New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team CHAPTER TWO: OVERVIEW OF HURRICANE KATRINA AND ITS AFTERMATH 2.1 Hurricane Katrina The path of Hurricane Katrina’s eye is shown in Figures 2.1 and 2.2. Hurricane Katrina crossed the Florida peninsula on August 25, 2005 as a Category 1 hurricane. It then entered the Gulf of Mexico, where it gathered energy from the warm Gulf waters, producing a hurricane that eventually reached Category 5 status on Sunday, August 28, shortly before making its second mainland landfall just to the east of New Orleans early on Monday, August 29, as shown in Figures 2.1 and 2.2. The Hurricane had weakened to a Category 4 level prior to landfall on the morning of August 29, and it weakened further as it came ashore. Because the eye of this hurricane passed just slightly to the east of New Orleans, the hurricane imposed unusually severe wind loads and storm surges (and waves) on the New Orleans region and its flood protection systems. 2.2 Overview of the New Orleans Flood Protection Systems Figure 2.3 shows the main study region. The City of New Orleans is largely situated between the Mississippi River, which passes along the southern edge of the main portion of the city, and Lake Pontchartrain, which fronts the city to the north. Lake Borgne lies to the east, separated from developed areas by open swampland. “Lake” Borgne is not really a lake at all; instead it is a bay as it is directly connected to the waters of the Gulf of Mexico. To the southeast of the city, the Mississippi River bends to the south and flows out through its delta into the Gulf of Mexico. The flood protection system that protects the New Orleans region is organized as a series of protected basins or “protected areas”, each protected by its own perimeter levee system, and these are “unwatered” by pumps. As shown in Figures 2.4 and 2.5, there are four main protected areas that comprise the New Orleans flood protection system of interest. A number of additional levee-protected units also exist in this area, but the focus of these current studies is the four main protected areas shown in Figures 2.4 and 2.5. These were largely constructed under the supervision of the U.S. Army Corps of Engineers, to provide improved flood protection in the wake of the devastating flooding caused by Hurricane Betsy in 1965. Figures 2.4 and 2.5 show the locations of most of the levee breaches and severely distressed (but non-breached, or only partially breached) levee sections covered by these studies. Levee breaches are shown with solid blue stars, and distressed sections as well as minor or partial breaches are indicated by red stars. The original base maps, and many of the stars, were graciously provided by the USACE (2005), and a number of additional blue and red stars have been added to the map in Figure 2.4 as a result of the studies reported herein. The yellow stars shown in these figures correspond to deliberate breaches made after Hurricane Katrina, to facilitate draining the flooded areas after the storm. 2-1 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team The pink shading in Figures 2.4 and 2.5 shows developed areas that were flooded, and the areas shaded with blue cross-hatching indicate undeveloped swamp land that was flooded. The deeper blue shading (near the east end of New Orleans East) denotes areas that still remained to be unwatered as late as September 28, 2005. As shown in these figures, approximately 85% of the metropolitan area of New Orleans was flooded during this event. As shown in Figure 2.4, the Orleans East Bank (Metro Orleans) section is one contiguously protected section. This protected unit contains the downtown district, the French Quarter, the Garden District, and the “Canal” District. The northern edge of this protected area is fronted by Lake Pontchartrain on the north, and the Mississippi River passes along its southern edge. The Inner Harbor Navigation Canal (also locally known as “the Industrial Canal”) passes along the east flank of this protected section, separating the Orleans East Bank protected section from New Orleans East (to the northeast) and from the Lower Ninth Ward and St. Bernard Parish (directly to the east.) Three large drainage canals extend into the Orleans East Bank protected section from Lake Pontchartrain to the north, for the purpose of conveying water pumped north into the lake by large pump stations within the city. These canals, from west to east, are the 17th Street Canal, the Orleans Canal, and the London Avenue Canal. A second protected section surrounds and protects New Orleans East, as shown in Figure 2.4. This protected section fronts Lake Pontchartrain along its north edge, and the Inner Harbor Navigation Canal (IHNC) along its west flank. The southern edge is fronted by the Mississippi River Gulf Outlet channel (MRGO) which co-exists with the Gulf Intracoastal Waterway (GIWW) along this stretch. The eastern portion of this protected section is currently largely undeveloped swampland, contained within the protective levee ring. The east flank of this protected section is fronted by additional swampland, and Lake Borgne is located slightly to the southeast. The third main protected section contains both the Lower Ninth Ward and St. Bernard Parish, as shown in Figure 2.4. This protected section is also fronted by the Inner Harbor Navigation Canal on its west flank, and has the MRGO/GIWW channel along its northern edge. At the northeastern corner, the MRGO bends to the south (away from the GIWW channel) and fronts the boundary of this protected area along the northeastern edge. Open swampland occurs to the south and southeast. Lake Borgne occurs to the east, separated from this protected section by the MRGO channel and by a narrow strip of undeveloped marshland. The main urban areas occur within the southern and western portions of this protected area. The fairly densely populated Lower Ninth Ward is located at the west end, and St. Bernard Parish along approximately the southern half of the rest of this protected area. The northeastern portion of this protected section is undeveloped marshy wetland, as indicated in Figure 2.4. A secondary levee, operated and maintained by local levee boards, separates the undeveloped marshlands of the northeastern portions of this protected area from the Ninth Ward and St. Bernard Parish urban areas. The fourth main protected area is a narrow, protected strip along the lower reaches of the Mississippi River heading south from St. Bernard Parish to the mouth of the river at the Gulf of Mexico, as shown in Figure 2.5. This protected strip, with “river” levees fronting the Mississippi River and a second, parallel set of “storm” levees facing away from the river forming a protected corridor less than a mile wide, serves to protect a number of small communities as well as utilities and pipelines. This protected corridor also provides protected access for workers, supplies and 2-2 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team gas and oil pipelines servicing the large offshore oil fields out in the Gulf of Mexico. This will be referred to in this report as “the Plaquemines Parish” levee protected zone. The current perimeter levee and floodwall defense systems for these four protected areas were largely designed and constructed under the supervision of the U.S. Army Corps of Engineers in the wake of the catastrophic flooding caused by Hurricane Betsy of 1965. These flood protection improvements typically involved either new levee construction, or raising existing levee defenses and/or adding new floodwalls, to provide storm flood protection for higher elevations of storm surge waters (and waves) at locations throughout the region. 2.3 Overview of Flood Protection System Performance During Hurricane Katrina 2.3.1 Storm Surge During Hurricane Katrina The regional flood protection system had been designed to safely withstand the storm surges and waves associated with the Standard Project Hurricane, which was intended to represent a scenario roughly “typical” of a rapidly moving Category 3 hurricane passing close to the New Orleans metropolitan region. Chapter 12 (Section 12.5.1) presents a more detailed discussion of the “Standard Project Hurricane”, and the criteria for which the regional flood protection system was designed. In simple terms, the system was intended to have been designed to safely withstand storm surge levels (plus waves) to specified elevations at various locations, as shown in Figures 2.6 and 2.7. In general, the “Standard Project Hurricane” provided for design to safely withstand storm surge rises (plus waves) to prescribed elevations at various locations throughout the system. The levels selected correspond generally to the storm surge level (mean peak storm surge water elevation, without waves) associated with the “Standard Project Hurricane” conditions plus an additional allowance for most (but not always all) of expected additional wave run-up. As shown in Figures 2.6 and 2.7, this resulted in a targeted protection level of about elevation +17 feet to +19 feet (MSL), or 17 to 19 feet above Mean Sea Level, at the eastern flank of the system, and + 13.5 feet to +18 feet (MSL) along much of the southern edge of Lake Pontchartrain. The storm surge levels within the various drainage canals and navigational channels varied, and the storm surge levels for design were typically on the order of Elev. + 14 feet to + 16 feet (MSL) along the GIWW and IHNC channels, and Elev. + 12.5 feet to + 14.5 feet (MSL) along the 17th Street, Orleans, and London Avenue Canals in the “Canal District”. There is some minor confusion as to the most recent “Standard Project Hurricane”, and the most recent storm surge design levels at some locations; the values indicated in Figure 2.6 are an interpretation by the Government Accountability Office (GAO, 2006) based in part on initial research by the staff of the New Orleans Times Picayune, and the values shown in Figure 2.7 have been added to this figure by our team, and are our own current best interpretation. The situation is further clouded a bit, as the actual targeted levee and floodwall heights along a given section also varied slightly as a function of waterside topography, obstacles and vegetation, levee geometry, orientation and potential wind fetch (distance of potential wind travel across the top of open water), etc. as these would affect the potential run-up heights of storm waves. Variations for these types of issues were typically minor, on the order of two feet or less. 2-3 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team There is, however, no “typical” hurricane, nor associated storm surge, and the actual wind, wave and storm surge loadings imposed at any location within the overall flood protection system during an actual hurricane are a function of location relative to the storm, wind speed and direction, orientation of levees, local bodies of water, channel configurations, offshore contours, vegetative cover, etc. These loadings vary over time, as the storm moves progressively through the region. Figures 2.8 and 2.9 show plots of storm surge levels resulting from numerical modeling simulations performed by the LSU Hurricane Research Center, for two different points in time during Hurricane Katrina, based on analyses of the storm track, wind speeds, regional topography and local conditions (marsh growth, soil stiffness, offshore contours, etc.) (Louisiana State University Hurricane Center, 2005.) The water levels shown in Figures 2.8 and 2.9 were predicted using a regionally calibrated numerical model, and the results shown in Figure 2.8 represent a point in time when the eye of the hurricane was first approaching the coast from the Gulf of Mexico, and those shown in Figure 2.9 correspond to a time when the eye of the storm was passing slightly to the east of New Orleans. These calculations are part of an overall single analysis of storm surge levels throughout the region, and throughout the continuous period of time as the storm approached and then passed through the region. Based on actual field observations and measurements of