleans and St. Bernard Parishes lies below mean sea level. At the intersection of Morrison Road and Blueridge Court (located in lake fringe deposits of eastern Orleans Parish), for example, which is presently about 2.6 m below local mean sea level, the cumulative effects of land subsidence, sealevel rise, and storm surge from a Category 3 hurricane at the end of this century place storm surge 6 to 7 m above the land surface (fig. 2). Such a storm would exceed the design capacity of the existing floodprotection levees. The storm surge of a Category 5 hur­ ricane, generally greater than 5 m (National Oceanic and Atmospheric Administration, 2002), would pose more serious flooding danger. Hurricane Camille, a Cat­ egory 5 hurricane that made landfall in Mississippi in 1969, increased water levels in coastal Mississippi by as much as 7 m (U.S. Army Corps of Engineers, 1970). Landfall of a Category 5 hurricane in New Orleans would place the Morrison Road/Blueridge Court inter­ section at least 9 m below storm-surge level today and, based on the same sea-level rise and land-subsidence trends discussed above, at 10.5 m or more below stormsurge level by the end of the 21st century. In addition to the decline in land-surface altitude, the loss of marshes and barrier islands that dampen storm surge and waves during hurricanes increases the risks of flood disaster in New Orleans and vicinity. Since 1940, approximately 1 million acres of coastal wetlands have been converted to open water in southern Louisiana as a result of natural and human-induced environmental change (Burkett and others, 2001). The extensive loss of coastal marshes and bald cypress for­ ests that once flanked the hurricane-protection levees of St. Bernard and Plaquemines Parishes has increased the threat of storm-surge flooding for the 94,000 residents in the southern part of the New Orleans MSA. Several barrier island and wetland restoration projects are planned by the State of Louisiana, local governments, and Federal agencies. ADAPTATIONS THAT MINIMIZE FLOODING Most of the New Orleans MSA is protected from flooding by levees constructed since 1879 by local sponsors and the U.S. Army Corps of Engineers under five different Congressional authorizations. Levee design heights range from about 4.5 to 6 m above mean sea level. The levees along the Lake Pontchartrain shoreline are designed at a height that exceeds the surge and waves of a Category 3 hurricane. The levee design criteria assume no increase in mean sea level and no subsidence (Alfred C. Naomi, U.S. Army Corps of Engineers, oral commun., 2001). The city of New ADAPTATIONS THAT MINIMIZE FLOODING 67 68 Sea-Level Rise and Subsidence: Implications for Flooding in New Orleans, Louisiana METERS ABOVE OR BELOW MEAN SEA LEVEL 100 yr subsidence Subsidence + Subsidence + sea-level rise sea-level rise + (mid-range, 0.48 m) storm surge* Present elevation at Morrison & Blueridge Net effect on storm surge water level Lake fringe Artificial fill Alluvial soils Natural levees Figure 2. Local subsidence and sea-level projections for New Orleans and vicinity through 2100. Arrow on right represents the cumulative influence of land-surface elevation change and sea-level rise on storm-surge level *(Category 3 hurricane) at the Morrison Road/Blueridge Court intersection, which is presently about 2.6 m below mean sea level. -3 -2 -1 0 1 2 3 4 5 Subsidence Observations Based on Traditional Geodetic Techniques, and Numerical Models Orleans is drained by an extensive network of drainage canals (108 km of surface and subsurface canals) with 22 pumping stations. Most of the stormwater drainage is pumped over the flood-protection levees into Lake Pontchartrain (Sewerage and Water Board of New Orleans, 2001). The following adaptation strategies would aid in reducing, but not eliminate, the vulnerability of the New Orleans MSA to flood disaster: 1. Upgrade levees and drainage systems to withstand Category 4 and 5 hurricanes. 2. Design and maintain flood protection on the basis of historical and projected rates of local subsid­ ence, rainfall, and sea-level rise. 3. Minimize drain-and-fill activities, shallow subsur­ face fluid withdrawals, and other human devel­ opments that enhance subsidence. 4. Improve evacuation routes. 5. Protect and restore coastal defenses. 6. Encourage flood proofing of buildings and infra­ structure. 7. Develop flood-potential maps that integrate local elevations, subsidence rates, and drainage capa­ bilities (for use in the design of ordinances, greenbelts, and other flood-damage reduction measures). GPS SOLUTIONS FOR MONITORING SUBSIDENCE IN LOUISIANA Accurate monitoring of land subsidence over time is vital to providing data for calibrating models of land subsidence and predicting subsidence, as well as pro­ viding information for planning, constructing, and maintaining infrastructure and levees. Historically, geodetic differential leveling has been used to measure subsidence in the New Orleans MSA; it was very accu­ rate but also very expensive. Over the past decade, GPS surveying techniques have proven to be so efficient and accurate that they are now routinely used in place of classical line-of-sight surveying methods for establish­ ing horizontal control. Understandably, interest has also been growing in using GPS techniques to establish accurate vertical control. Progress, however, has been hampered due to difficulties in obtaining sufficiently accurate geoid height differences to convert GPS- derived ellipsoid height differences to accurate orthometric height differences. These factors have recently been resolved, mak­ ing GPS-derived orthometric heights a viable alter­ native to classical line-of-sight geodetic differential leveling techniques for many applications. Additional information is available at the following web sites on the topics of • completion of the general adjustment of NAVD 88 (http://www.ngs.noaa.gov/PUBS_LIB/NAVD88/ navd88report.htm), • development of NGS guidelines for establishing GPS-derived ellipsoid heights to meet 2- and 5-cm standards (http://www.ngs.noaa.gov/ PUBS_LIB/NGS-58.html), and • computation of an accurate, nationwide, highresolution geoid model, GEOID99 (http://www.ngs.noaa.gov/GEOID/). A cooperative study between the HarrisGalveston Coastal Subsidence District (HGCSD) and the NGS is using GPS methods to measure subsidence at a fraction of the cost of the previous method. Due to the magnitude of subsidence in the Houston-Galveston region of southeastern Texas, there are no stable bench­ marks in the area. Therefore, stable borehole extensom­ eters were equipped with GPS antennas to provide a reference frame to measure subsidence at other stations in the area. These stations are known as local GPS Continuously Operating Reference Stations (CORS). The NGS/HGCSD project uses dual-frequency, full-wavelength GPS instruments and geodetic anten­ nas. Data are collected at 30-second sampling intervals and averaged over long periods, generally 24 hours. The goal is to yield differential vertical accuracy of less than 10 mm in a totally automated mode operated by HGCSD personnel. Data have now been collected from the three CORS sites and four portable GPS measuring stations called Port-A-Measures (PAMS), at 20 sites, for more than 4 years in the Houston-Galveston region. Results between CORS and PAMS indicate that some geodetic monuments are subsiding as much as 70 mm/yr and correlate well with extensometer data. The joint NGS/HGCSD GPS subsidence project is described in more detail by Zilkoski and others (p. 13 of these proceedings). Louisiana’s greatest environmental problem is the continuing loss of its coast. To address these problems, NGS in partnership with Louisiana State University GPS SOLUTIONS FOR MONITORING SUBSIDENCE IN LOUISIANA 69 Subsidence Observations Based on Traditional Geodetic Techniques, and Numerical Models through a newly created Louisiana Spatial Reference Center (LSRC) is building a statewide network of GPS CORS similar to the HGCSD network. Like the HGCSD network, the LSRC GPS CORS will be refer­ enced to the National Oceanic and Atmospheric Admin­ istration (NOAA) national CORS. The national GPS CORS will provide the framework for the LSRC CORS to measure yearly subsidence rates at the 10-mm level. In addition to the continuously operating GPS CORS and PAMS, specially designed GPS network surveys adhering to NGS guidelines will be performed to esti­ mate the subsidence in local areas. CONCLUSIONS Increases in mean sea level, coupled with the cur­ rent low altitude of the land surface and land-subsidence 70 trends in the region, portend serious losses of life and property in the New Orleans MSA unless flood-control levees and drainage systems are upgraded. The mainte­ nance of barrier islands and wetlands that flank New Orleans to the south, west, and east is another adapta­ tion that will likely minimize the potential loss of life and property due to flooding. The changes in sea level that are predicted to accompany increasing global tem­ perature are statistically and practically significant to those responsible for designing flood-control works and coastal protection strategies for New Orleans, Houston, Amsterdam, and other rapidly subsiding coastal areas. The application of GPS technology for determining orthometric height differences should enhance the util­ ity and cost effectiveness of land-subsidence monitoring in flood-protection design. Sea-Level Rise and Subsidence: Implications for Flooding in New Orleans, Louisiana Objectives: This module aims to enhance students’ understanding of the catastrophic impact of Hurricane Katrina on the New Orleans flood protection systems. Case History: Katrina remains one of the deadliest and costliest hurricanes in U.S. history, with more than 1,800 lives lost and damages estimated at over $100 billion. When the levees failed, about 80 percent of New Orleans flooded. More than 1 million people across the Gulf Coast were forced to leave their homes — many never returned. An independent levee investigation team final report was issued on July 31, 2006. This report contained the observations and findings of an investigation by an independent team of professional engineers and researchers with a wide array of expertise (Report Link) Please review the following materials: − − − − − Overview of Hurricane Katrina and its aftermath (Chapter 2, posted on Canvas) Geology of the New Orleans Regions (Chapter 3, posted on Canvas) History of the New Orleans Flood Protection System (Chapter 4, posted on Canvas) Summary of Engineering Lessons (Chapter 11, ) Sea-Level Rise and Subsidence: Implications for Flooding in New Orleans, Louisiana (USGS Paper) assigment The geology underlying New Orleans is very complex due to both natural and man-made processes. The Mississippi river is the most dominant natural feature shaping the geology of New Orleans. Land subsidence and sea level rise are the supporting natural processes. Settlement and erosion characteristics of the sediments were altered by the manmade processes of urbanization and dewatering. The major design flaws of the levees, intensity of Hurricane Katrina and the coupling of these various mechanisms led to one of the most severe crises in the history of the United States. Hurricane Katrina was a major disaster from which the city of New Orleans may never recover. However, engineers and geologists can learn from the failure and try not to repeat the mistakes of their predecessors. Please put together a technical note (4 to 6 singles spaced pages including the figures and tables) summarizing your opinions and suggestions. In your work, please do not copy-paste from external sources, please do not pull text verbatim from other sources, and please make proper citations. I’ll use the Turnitin Software for Plagiarism Review.
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