maximum storm surge levels at more than 100 locations throughout the region, this global analysis of storm surge levels is expected to be accurate (relative to surge levels that actually occurred) within approximately ± 15% at all locations of interest for these current studies (IPET, 2006.) Predicted and actual storm surge heights varied over time, at different locations, and the water levels shown in Figures 2.8 and 2.9 do not represent predictions of the peak storm surges noted at all locations. Instead, these images show calculated conditions at two interesting points in time when: (a) [Fig. 2.8] the initial large surge was being driven up against the coast of the Gulf of Mexico in the New Orleans region by the approaching storm, and (b) [Fig. 2.9] at a particularly critical moment when a large storm surge had first “inflated” (raised the level of) Lake Borgne, then the locally prevailing westward swirl of the counterclockwise hurricane winds threw the risen waters of Lake Borgne westward over the adjacent levees protecting eastern flanks of the New Orleans East and St. Bernard/Lower Ninth Ward protected areas, as shown schematically in Figure 2.11. These types of storm surge modeling calculations are being performed by a number of research and investigation teams, and are constantly being calibrated and updated based on actual field measurements of high water marks, etc. The USACE’s IPET investigation team are devoting significant effort to these types of hydrodynamic analytical “hind-casts”, and the IPET back analyses provided to date to our UC Berkeley-led ILIT study team are in good agreement with the storm surge predictions shown in Figures 2.8 and 2.9 at most locations of interest for these studies (IPET; Draft Final Report, June 1, 2006). Figure 2.10 shows an aggregate summary of the calculated peak storm surges, at any point in time during Hurricane Katrina, based on similar calculations performed by the IPET study (IPET; March, 2006). These calculations are very similar to those developed by the Louisiana investigation team, and both the IPET and Team Louisiana analyses will be used as a partial basis for estimation of storm surge levels and wave conditions in these current studies. The maximum flood stages calculated (predicted) by the two sets of analyses are generally in good agreement at 2-4 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team most points of interest. Agreement regarding storm waves is also generally good, but the differences between the two sets of predicted storm waves are a bit more significant at a few locations of interest. Discussions of the IPET and Team Louisiana hydrodynamic storm surge and storm wave calculations will be presented, in more detail, at locations of interest in the chapters that follow. It should be noted that a number of different datums have been used as elevation references throughout the historic development of the New Orleans regional levee systems, and this situation is further complicated by ongoing subsidence in the region. This investigation has elected to resolve these differences between different datums, and to refer to all elevations in this report (as consistently as possible) in terms of elevation with respect to the NAVD88 (2004.65) datum; approximately “mean sea level” in the region. This particular version of the NAVD88 datum is currently thought to be within about 3-inches of Mean Sea Level (MSL) in the New Orleans region. For a more in-depth discussion of differences between the various datums used in the greater New Orleans region, please see IPET Interim Report No. 2 (IPET; March, 2006). 2.3.2 Overview of the Performance of the Regional Flood Protection System Hurricane Katrina, as expected, produced a large onshore storm surge from the Gulf of Mexico. As shown in Figures 2.8 through 2.10 this produced significant overtopping of storm levees along the lower Mississippi River reaches in the Plaquemines Parish area, and numerous levee breaches occurred in this area, as shown previously in Figure 2.5. In simple terms, the “storm” levees of Plaquemines Parish were largely overwhelmed by the large storm surge; they were overtopped by the storm surge and by the large storm waves that accompanied the average rise (storm surge) in water levels. Fortunately, the Plaquemines Parish protected corridor is only sparsely populated, and the local inhabitants were acutely aware of the risk that they faced so that evacuation in advance of the storm was unusually complete. Plaquemines Parish was largely inundated by the massive storm surge and the numerous resulting levee breaches. Most breaches appear to have been primarily the result of overtopping and erosion, and it is interesting to note that these breaches occurred mainly in the “storm” levees, while the “river” levees often better withstood the storm surge (and waves) without catastrophic erosion. The devastation within Plaquemines parish produced by this flooding was very severe, as described in Chapter 5. By approximately 7:00 a.m. on the morning of Monday, September 29, most of Plaquemines Parish was under water. A more detailed discussion of the performance of the flood protection systems in the Plaquemines Parish area is presented in Chapter 5. As the storm surge began to raise the water levels throughout the New Orleans region, it began to raise the water levels within the GIWW, MRGO and IHNC channels. As the water level within the IHNC began to rise, the first “breach” within the metropolitan New Orleans region (north of Plaquemines Parish) occurred at about 5:00 a.m. somewhere along the IHNC. This was evidenced by a pronounced, and short-lived, decrease in the rate of water level rise at two gage stations along the IHNC at this point in time. There are several breaches along this section of the IHNC that might have accounted for this observed water level gage behavior, and this is discussed in Chapter 8. This was a “non-catastrophic” failure; although the breach eroded and became enlarged by the flow, the “lip” of the breach remained above sea level. As a result, 2-5 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team although water flowed for a while into the protected area, this flow later stopped as the storm surge subsequently subsided. Simple calculations, based on flood stages and breach sequences and dimensions, suggest that less than 5% of the water that eventually flowed into the main Orleans East Bank (downtown) protected zone entered through this breach. The large onshore storm surge also raised water levels within Lake Borgne (which is directly connected to the Gulf.) Lake Borgne rose up, and outgrew its normal banks. As the storm then passed to the east of New Orleans, the prevailing counterclockwise swirl of the storm winds drove the waters of Lake Borgne as a large storm surge to the west, against the eastern flank of the regional flood protection systems as shown schematically in Figure 2.11. This produced a storm surge estimated at approximately +16 to +18 feet (MSL), as shown in Figures 2.9 and 2.10. This storm surge level exceeded the crest heights of the levees along a nearly 11-mile long stretch of the northeastern edge of the St. Bernard/Lower Ninth Ward protected area. The levees along this frontage were intended to be built to provide protection to a level of approximately +17.5 feet (MSL), but at the time of Hurricane Katrina many of the levees along this frontage had crest elevations approximately 2 to 4 feet lower than that. This was because the levees along this frontage had not yet been completed. These were “virgin” levees, being constructed on swampy foundation soils that had not previously had significant levees before. Accordingly, the swampy shallow foundation soils were both weak and compressible, and the levees were being constructed in stages to allow time for consolidation and settlement of the foundations soils. This process also allowed time for the drying of the very wet locally excavated soils used for some portions of the levee embankment fills, and also for increases in strength of the underlying foundation soils as they compressed under the weights of the growing levees. Construction of the first phase of the levees along this frontage began in the late 1960’s. The last major work in this area prior to Katrina had been the construction of the third phase, in 1994-95. Since that time, the USACE had been waiting for Congressional appropriation of the funds necessary to construct the final stage (to the full design height, with allowance for anticipated future settlements.) Now it is too late. In addition to the levees along this frontage being well below design grade, the manner of construction and the materials used were non-typical of most other USACE levees in the region. Ordinarily, the USACE requires the use of “cohesive” (clayey) soils to create an embankment fill that is both strong and relatively resistant to erosion. The levees along the “MRGO” frontage at the northeast edge of the St. Bernard Parish/Ninth Ward protected area were instead “sand core” levees (USACE, 1966). These levees were constructed using locally available soils, including dredge spoils from the excavation of the adjacent MRGO channel. This is a region with predominantly marshy deposits, consisting largely of organic soils and soft paludal swamp clays with very high water contents. Beneath these generally poor surficial soils, the most common materials occurring at shallow, relatively accessible depths tend to be predominantly sandy soils that are highly erodeable and generally unsuitable for levee embankment fill. A decision was made, however, to attempt to use the locally available soils rather than importing higher quality soil fill materials. The USACE Design Memorandum describing this design refers to these as “sand core” levees (USACE, 1966). 2-6 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team The levees along this MRGO frontage section (along the northeastern edge of the St. Bernard protected area) were, in the end, constructed using large volumes of the spoil material excavated during the dredging of the adjacent MRGO shipping channel, and they contained unusually large quantities of highly erodeable sandy soils. In addition, some of the more cohesive (clayey) soils were too wet to be compacted effectively, and some sections of the embankments remained wet and soft for many years after construction. Chapter 6 presents a more detailed discussion of the erodeability of the levee embankments along the MRGO frontage. In simple terms, these levees were unusually massively erodeable, and this (combined with their lack of crest height) caused them to be unusually rapidly eroded as the storm surge from Lake Borgne approached and passed over, and through, these levees. Based on analytical storm surge analyses and analytical “hindcasts” performed by various investigation teams, as well as eyewitness reports and timings of flooding and damages in St. Bernard Parish and the Ninth Ward, it is estimated that the storm surge passed over and through the MRGO levee frontage between approximately 6:00 to 7:00 a.m. The storm surge along the northeastern frontage of the St. Bernard Parish protected area peaked at approximately 7:30 to 8:00 a.m. (see Figure 2.9.) By the time the storm surge peaked along this important frontage, however, the unfinished “sand core” levees fronting Lake Borgne had been massively eroded and the brunt of the storm surge passed over and through the levees and raced across the undeveloped swamplands shown in Figure 2.11 towards the developed areas of St. Bernard Parish. This is illustrated schematically in Figure 2.11. The levees along this frontage were so badly eroded, and so rapidly, that they did little to impede the passage of the storm surge which then crossed the roughly 7 to 10 miles of open swamp and reached the secondary levee that separates the northern (undeveloped) swampy section of this protected area from the populated southern section. The secondary levee had not been intended to face the full fury of a storm surge of this magnitude; it had been assumed that the MRGO frontage levees would absorb much of the energy and provide more resistance. Accordingly, the storm surge passed over the secondary levee (which had lesser typical crest heights of only + 7.5 feet to + 10 feet, MSL) and washed into the populated regions of St. Bernard Parish. A number of minor breaches were produced by the overtopping (and erosion) of this secondary levee, but it is interesting to note that although this secondary levee must have been massively overtopped along much of its length, relatively little erosion damage resulted. The secondary levee was properly constructed, using compacted clayey soils, and the resulting levee embankment generally performed well with regard to resisting erosion. It was not, however, tall enough to restrain the massive overtopping from the storm surge which had passed so easily through the MRGO frontage levees. The resulting carnage in St. Bernard Parish was devastating. A wall of water raced over the secondary levee; pushing homes laterally (Figure 2.16), flipping cars like toys and leaving them leaning against buildings, and driving large shrimp boats deep into the heart of residential neighborhoods (see Chapter 6.) The flooding of St. Bernard Parish was unexpectedly rapid. The peak depth of flooding in St. Bernard Parish was also unexpectedly deep because the floodwaters were pushed by the still rising storm surge (rather than having to flow more slowly, over time, through more finite breaches as the storm surge subsided; as occurred in most other parts of the greater New Orleans area) so that the top of the floodwaters at their peak within the developed areas were at an elevation well above mean sea level (approximately Elev. +12 feet, MSL.) 2-7 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team Indeed, after the storm surge subsided, “notches” were excavated through a number of local levees to let floodwaters drain under gravity loading from the significantly “plus mean sea level” flooding entrapped in some areas. Figure 2.12 shows a plot of the locations where dead bodies were retrieved after the disaster as of December 2005. This map shows locations for only approximately 960 of the approximately 1,296 official deaths (to date) in the greater New Orleans area, but this map serves well to show the general distribution of deaths attributed to the flooding produced by this event. As shown in Figure 2.12, approximately 30% of these deaths occurred in St. Bernard Parish. In addition to those who perished, considerable damage was done to many thousands of homes and businesses in this area (see Chapter 6.) The same storm surge from Lake Borgne that topped and eroded the levees along the “MRGO” frontage also pushed westward over the southeastern corner of the New Orleans East protected section, as shown in Figures 2.9 through 2.11, and this produced overtopping and a number of breaches, as shown previously in Figure 2.4. This was a principal source of the catastrophic flooding that subsequently made its way across the local undeveloped swamplands and into the populated areas of New Orleans East. Like the MRGO levee frontage discussed above, large portions of this levee frontage section had been constructed using materials excavated from the adjacent shipping channel (in this case the GIWW channel), and large portions of the levee were comprised of highly erodeable sandy and lightweight shell sand fill. This storm surge from Lake Borgne also passed westward into a V-shaped “funnel” as it entered the shared GIWW/MRGO channel that separates the St. Bernard and New Orleans East protected areas, and this in turn resulted in an elevated surge of water that passed westward along the waterway to its juncture (at a “T”) with the IHNC channel, overtopping a number of levees and floodwalls on both the north and south sides of this east-west trending channel and producing levee distress and several breaches (as shown in Figures 2.4 and 2.11.) After reaching the “T” intersection with the IHNC channel, the surge then passed to the north and south (from the “T”) along the IHNC channel, periodically overtopping many (but not all) of the sections of levees and floodwalls lining the east and west sides of the IHNC, and causing a number breaches as shown in Figures 2.4 and 2.11. By about 6:45 to 7:00 a.m. overtopping (by up to as much as 1 to 2 feet at it’s peak at most locations) was occurring along a number of levee and floodwall sections lining the IHNC channel. This overtopping did not occur at all locations, and was only of limited duration (typically several hours or less) where it did occur. A pair of major breaches occurred at the west end of the Lower Ninth Ward as this overtopping occurred along the IHNC, and the larger of these two breaches is shown (roughly seven weeks later, after construction of an interim repair embankment just outside the breach) in Figure 2.13. A large barge passed in through this breach, and can be seen in the rear of the photo. It is worth noting the tremendous scour-induced damage to the homes immediately inboard of this massive breach; most of the homes in Figure 2.13 were washed off of their foundations and transported laterally (often in pieces) by the inrushing floodwaters. A more detailed examination of the two large breaches at the west end of the Ninth Ward is presented in Chapter 6; Sections 6.4 and 6.5. The large breaches at the west end of the Lower Ninth Ward appear to have occurred by approximately 7:45 a.m. (Louisiana State University Hurricane Center, 2006.) 2-8 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team Like St. Bernard Parish, the breaches at the west end of the Lower Ninth Ward occurred before the storm surge peaked (at about 8:30 a.m. in the IHNC channel), so the Lower Ninth Ward was flooded to a level well above mean sea level before the storm surge subsequently subsided. This neighborhood, which had ground surface elevations of generally between about -3 to -6 feet (MSL) was flooded to elevations of up to as much as 10 to 12 feet above sea level. The resulting carnage, in terms of both loss of life (as shown in Figure 2.12) and destruction of homes and businesses was considerable, as the flooding rose above the tops of many of the one-story homes in this densely packed neighborhood. The protected area of New Orleans East, directly to the north of the St. Bernard Parish/Ninth Ward protected area, had been breached at its southeastern corner by the initial storm surge and lateral rush from Lake Borgne (as shown schematically in Figure 2.11) by about 6:00 to 7:00 a.m., though the resulting breaches were confined to several locations so that the inflowing waters began to make their way across the undeveloped swamplands of the eastern portion of this protected area and timing is thus difficult to pin down with exactitude. The storm surge then passed laterally along the GIWW/MRGO east-west channel and produced another finite breach on the north side of this channel and several additional distressed sections. This breach added to the sources of water beginning to flow into this protected area. The surge that passed west along the GIWW/MRGO east-west channel then pushed north along the IHNC, and produced several additional breaches and distressed sections, of varying severity, along the IHNC frontage as shown in Figure 2.4. These, too, added to the flow into the protected area of New Orleans East. The lateral storm surge that passed westward along the east-west trending GIWW/MRGO channel between New Orleans East and St. Bernard Parish also attacked the west side of the IHNC channel, at the eastern edge of the main Orleans East Bank (downtown New Orleans) protected area. This produced three additional breaches along this frontage, as shown in Figures 2.4 and 2.11. Floodwaters began to flow into the main New Orleans metropolitan (downtown) protected area through these breaches between approximately 7:00 to 8:30 a.m. Although three of these breaches were relatively significant, all three breaches along this frontage failed to scour to significant depths. As a result, all three either had “lips” with lowest elevations above mean sea level, or there were points along the path from the IHNC to the breach that were above mean sea level. Accordingly, although all three breaches allowed some flow of water into the main Orleans East Bank (downtown) protected area, they allowed only limited flow and this flow stopped as the storm surge subsequently subsided. It would be the subsequent breaches in the drainage canals, to the northwest (along the edge of Lake Pontchartrain) that would prove to be devastating for this main (downtown) protected area. As the hurricane then passed northwards to the east of New Orleans, the counterclockwise direction of the storm winds also produced a well-predicted storm surge southwards towards the south shore of Lake Pontchartrain. The lake level rose, but mainly stayed below the crests of most of the lakefront levees. The lake rose approximately to the tops of the lakefront levees at a number of locations, especially along the shoreline of New Orleans East, and there was moderate overtopping (or at least storm wave splash-over) and some resulting erosion on the crests and inboard faces of some lakefront levee sections along the Lake frontage. Significant overtopping occurred over a long section of concrete floodwall near the west end of the New Orleans East protected area lakefront (behind the Old Lakefront Airport), where the floodwall appears to have 2-9 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team been inexplicably lower than the adjacent earthen levee sections. This, too, added to the flow into the New Orleans East protected area, which was now continuing to fill with water even as the original storm surges subsided. Farther to the west, the storm surge along the Pontchartrain lakefront (which peaked at about 9:00 to 9:30 a.m. at an elevation of about +10 feet, MSL) did not produce water levels sufficiently high as to overtop the crests of the concrete floodwalls atop the earthen levees lining the three drainage canals that extend from just north of downtown to Lake Pontchartrain; the 17th Street Canal, the Orleans Canal, and the London Avenue Canal. Three major breaches occurred along these canals, however, and these produced significant flooding of large areas within the Orleans East Bank protected area (as shown in Figure 2.4.) Figure 2.13 shows military helicopters lowering oversized bags of gravel into the levee breach on the east side of the 17th Street Canal, near the north end of the canal. Note that the flood waters have equilibrated, and that there is no net flow through the breach at the time of this photo. The first breach along the drainage canals occurred near the south end of the London Avenue canal, between about 7:00 to 8:00 a.m. The second breach occurred near the north end of the London Avenue canal, and the best current estimates of the timing of this breach are between about 7:30 to 8:30 a.m. The third major breach occurred near the north end of the 17th Street canal. The main breach here occurred between about 9:00 to 9:15 a.m., but this may have been preceded by earlier visually observable distress at this same location. All three of these breaches rapidly scoured to depths well below mean sea level, so they continued to transmit water into the main Orleans East Bank (downtown) protected area after the storm surges subsided. A more detailed discussion and analyses of these catastrophic drainage canal breaches are presented in Chapter 8. The resulting flooding of the main Orleans East Bank (Downtown) protected area was catastrophic, and resulted in at least 588 of the approximately 1,293 deaths attributed (to date) to the flooding of New Orleans by this event. Contributions to this flooding came from the overtopping and breaches along the IHNC channel at the east side of this protected area, but the majority of the flooding came from the three catastrophic failures along the drainage canals at the northern portion of this protected area. In addition, one of the drainage canals (the Orleans Canal) had not yet been fully “sealed” at its southern end, so that floodwaters flowed freely into New Orleans during the storm surge through this unfinished drainage canal. A section of levee and floodwall approximately 200 feet in length had been omitted at the southern end of this drainage canal, so that despite the expense of constructing nearly 5 miles of levees and floodwalls lining the rest of this canal, as the floodwaters rose along the southern edge of lake Pontchartrain, the floodwaters did not rise fully within the Orleans canal; instead they simply flowed freely into downtown New Orleans. Chapters 4 through 8 present a more detailed discussion of the performance of the flood protection systems nominally intended to protect the main Orleans East Bank area, and studies of the major failures and near failures within this critical area. By approximately 9:30 a.m. the principal levee failures had occurred, and most of New Orleans was rapidly flooding. 2 - 10 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team 2.3.3 Brief Comments on the Consequences of the Flooding of New Orleans The consequences of the flooding of major portions of all four levee-protected areas of New Orleans were catastrophic. Approximately 85% of the metropolitan area of greater New Orleans was flooded, as shown in Figures 2.4 and 2.5. In Figure 2.4, the flooded areas are shown in pink, and those that remained still to be “unwatered” as late as September 28th are shown in darker blue. The blue cross-hatched areas were open, undeveloped swamplands, and these were also flooded but were not counted in determining the 85% flooding figure. Large developed areas within all of the four main “protected areas” were flooded, and most remained inundated for two to three weeks before levee breaches could be repaired and the waters fully pumped out. Figure 2.15 shows the approximate depth of flooding that remained on September 2nd, four days after Hurricane Katrina, in the St. Bernard Parish and Lower Ninth Ward protected area, based on an estimated surface water elevation of approximately +5 ft. (MSL) at that time. This is a significantly lower flood level than the estimated peak flooding to an elevation of up to +10 to 12 feet above mean sea level during the actual hurricane. The undeveloped swampland to the north of the populated areas can be seen in this Figure to also still be flooded on September 2nd, but the flood depths are not indicated. Figure 2.16 shows the approximate depth of flooding that remained on September 2nd, again four days after the hurricane, in the New Orleans East protected area. As this protected area filled slowly during and after the hurricane, and as it was “unwatered” relatively slowly over the days and weeks that followed, this represents nearly the full depth of flooding in this area. Figure 2.17 shows the approximate depth of flooding of the main Orleans East Bank (downtown) protected area on September 2nd. Like the New Orleans East protected area, this large protected “basin” filled relatively slowly over time. By September 2nd, the breaches had not yet all been closed by emergency repairs, so the depths of flooding in Figure 2.17 represent the nearly the full depth of flooding at its worst in this area. Neighborhoods that were inundated exhibit stark evidence of this catastrophic flooding. Water marks, resembling oversized bathtub rings, line the sides of buildings and cars in these stricken neighborhoods, as shown in Figure 2.18. Household and commercial chemicals and solvents, as well as gasoline, mixed with the salty floodwaters in many neighborhoods, and at the time of this investigation’s first field visits shortly after the event the paint on cars below the watermarks on adjacent buildings had been severely damaged, and bushes and shrubs were browned below the watermarks, but often starkly green above. Driving through neighborhoods that had been flooded, there was often the impression that one was viewing a television screen where the color of the picture was somehow distorted or altered below a horizontal line; the level at which the floodwaters had been ponded. The devastation in these neighborhoods, and its lateral extent across many miles of developed neighborhoods, was stunning even to the many experienced members of our forensic teams that had seen numerous devastating earthquakes, tidal waves, and other major disasters. Close to major breaches, the hydraulic forces of the inflowing floodwaters often had devastating effect on the communities. Figure 2.13 shows the devastation immediately inboard 2 - 11 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team from the large breach at the west end of the Ninth Ward site after the area had been unwatered. Note the numerous empty slabs where homes had been stripped away and scattered, mostly in pieces, across a large area. Figure 2.19 shows another aspect of the flooding. This photograph shows a region within St. Bernard Parish in which some of the homes were transported from their original locations by the floodwaters, and then deposited in new locations. Figure 2.20 shows a number of homes in the Plaquemines Parish polder that were carried across the narrow polder (from left to right in this photograph) as the west side (left side of photo) “hurricane levee” or back levee was breached, and were then deposited on the crest of the Mississippi River levee. The water side slope face of the Mississippi River levee is clearly shown in this photograph, as evinced by the concrete slope face protection on the outboard side of the riverfront levee in the right foreground of the figure. Figures 2.18 through 2.25 show examples of the devastation that occurred within the stricken flooded areas. The spray painted markings on the sides of the buildings in these areas were left by search and rescue teams, and they denote a number of important findings within each dwelling, including toxic contamination, etc. The most important numbers are those centered at the base of the large “X”, as these denote the number of dead bodies found within the building. In most cases this number was “0”, as for example in Figures 2.18 and 2.22. But this was not always the case. Figure 2.24 shows the outside of a dwelling in the Ninth Ward with a “3” beneath the X, indicating three deaths within. This was a housing unit, and the wheelchair ramp from the front door is askew at the bottom of the photograph. Figure 2.25 shows the muddy devastation, and a wheelchair, within this flooded structure. Figure 2.26 gives another sense of perspective regarding the terrible and pervasive devastation wreaked by the flooding of large urbanized areas. This photo shows the flooding of an area of New Orleans East, but it could just as well be any of a number of large areas of New Orleans. Figure 2.27 gives a similar sense of perspective. In this photo, the flooded Lower Ninth Ward is in the foreground, and virtually every neighborhood shown (including those in the far background behind the tall downtown buildings) is flooded, excepting only the small area occupied by the tall buildings of the downtown area. At the time of the writing of this report, the death toll from the flooding of New Orleans has risen to 1,293. It is expected to continue to climb a bit higher as some of those currently listed as “missing” will likely have been drawn out into the swamps and the Gulf by the floodwaters. Loss projections continue to evolve, but estimates of overall losses have now climbed to the $100 to $ 200 billion range for the metropolitan New Orleans region. The members of this investigation team extend their hearts and their deepest condolences to those who were devastated by Hurricane Katrina, and by the flooding of most of New Orleans. The suffering and losses of those most intimately involved are almost beyond comprehension. It must be the goal and objective of all of us that a catastrophe of this sort never be allowed to happen again. 2 - 12 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team 2.4 References Interagency Performance Evaluation Task Force, (2006), “Performance Evaluation, Status and Interim Results, Report 2 of a Series, Performance Evaluation of the New Orleans and Southeast Louisiana Hurricane Protection System,” Interim Report No. 2, March 10, 2006. Interagency Performance Evaluation Task Force, (2006), “Performance Evaluation of the New Orleans and Southeast Louisiana Hurricane Protection System ”, Draft Final Report, June 1, 2006. Louisiana State University Hurricane Center, (2005), “Hurricane Katrina Advisory #22,” available from: http://hurricane.lsu.edu/floodprediction/katrina22/; date accessed: November 10, 2005. United States Army Corps of Engineers, (1967), “Lake Pontchartrain, LA, and Vicinity, Chalmette Area Plan, Hurricane Protection Levee First Lift, Sta. 594+00 – Sta. 770+00 (Not Continuous),” File No. H-8-24100, May 11, 1967. United States Army Corps of Engineers, (1966), “Lake Pontchartrain, LA. and Vicinity, Chalmette Area Plan, Design Memorandum No. 3, General Design,” November, 1966. 2 - 13 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team New Orleans Source: http://flhurricane.com/googlemap Figure 2.1: Location of New Orleans, and map of the path of the eye of Hurricane Katrina. 2 - 14 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team New Orleans New Orleans Source: Mashriqui, 2006 Figure 2.2: Traced path of the eye of Hurricane Katrina at landfall in the New Orleans area. 2 - 15 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team Principal Study Area Lake Pontchartrain Lake Borgne Mississippi River Source: ESRI North American Thematic Basemap, ArcGIS 9.0 Figure 2.3: The greater New Orleans region levee and flood protection system Study Area. 2 - 16 2 - 17 Figure 2.4: Map showing principal features of the main flood protection rings or “protected areas” in the New Orleans area. Source: Modified after USACE, 2005 Independent Levee Investigation Team New Orleans Levee Systems Hurricane Katrina July 31, 2006 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team Source: Modified after USACE, 2005 Figure 2.5: Map showing the levee protected areas along the lower reaches of the Mississippi River (in the Plaquemines Parish Area.) 2 - 18 2 - 19 Figure 2.6: Map showing design flood stage elevations throughout the New Orleans region. Source: Graphic by Emmet Mayer III/emayer@timespicayune.com (2005) Independent Levee Investigation Team New Orleans Levee Systems Hurricane Katrina July 31, 2006 2 - 20 Figure 2.7: Map showing the design flood stage levels for selected locations in the New Orleans Area. Source: Modified after USACE, 2005 Independent Levee Investigation Team New Orleans Levee Systems Hurricane Katrina July 31, 2006 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team Source: http://hurricane.lsu.edu/floodprediction/ Figure 2.8: Calculated storm surge against the coast at about 7:30 am (CDT), August 29, 2006. 2 - 21 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team Source: http://hurricane.lsu.edu/floodprediction/ Figure 2.9: Map of calculated storm surge levels, at time when the eye of the storm passed close to the east of New Orleans at about 8:30 am (CDT). 2 - 22 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team S Source: IPET Interim Report No. 2; April, 2006 Figure 2.10: Map showing calculated aggregate maximum storm surge levels (maximum values at any point in time). 2 - 23 2 - 24 Figure 2.11: Storm surge overtopping the eastern flank of the regional flood protection system at the northeast edge of the St. Bernard Parish and Ninth Ward protected areas. Source: Modified after USACE, 2005 Independent Levee Investigation Team New Orleans Levee Systems Hurricane Katrina July 31, 2006 2 - 25 Figure 2.12: Map showing locations of confirmed deaths (as of December 2005) as a result of Hurricane Katrina. Source: Times Picayune (2005) Independent Levee Investigation Team New Orleans Levee Systems Hurricane Katrina July 31, 2006 2 - 26 Figure 2.13: Oblique view of the (south) levee break at the Inner Harbor Navigation Canal into the lower Ninth Ward. Photograph by Les Harder Independent Levee Investigation Team New Orleans Levee Systems Hurricane Katrina July 31, 2006 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team Photo courtesy of the U.S. Army Corps of Engineers Figure 2.14: Initial closure of the large breach at the north end of the 17th Street Canal. 2 - 27 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team Source: LSU Hurricane Center, 2006 Figure 2.15: Depth of flooding of New Orleans East on September 2nd (4 days after Hurricane Katrina) Source: LSU Hurricane Center, 2006 Figure 2.16: Depth of flooding of St. Bernard Parish and the Lower Ninth Ward on Sept. 2nd (4 days after Hurricane Katrina). 2 - 28 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team Source: LSU Hurricane Center, 2006 Figure 2.17: Depth of flooding of the Orleans East Bank (Downtown) protected area on September 2nd (4 days after Hurricane Katrina). 2 - 29 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team Photograph by Rune Storesund Figure 2.18: High water marks remain on structures after temporary levee repairs have been completed and flood waters have been pumped out. Photograph by Les Harder Figure 2.19: Flooded neighborhood in St. Bernard Parish, showing homes floated off their foundations and transported by floodwaters. 2 - 30 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team . Photograph by Les Harder Figure 2.20: Homes in Plaquemines Parish carried from left to right in photo and strewn across the crown of the Mississippi Riverfront levee. Photograph by Rune Storesund Figure 2.21: Damage to a residential neighborhood in the 17th Street Canal area due to flooding. 2 - 31 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team Photograph by Rune Storesund Figure 2.22: Search and rescue markings on a residence in the Canal District. Photograph by Rune Storesund Figure 2.23: Another view of flooding damage in the Canal District. 2 - 32 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team Photograph by Les Harder Figure 2.24: Search and rescue team markings on a building in the lower Ninth Ward where three inhabitants died. Photograph by Les Harder Figure 2.25: View inside structure shown previously in Figure 2.21. 2 - 33 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team Photo Courtesy of http://www.wwltv.com/sharedcontent/breakingnews/slideshow/083005_dmnkatrina/7.html Figure 2.26: Neighborhood in New Orleans East fully flooded. Photo courtesy of http://www.wwltv.com/sharedcontent/breakingnews/slideshow/083005_dmnkatrina/7.html Figure 2.27: View of the City of New Orleans at the peak of the flooding. 2 - 34 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team CHAPTER THREE: GEOLOGY OF THE NEW ORLEANS REGION 3.1 General Overview of the Geology of New Orleans 3.1.1 Introduction Hurricane Katrina brought devastation to New Orleans and the surrounding Gulf Coast Region during late August 2005. Although there was wind damage in New Orleans, most of the devastation was caused by flooding after the levee system adjacent to Lake Pontchartrain, Lake Borgne and Inner Harbor areas of the city systematically failed. The storm surge fed by winds from Hurricane Katrina moved into Lake Pontchartrain from the Gulf of Mexico through Lake Borgne, backing up water into the drainage and navigation canals serving New Orleans. The storm surge overwhelmed levees surrounding these engineered works, flooding approximately 80% of New Orleans. Although some levees/levee walls were overtopped by the storm surge, the London Avenue and 17th Street drainage canal walls were not overtopped. They appear to have suffered foundation failures when water rose no higher than about 4 to 5 feet below the crest of the flood walls. This occurrence has led investigators to carefully investigate and characterize the foundation conditions beneath the levees that failed. A partnership between the U.S. Geological Survey’s Mid-Continent Geologic Science Center and the University of Missouri – Rolla, both located in Rolla, MO, was established in the days immediately after the disaster to make a field reconnaissance to record perishable data. This engineering geology team was subsequently absorbed into the forensic investigation team from the University of California, Berkeley, funded by the National Science Foundation. The team has taken multiple trips to the devastated areas. During these trips team members collected physical data on the levee failures, much of which was subsequently destroyed or covered by emergency repair operations on the levees. Our team also logged a series of subsurface exploratory borings to characterize the geological conditions present in and around the levee failure sites. 3.1.2 Evolution of the Mississippi Delta beneath New Orleans The Mississippi River drains approximately 41% of the Continental United States, a land area of 1.2 million mi2 (3.2 million km2). The great majority of its bed load is deposited as subaerial sediment on a well developed flood plain upstream of Baton Rouge, as opposed to subaqueous deposits in the Gulf of Mexico. The Mississippi Delta has been lain down by an intricate system of distributary channels; that periodically overflow into shallow swamps and marshes lying between the channels (Figure 3.1, upper). The modern delta extends more or less from the present-day position of Baton Rouge (on the Mississippi River) and Krotz Springs (on the Atchafalaya River). The major depositional lobes are shown in Figure 3.1 (lower). Between 12,000 to 6,000 years ago sea level rose dramatically as the climate changed and became warmer, entering the present interglacial period, which geologists term the 3-1 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team Holocene Epoch (last 11,000 years). During this interim, sea level rose approximately 350 feet, causing the Gulf of Mexico to retreat into southeastern Louisiana inundating vast tracts of coastline. By 7,000 years ago sea level had risen to within about 30 feet of its present level. By 6,000 years ago the Gulf had risen to within 10 to 15 feet of its present level. The modern Mississippi Delta is a system of distributary channels that have deposited large quantities of sediment over the past 6,000 to 7,000 years (Figure 3.1 –upper). Six major depositional lobes, or coalescing zones of deposition, have been identified, as presented in Figure 3.1 (lower). In southeastern Louisiana deltaic sedimentation did not begin until just the last 5,000 years (Saucier, 1994). Four of these emanate from the modern Mississippi River and two from the Atchafalaya River, where the sediments reach their greatest thickness. The St. Bernard Delta extending beneath Lake Borgne, Chandeleur and Breton Sounds to the Chandeleur and Breton Shoals was likely deposited between 600 and 4,700 years ago. The 50+ miles of the modern Plaquemines-Balize Delta downstream of New Orleans has all been deposited in just the last 800 to 1,000 years (Darut et al. (2005). During this same period (last 7,000 years) the Mississippi River has advanced its mouth approximately 200 river miles into the Gulf of Mexico. The emplacement of jetties at the river’s mouth in the late 1870’s served to accelerate the seaward extension of the main distributary passes (utilized as shipping channels) to an average advance of about 70 meters per year, or about six times the historic rate (Coleman, 1988; Gould, 1970). The combination of channel extension and sea level rise has served to flatten the grade of the river and its adjoining flood plains, diminishing the mean grain size of the river’s bed load, causing it to deposit increasing fine grained sediments. Channel sands are laterally restricted to the main stem channel of the Mississippi River, or major distributary channels, or “passes”, like the Metairie-Gentilly Ridge. The vast majority of the coastal lowland is infilled with silt, clay, peat, and organic matter. Geologic sections through the Mississippi Embayment show that an enormous thickness of sediment has been deposited in southern Louisiana (Figure 3.2). During the Quaternary Period, or Ice Ages, (11,000 to 1.6 million years ago) the proto Mississippi River conveyed a significantly greater volume of water on a much steeper hydraulic grade. This allowed large quantities of graveliferous deposits beneath what is now New Orleans, reaching thicknesses of up to 3600 feet (Figures 3.2 and 3.3). These stiff undifferentiated Pleistocene sands and gravels generally lie between 40 and 150 feet beneath New Orleans, and much shallower beneath Lake Pontchartrain and Lake Borgne (as one approaches the Pleistocene outcrop along the North Shore of Lake Pontchartrain). Just south of the Louisiana coast, the Mississippi River sediments reach thicknesses of 30,000 feet or more. The enormous weight of this sediment mass has caused the earth’s crust to sag in this area, resulting in a structure known as the Gulf Geosyncline (Figure 3.2). Flow of mantle material from below the Gulf Geosyncline is causing an uplift along about the latitude of Wiggins, MS. This is one cause of subsidence in South Louisiana (discussed in Section 3.7.2). Figure 3.4 presents a generalized geologic map of the New Orleans area, highlighting the salient depositional features. Depth contours on the upper Pleistocene age (late Wisconsin glacial stage) horizons are shown in red. Sea level was about 100 feet lower than present 3-2 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team about 9000 years ago, so the -100 ft contour represents the approximate shoreline of the Gulf at that time, just south of the current Mississippi River channel. Figure 3.5 presents a more detailed view of the dissected late Wisconsin stage erosional surface beneath New Orleans. This system emanates from the Lake Pontchartrain depression and reaches depths of 150 feet below sea level where it is truncated by the modern channel of the Mississippi River, which is not as deeply incised. A veneer of interdistributary deltaic deposits covers this older surface and is widely recognized for having spawned differential settlement of the cover materials where variations in thickness are severe, such as the Garden District. 3.1.3 Pine Island Beach Trend Relict beach deposits emanating from the Pearl River are shown in stippled yellow on Figure 3.4. Saucier (1963) named these relic beaches the Pine Island and Miltons Island beach trends. These sands emanate from the Pearl River between Louisiana and Mississippi, to the northeast. The Miltons Island Beach Trend lies beneath the north shore of Lake Pontchartrain, while the Pine Island Beach Trend runs northeasterly, beneath the Lakeview and Gentilly neighborhoods of New Orleans up to the Rogolets. The Pine Island Beach Trend is believed to have been deposited when sea level had almost risen to its present level, about 4500 years ago. At that juncture, the rate of sea level rise began to slow and there was an unusually large amount of sand being deposited near the ancient shoreline by the Pearl River, which was spread westerly by longshore drift, in a long linear sand shoal, which soon emerged into a beach ridge along a northeast-southwest trend (Saucier, 1963). The subsequent development of accretion ridges indicate that shoreline retreat halted and the beach prograded southwestward, into what is now the Gentilly and Lakeview areas. By about 5,000 years ago, the beach has risen sufficiently to form a true barrier spit anchored to the mainland near the present Rigolets, with a large lagoon forming on its northern side (what is now Lake Pontchartrain, which occupies an area of 635 mi2). Sometime after this spit formed, distributaries of the Mississippi River (shown as yellow bands on Figure 3.4) began depositing deltaic sediments seaward of the beach trend, isolating it from the Gulf of Mexico. The Pine Island Beach Trend was subsequently surrounded and buried by sediment and the Pine Island sands have subsided 25 to 45 feet over the past 5,000 years (assuming it once stood 5 to 10 feet above sea level). The distribution of the Pine Island Beach Trend across lower New Orleans is shown in Figure 3.6. The Pine Island sands reach thicknesses of more than 40 feet in the Gentilly area, but diminish towards the Lakeview area, pinching out near the New Orleans/Jefferson Parish boundary (close to the 17th Street Canal breach). The Pine Island beach sands created a natural border that helped form the southern shoreline of Lake Pontchartrain, along with deposition by the Mississippi River near its present course. Lake Pontchartrain was not sealed off entirely until about 3,000 years ago, by deposition in the St. Bernard’s Deltaic lobe (Kolb, Smith, and Silva, 1975). The Pine Island Beach Trend peters out beneath Jefferson Parish, as shown in Figures 3.4 and 3.6. 3.1.4 Interdistributary Zones Most of New Orleans’ residential areas lie within what is called an interdistributary zone, underlain by lacustrine, swamp, and marsh deposits, shown schematically in Figure 3.7. 3-3 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team This low lying area rests on a relatively thin deltaic plain, filled with marsh, swamp, and lacustrine sediments. The drainage canals were originally constructed between 1833-78 on interdistributary embayments, which are underlain by fat clays deposited in a quiet water, or paludal, environment (Kolb and Van Lopik, 1958). Interdistributary sediments are deposited in low lying areas between modern distributory channels and old deltas of the Mississippi River, shown schematically in Figure 3.8. The low angle bifurcation of distributary streams promotes trough-like deposits that widen towards the gulfward. Sediment charged water spilling over natural channel levees tends to drop its coarse sediment closest to the channel (e.g. Metairie and Gentilly Ridges) while the finest sediment settles out in shallow basins between the distrubutaries. Finegrained sediment can also be carried into the interdistributary basins through crevasse-splays well upstream, which find their way into low lying areas downstream. Storms can blow sediment-laden waters back upstream into basins, while hurricanes can dump sediment-laden waters onshore, though these may be deposited in a temporarily brackish environment. Considerable thickness of interdistributary clays can be deposited as the delta builds seaward. Kolb and Van Lopik (1958) noted that interdistributary clays often grade downward into prodelta clays and upward into richly organic clays of swamp or marsh deposits. The demarcation between clays deposited in these respective environments is often indistinct. True swamp or marsh deposits only initiate when the water depth shallows sufficiently to support vegetation (e.g. cypress swamp or grassy marsh). The interdistributary zone is typified by organic clays, with about 60% by volume being inorganic fat clays, and 10% or less being silt (usually in thin, hardly discernable stringers). Kolb and Van Lopik (1958) reported cohesive strengths of interdistributary clays as ordinarily being something between 100 and 400 psf. These strengths, of course, depend also on the past effective overburden pressure. Careful logging is required to identify the depositional boundary between interdistributary (marsh and swamp) and prodelta clays (Figure 3.9). The silt and fine sand fractions in interdistributary materials are usually paper-thin partings. Prodelta clays are typified by a massive, homogeneous appearance with no visible planes or partings. Geologically recent interdistributary clays, like those in lower New Orleans, also tend to exhibit underconsolidation, because they were deposited so recently. Interdistributary clays in vicinity of South Pass (45 miles downstream of New Orleans) exhibit little increase in strengths to depths of as much as 375 ft. This is because these materials were deposited rapidly, during the past 600 to 1,000 years, and insufficient time has passed to allow for normal consolidation, given the low drainage characteristics of the units. This phenomenon was noted and analyzed for offshore clays by Terzaghi (1956). The older prodelta clays underlying recent interdistributary clays tend to exhibit almost linear increase of density and strength with depth, because these materials were deposited very slowly. So, the environment of deposition greatly impacts soil strength. 3.1.5 Paludal environments Paludal environments on the Mississippi River deltaic plain are characterized by organic to highly organic sediments deposited in swamps and marshes. Paludal environments 3-4 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team are typified half-land and half-water, with water depths seldom exceeding two feet above mean gulf level. 90% of New Orleans is covered by swamp or marsh deposits (excluding filled areas). Lacustrine (lake) and tidal channel deposits can be complexly intermingled with swamp and marsh deposits. 3.1.5.1 Marshes More than half of the New Orleans area was once covered by marshes, essentially flat areas where the only vegetation is grasses and sedges. Tufts of marsh grass often grow with mud or open water between them. When these expanses are dry, locals often refer to them as “prairies.” As the marshes subside, grasses become increasingly sensitive to increasing salinity. As grasses requiring fresh water die out, these zones transition into a myriad of small lakes, eventually becoming connected to an intricate network of intertidal channels that rise and fall with diurnal tides. These are often noted on older maps as “brackish” or “sea marshes” to discern them from adjoining fresh water swamps and marshes (Figure 3.9). Marsh deposits in New Orleans are typically comprised of organic materials in varying degrees of decomposition. These include peats, organic oozes, and humus formed as marsh plants die and are covered by water. Because the land is sinking, subaerial oxidation is limited, decay being largely fomented by anerobic bacteria. In stagnant water thick deposits consisting almost entirely of organic debris are commonplace. The low relative density of these materials and flooded nature provides insufficient effective stress to cause consolidation. As a consequence, the coastal marsh surface tends to “build down,” as new vegetation springs up each year at a near-constant elevation, while the land continues to subside. In areas bereft of inorganic sediment, thick sequences of organic peat will accumulate, with low relative density. If the vegetation cannot keep pace with subsidence, marine waters will inundate the coastal marsh zone, as noted in the 1849 map in Figure 3.10. Peats are the most common variety of marsh deposits in New Orleans. They usually consist of brown to black fibrous or felty masses of partially decomposed vegetative matter. Materials noted on many of the older boring logs as “muck” or “swamp muck” are usually detrital organic particles transported by marsh drainage or decomposed vegetative matter. These mucks are watery oozes that exhibit very low shear strength and cannot support any appreciable weight. Inorganic sediments may also accumulate in marshes, depending on the nearness of a sediment source(s). Common examples are sediment-laden marine waters and muddy fluvatile waters. Brackish marsh deposits interfinger with fresh water deposits along the southern shore of Lake Pontchartrain, but dominate the shoreline around Lake Borgne. Floating marsh materials underlie much of the zone along old watercourses, like Bayou St. John and Bayou des Chapitoulas. Kolb and Van Lopik (1958) delineated four principal types of marsh deposits in New Orleans: 1. Fresh water marsh consists of a vegetative mat underlain by clays and organic clays. Fresh water marshes generally form as a band along the landward border of established marshes and in those areas repeatedly subjected to fresh water inundation. In most instances an upper mat of roots and plant parts at least 12 inches thick overlies fairly soft organic clays, 3-5 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team which become firmer and less organic with depth. Peat layers are often discontinuous and their organic content is usually between 20 and 50%. 2. Floating marsh or flotant is a vegetative mat underlain by organic ooze. This is sometimes referred to as a “floating fresh marsh” or “floating three-cornered grass marsh.” The vegetative mat is typically between 4 and 14 inches thick, floating on 3 to 15 ft of finely divided muck or organic ooze, grading into clay with depth. The ooze often consolidates with depth and grades into a black organic clay or peat layer. 3. Brackish-fresh water marsh sequence consists of a vegetative mat underlain by peat. The upper mat of roots and recent marsh vegetation is typically 4 to 8 inches thick and underlain by 1 to 10 ft of coarse to medium textured fibrous peat. This layer is often underlain by a fairly firm, blue-grey clay and silty clay with thick lenses of dark grey clays and silty clays with high organic contents. The great majority of marsh deposits in New Orleans are of this type, with a very high peat and humus content, easily revealed by gravimetric water content and/or dry bulk density values. 4. Saline-brackish water marsh is identified by a vegetative mat underlain by clays. These are sometimes termed “drained salt marshes” on older maps. The typical sequence consists of a mat of roots, stems, and leaves from 2 to 8 inches thick, underlain by a fairly firm blue-grey clay containing roots and plant parts. Tiny organic flakes and particles are disseminated through the clay horizon. The clays tend to become less organic and firmer with depth. The saline to brackish water marsh occupies a belt ½ to 8 miles wide flanking the present day shoreline, along the coast. The strengths of marsh deposits are generally quite low, depending on their water content. Embankments have been placed on vegetative mats underlain by ooze, supporting as much as 2 or 3 psi of loading, provided it is uniformly applied over reasonable distances, carefully (Kolb and Van Lopik, 1958). Field observations of sloped levees founded on such materials indicate failure at heights of around 6 feet, which exert pressures close to those cited above. 3.1.5.2 Swamps Before development, swamps in the New Orleans were easily distinguishable from marshes because of the dense growth of cypress trees. All of the pre-1900 maps make reference to extensive cypress marshes in lower New Orleans, between the French Quarter and Lake Pontchartrain (Figure 3.11). Encountering cypress wood in boreholes or excavations is generally indicative of a swamp environment. These cypress swamps thrived in 2 to 6 feet of water, but cannot regenerate unless new influx of sediment is deposited in the swamp, reducing the water depth. Brackish water intrusion can also cause flocculation of clay and premature die out of the cypress trees. Two layers of cypress swamp deposits are recognized to extend over large tracts of New Orleans (WPA-LA, 1937). The upper layer is the historic swamp occupying the original ground surface where infilling has occurred since the founding of the city in 1718; and the second; is a pervasive layer of cypress tree stumps that lies 20 to 30 feet below the ground surface, around -25 ft MGL (Mean Gulf Level). This older cypress forest was undoubtedly 3-6 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team killed off and buried in a significant pre-historic flood event, fomented by considerable deposition of inorganic sediment. This sudden influx of sediment may have come from a crevasse-splay along the Mississippi River upstream of New Orleans, as in most of the damaging floods that befell the city prior to 1849. There are two principal types of swamps in the New Orleans area, inland swamps and mangrove swamps. Inland swamps typically occupy poorly drained areas enclosed by higher ground; either natural levee ridges (like Metairie Ridge) or, much older (Pleistocene age) Prairie Terraces. These basins receive fresh water from overflow of adjacent channels during late spring and early summer runoff. The trees growing in inland swamps are very sensitive to increases in salinity, even for short-lived periods. Continued subsidence allows eventual encroachment of saline water, gradually transforming the swamp to a grassy marsh. The relative age of the tree die-off is readily seen in the form of countless dead tree trunks, followed by stumps, which become buried in the marsh that supersedes the swamp. As a consequence, a thin veneer of marsh deposits often overlies extensive sequences of woody swamp deposits. The converse is true in areas experiencing high levels of sedimentation, such as those along the historic Mississippi and Atchafalaya River channels, where old brackish water marshes are buried by more recent fresh water swamp deposits. Swamp deposits typically contain logs, stumps, and arboreal root systems, which are highly permeable and conductive to seepage. Mangrove swamps are the variety that thrives in salt water, with the two principal varieties being black and honey mangrove. Mangrove swamps are found along the distal islands of the Mississippi Delta, such as Timbalier, Freemason North, and the Chandeleur Islands, well offshore. Mangrove swamps also fringe the St. Bernard Marsh, Breton and Chandeleur Sounds, often rooting themselves on submerged natural levees. Mangrove swamps can reach heights of 20 to 25 feet in Plaquemines Parish. A typical soil column in a mangrove swamp consists of a thin layer of soft black organic silty clay with interlocking root zone that averages 5 to 12 inches thick. Tube-like roots usually extend a few inches above the ground surface. Thicknesses of five feet or more are common. Where they grow on sandy barrier beaches, the mangrove swamps thrive on the leeward side, where silts and clays intermingle with wash-over sands off the windward side, usually mixed with shells. Surficial swamp deposits provide the least favorable foundations for structures and man-made improvements, like streets and buried utilities. Kolb and Saucier (1982) noted that the amount of structural damage in New Orleans was almost directly proportional to the thickness of surficial organic deposits (swamps and marshes). This peaty surface layer reaches thicknesses of up to 16 ft, as shown in Figure 3.12. Most of this foundation distress is attributable to differential settlement engendered by recent de-watering (discussed in Section 3.7.4). 3.1.5.3 Lacustrine Deposits Lacustrine deposits are also deposited in a paludal environment of deltaic plains. This sequence most often occurs as marshes deteriorate (from lack of sediment) or subside (or both). These lakes vary in size, from a few feet in diameter to the largest, Lake Salvador (a few miles southwest of New Orleans), which measures 6 by 13 miles. Lake Pontchartrain (25 3-7 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team x 40 miles) is much larger, but is not a true marshland lake. The depths of these lakes vary from as little as 1.5 feet to about 8 feet (Lake Pontchartrain and Lake Borgne average 15 and 10 feet deep, respectively). Small inland lakes within the marsh environment usually evolve from subsidence and erosion from wind shear and hurricane tides. Waves set up a winnowing action which concentrates the coarser material into the deepest portion of the lake. These lakes are generally quite shallow, often only a foot or two deep, even though up to a mile long. They are simply water-filled depressions on the underlying marsh, often identified in sampling by fine grained oozes overlying peats and organic clays of the marsh that preceded the transition to lake. The ooze become increasingly cohesive with age and depth, but is generally restricted to only 1 to 3 feet in thickness in small inland lakes. Transitional lakes are those that become larger and more numerous closer to the actively retreating shoreline of the delta. These lake waters are free to move with the tides and currents affecting the open water of adjacent bays and sounds. Fines are often winnowed from the beds of these lakes and moved seaward, leaving behind silts and fine sands. Sediments in these lakes are transitional between inland lakes and the largely inorganic silty and sandy materials flooring bays and sounds. Large inland lakes are the only lacustrine bodies where significant volumes of sediment are deposited. Principal examples would be the western side of Lake Borgne, Lake Pontchartrain, and Lake Maurepas, among others. Lacustrine clays form a significant portion of the upper 20 to 30 feet of the deltaic plain surrounding New Orleans. Lake Pontchartrain appears to have been a marine water body prior to the deposition of the Metairie Ridge distributary channel, which formed its southern shoreline, sealing it off from the Gulf. The central and western floor of Lake Pontchartrain is covered by clays, but the northern, eastern and southern shores are covered by silts and sands, likely due to the choppy wave-agitated floor of the shallow lake. Deeper in the sediment sequence oyster shells are encountered, testifying that saline conditions once existed when the lake was open to the ocean. The dominant type of mollusk within Lake Pontchartrain today is the clam Rangia cuneata, which favors brackish water. Dredging for shells was common in Lake Pontchartrain until the late 1970’s. During Hurricanes Katrina and Rita in 2005, wind shear removed extensive tracts of marsh cover, creating 118 square miles of new water surface in the delta. Forty-one square miles of shear-expanded pools were added to the Breton Sound Basin within Plaquemines Parish. This was more erosion and land loss than had occurred during the previous 50 years combined (Map USGS-NWRC 2006-11-0049). 3.1.6 Recognition Keys for Depositional Environments Marsh deposits are typified by fibrous peats; from three principal environments: 1) fresh water marshes; 2) floating marsh – roots and grass sitting on an ooze of fresh water; and 3) saltwater marshes along the coast. The New Orleans marsh tends to be grassy marsh on a flat area that is “building down,” underlain by soft organic clays. Low strength smectite clays 3-8 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team tend to flocculate during brackish water intrusions, most commonly triggered by hurricanes making landfall in the proximate area. Typical recognition keys for depositional environments have been summarized as follows. • • • • • • Cypress wood = fresh water swamp Fibrous peaty materials = marshes Fat Clays with organics; usually lacustrine. A pure fat clay has high water content (w/c) and consistency of peanut butter Interdistributary clays; paludal environments; lakes - Silt lenses when water is shallow and influenced by wind swept waves Lean clays CL Liquid Limit (LL) 70% Abandoned meanders result in complex mixtures of channel sands, fat clay, lean clay, fibrous peat, and cypress swamp materials, which can be nearly impossible to correlate linearly between boreholes. The New Orleans District of the Corps of Engineers has historically employed 3-inch diameter Steel Shelby tubes and 5-inch diameter piston sampler, referring to samples recovered from the 5-inch sampler as their “undisturbed samples.” These are useful for characterizing the depositional environment of the soils. The larger diameter “undisturbed” samples are usually identified on boring logs and cross sections in the New Orleans District Design Memoranda by the modifier “U” for “undisturbed” samples (e.g. Boring prefixes X-U, UMP-X, MUE-X, MUG-X, and MUW-X). 3.1.7 Holocene Geology of New Orleans The surficial geology of the New Orleans area is shown in Figure 3.13. The Mississippi River levees form the high ground, underlain by sands (shown as bright yellow in Figure 3.13). The old cypress swamps (shown in green) and grassy marshlands (shown in brown) occupied the low lying areas. The Mid-town area between the Mississippi and Metairie Ridge was an enclosed depression (shown in green) known as a “levee flank depression” (Russell, 1967). The much older Pleistocene age Prairie formation (shown in ochre) lies north of Lake Pontchartrain. This unit dips down beneath the city and is generally encountered at depths greater than 40 feet between the city (described previously). The levee backslope and former swamplands north of Metairie Ridge are underlain by four principal stratigraphic units, shown in Figure 3.14. The surface is covered by a thin veneer of recent fill, generally a few inches to several few feet thick, depending on location. This is underlain by peaty swamp and marsh deposits, which are highly organic and susceptible to consolidation. Entire cypress trunks are commonly encountered in exploratory borings, as shown in Figure 3.15. This unit contains two levels of old cypress swamps, discussed previously, and varies between 10 and 40 feet thick, depending on location. The clayey material beneath this is comprised of interdistributary materials deposited in a paludal (quiet water) environment, dominated by clay, but with frequent clay stringers. This unit pinches out in vicinity of the London Avenue Canal and increases in thickness to about 15 feet beneath the 17th Street Canal, three miles west. Occasional discontinuous lenses of pure clay are often encountered which formed through flocculation of the clay platelets when the swamp was inundated by salt water during severe hurricanes. 3-9 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team The area east of the Inner Harbor Navigation Canal (IHNC) is quite different (Figure 3.14), in that these deposits are dominated by fine-grained lacustrine deposits deposited in proto Lake Pontchartrain, and the Pine Island Sands are missing. These lacustrine materials extend eastward and are characterized by clays and silty clays with intermittent silt lenses and organics. The lacustrine facies is underlain by the distinctive Pine Island Beach Sand, described previously. These relict beach sands thicken towards the east, closer to its depositional source. They reach a maximum thickness of about 30 ft. It thins westward towards Jefferson Parish, where it is only about 10 feet thick beneath the 17th Street Canal, as shown in Figure 3.14. The Pine Island sands are easily identified by the presence of mica in the quartz sand, and were likely transported from the mouth of the Pearl River by longshore drift (Saucier, 1963). Broken shells are common throughout the entire layer. A bay sound deposit consisting of fine lacustrine clays begins just east of the Inner Harbor Navigation Canal; it begins near the 40 foot depth, has about a 10 foot thickness and continues to the west across the city, thickening along the way (Figure 3.14). It reaches its greatest thickness of about 35 feet just east of the 17th Street Canal. It is interesting to note that this area has experienced the greatest recorded settlement in the city, which may be attributable to dewatering of the units above this compressible lacustrine clay, increasing the effective stress acting on these materials (areas to the east are underlain by much more sand, which is less compressible). The Holocene age deposits reach their greatest thickness just east of the 17th Street Drainage Canal where they are 80 feet thick (Figure 3.5). Undifferentiated Pleistocene deposits lie below these younger deposits. For the most part, this area sits below sea level with the exception of the areas along old channels and natural levees. The Metairie-Gentilly Ridge lies above the adjacent portions of the city because it was an old distributary channel of the Mississippi River (Figure 3.1upper). The same is true for the French Quarter and Downtown New Orleans, which are built on the natural sand levee of the Mississippi River. Geology from the Inner Harbor Navigation Canal to the east becomes exceedingly complex. Although the surficial 10 feet consist of materials from an old cypress swamp, this is an area dominated by the Mississippi River and its distributaries, especially the old St. Bernard delta (See Figure 3.1-lower). Distributaries are common throughout the area and consist of sandy channels flanked by natural levees. 10-15 feet of interdistributary materials, mainly fine organic materials, are present between distributaries. Relic beaches varying in thickness from 10 to 15 feet are present below the interdistributary deposits. These beaches rest atop a 5-10 foot thick layer of nearshore deposits which are then followed by a thick sequence of prodelta clays leading out into the Gulf of Mexico. 3.1.8 Faulting and Seismic Conditions Subsidence of the Gulf Geosyncline has led to numerous “growth” faults in South Louisiana. One group, the Baton Rouge Fault Zone (shown in Figure 3.7), is currently active 3 - 10 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team and passes in an east-west direction along the north shore of Lake Pontchartrain. Localized faulting is also common near salt domes. There has been no known faulting in the New Orleans area which has been active in Holocene times. The area is seismically quiescent. The earthquake acceleration with a 10% chance of being exceeded once in 250 years is about 0.04g. 3.2 Geologic Conditions at 17th Street Canal Breach 3.2.1 Introduction The 17th St. Canal levee (floodwall) breach is one of New Orleans’ more interesting levee failures. It is one of several levees that did not experience overtopping. Instead, it translated laterally approximately 50 feet atop weak foundation materials consisting of organic-rich marsh and swamp deposits. Trees, fences, and other features on or near the levee moved horizontally but experienced very little rotation, indicating the failure was almost purely translational in nature. 3.2.2 Interpretation of Geology from Auger Borings A series of continuously sampled borings was conducted and logged using 3-inch Shelby tubes in the vicinity of the 17th St. Outlet Canal levee failure on 2-1-2006 (east side) and 2-7-2006 (west side) to characterize the geology of the materials serving as a foundation for the levee embankments and floodwalls. Drilling on the east bank took place just behind (east) of an intact portion of the levee embankment that had translated nearly 50 feet while drilling on the west side took place directly across the canal from the middle of the eastern breach. This drilling uncovered a wide range of materials below the embankments and provided insights into the failure. Drilling on the east side of the levee was started at approximately 2-3 feet above sea level. A thin layer of crushed rock fill placed by contractors working for the U.S. Army Corps of Engineers to provide a working surface at the break site was augered through before reaching the native materials. Upon drilling at the east side of the levee, organic matter was encountered almost immediately and a fetid swamp gas odor was noted. This organic matter consisted of low-density peat, humus, and wood fragments intermixed with fine sand, silt, and clay, possibly due to wind shear and wave action from prehistoric hurricanes. This area appears to have been near the distal margins of a historic slough, as shown in Figure 3.16. At 4-6 feet, highly permeable marsh deposits were encountered and drilling fluid began flowing from a CPT hole several feet away, indicative of almost instantaneous conductivity at this depth. The CPT was sealed with bentonite before proceeding to prevent further fluid loss. The bottom of this sample was recovered as a solid 3-inch core of orange-red cypress wood indicating that this boring had passed through a trunk of stump of a former, but geologically young, tree. A suspected slide plane was discovered at a depth between 8.3 and 11 feet below the ground surface depending on the location of the borings, indicative of an undulating slip surface. Gray plastic clays appeared to have been mixed with dark organics by shearing and 3 - 11 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team this zone was extremely mushy and almost soupy in texture. The water content was very high, on the order of 278%. Organic rich deposits continue to a depth of about 20 feet below the surface while showing an increasing clay and silt content. Most clays are highly plastic with a high water content although there are lenses of lower plasticity clay, silt, and some sand. The variability of grain sizes and other materials is likely due to materials churned up by prehistoric storms. The clays are usually gray in color but vary and are olive, brown, dark gray, and black depending on the type and amount of organic content. Some organic matter towards the base of this deposit was likely roots that grew down through the pre-existing clays and silts or tree debris and that were mixed by prior hurricanes. Some woody debris came up relatively free of clays and closely resembled cypress mulch sold commercially for landscaping purposes. Full recoveries of material in this zone were rarely achieved in this organic rich zone. It appears that the low-density nature (less than water) of these soils caused them to compress due to sampling disturbances. Most material below 21 feet was gray plastic clay varying from soft to firm and nearly pure lacustrine in origin. This clay included many silt lenses which tended to be stiffer and had some organics at 26 feet. It is likely that the silt and organics were washed into an otherwise quiet prehistoric Lake Pontchartrain by storms. Sand and broken shells showed up at 30 feet in depth and continued to increase in quantity and size until 35.5 feet when the material became dirty sand with very little cohesion. This hole was terminated at 36 feet. These sands appear to be the Pine Island Beach Trend deposits, described in Section 3.1.3. The geologic conditions beneath the 17th Street Canal breach are shown in Figures 3.17 thru 3.20. Figure 3.17 shows the relative positions of the cross sections presented in Figures 3.18 and 3.19. Figure 3.18 is a geologic section through the 17th Street Canal breach, extending into the canal. It was constructed using Brunton Compass and tape techniques commonly employed in engineering geology (Compton, 1962). In this section the landside of the eastern levee embankment translated laterally about 48 feet. The levee had two identifiable fill horizons, separated by a thin layer of shells, likely used to pave the old levee crest or the road next to the levee prior to 1915 (similar to the conditions depicted in Figure 4.18). A distinctive basal rupture surface was encountered in al the exploratory borings, as depicted in Figures 3.18 and 3.19. This rupture surface was characterized by the abrupt truncation of organic materials, including cypress branches up to two inches in diameter (shown in the inset of Figure 3.18). The rupture surface was between ¾ and 1 inch thick, and generally exhibited a very high water content (measured as 279% in samples recovered and tested). This material had a liquid consistency with zero appreciable shear strength. It could only be sampled within more competent materials in the Shelby Tubes. A brecciated zone three to four inches thick was observed in samples immediately above the rupture surface. This contained chunks of clay with contrasting color to the matrix materials, and up to several inches across, along with severed organic materials. The geologic cross section portrayed in Figure 3.19 was taken on the north side of the same lot, using the same Brunton Compass and tape technique. It was located between 80 and 100 feet north of the previous section described above, as shown in Figure 3.17. In this 3 - 12 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team location the landside of the levee embankment translated about 52 feet laterally, to the east. These offsets were based on tape measurement made from the chain link right-of-way fences along the levee crest. No less than four distinct thrust planes were identified in the field, suggesting a planar, translational failure mode, as sketched in the cross section. As with the previous section, the old swamp deposits are noticeably compressed beneath the levee embankment, likely due to fill surcharge and the fact that the drainage canals have never been drained over their lifetime (in this case, since 1858 or thereabouts, described in Section 4.6). This local differential settlement causes the contact between the swamp deposits and the underlying lacustrine clays to dip northerly, towards the sheetpile tips supporting the concrete I-walls constructed in 1993-94. There was ample physical evidence that extremely high pore pressures likely developed during failure and translation of the levee block, in the form of extruded bivalve shells littering the ground surface at the second toe thrust, as shown in Figure 3.20 and indicated on the cross section (Figure 3.19). Planar translational failures are typical of situations where shear translation occurs along discrete and semi-continuous low strength horizons (Cruden and Varnes, 1996). Additional evidence of translation is the relatively intact and un-dilated nature of the landside of the failed levee embankment, upon which the old chain link right-of-fence was preserved, as well as a substantial portion of the access road which ran along the levee crest, next to the concrete I-wall. Wherever we observed the displaced concrete I-wall in this area it was solidly attached to the Hoesch 12 steel sheetpiles, each segment of which was about 23 inches wide (as measured along the wall alignment) and 11 inches deep, with an open Z-pattern. The thickness of the sheets were about 7/16ths of an inch. The observed sheetpiles interlocks were all attached to one another. The entire wall system was quite stiff and fell backward (towards the canal) after translating approximately the same distance as the landslide of the levee embankment. The sheetpiles and attached I-walls formed a stiff rigid element. The sheetpiles were 23 ft-6 inches long and were embedded approximately 2 to 3 feet into the footings of concrete I-walls. The geology of the opposite (west) bank was relatively similar except that the organics persist in large quantities, to a depth of 36 feet. The marsh deposits appeared deeper here and root tracks filled with soft secondary interstitial clay persisted to a depth of 39 feet. Sand and shells were first encountered at 40 feet and cohesionless sand was found at 41 feet. This hole was terminated at 42 feet. 3.2.3 Interpretation of Data from CPT Soundings Six distinctive geologic formations are identified studying the Cone Penetrometer Test (CPT) soundings which were done in the vicinity of 17th Street Canal: Fill, swamp/marsh deposits, Intermixing deposits, lacustrine deposits, Pine Island beach sand deposits and Bay Sound deposits. The description and coverage of these geologic formations from CPT soundings are explained in the following paragraphs. These unit assignments are shown graphically in Figure 3.21. FILL: Fill is not present in all CPT soundings. It is characterized by stiff silty clay to sandy clay and sandy silt with some silt lenses. It is differentiated from the swamp deposits by having little or no organic matter in its content. Along the breached area, the fill appears to be missing in the CPT soundings. Fill thickness is around 10 ft (down to -8 ft below sea level) on 3 - 13 New Orleans Levee Systems Hurricane Katrina July 31, 2006 Independent Levee Investigation Team the west bank of the 17th street Canal. Just north of the breached area (east bank), the thickness of the fill ranges from 14 ft to 16 ft (down to -10 ft). Fill materials for the drainage canals appear to have been placed in three sequences: 1) during the original excavation of the various canals, between 1833-1878; 2) after the 1915 Grand Isle Hurricane; and 3) after the October 1947 hurricane (the history of the drainage canals is described in Chapter 4, Section 4.6). SWAMP/MARSH DEPOSITS: Marsh deposits consists of soft clays, organic clays usually associated with organic material (wood and roots). The organic materials are readily identifiable by observing the big jumps in the friction ratios of the CPT’s. The thickness of swamp/marsh deposits is around 9.5 ft on the west bank of the canal and 4 to 6 ft on the east bank of the canal. The depth at which swamp/marsh deposits encountered on banks ranges from approximately -8.5' (on the west side) to -10' (on the east side), using the NAVDD882004.65 datum. INTERMIXING ZONE: This zone consists of mixture of soft clays, silt lenses with little or no organic material. The thickness of intermixing zone ranges from 3 ft to 8.5 ft on the east bank of the canal. No intermixing zone is interpreted on the west ban...
Purchase answer to see full attachment