CHAPTER FIVE: THE LOWER MISSISSIPPI REGION AND PLAQUEMINES PARISH 5.1 Overview Plaquemines Parish is the area where the last portion of the Mississippi River flows out into the Gulf of Mexico (see Figures 2.6 and 5.1). Extending southeast from New Orleans, Plaquemines Parish straddles both sides of the lower reaches of the Mississippi River for about 70 miles out to the river s mouth in the Gulf. 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 and supplies servicing the large offshore oil fields out in the Gulf of Mexico. It is an area that is sparsely populated, with a population of only about 27,000 people in the entire parish just prior to s arrival (see Plaquemines Parish Government Website: http://www.plaqueminesparish.com). Most of these people live in small, unincorporated towns and villages along the river. Not only are these communities subject to potential flooding from the Mississippi River, but they are also vulnerable to flooding from hurricane surges because the parish extends so far out into the Gulf from the mainland. For flood protection from the Mississippi River, large federal project levees were constructed along both sides of the river with design crest elevations of approximately +25 feet (MSL). For many of the communities lying closely alongside the Mississippi River levees, hurricane or back levees were also constructed behind them to protect them from hurricane surges coming from the Gulf. These hurricane levees were constructed with lesser crest heights than the river levees, and typically had crest heights on the order of +17 to +18 feet (MSL). Thus, many of the homes in these areas are sandwiched between two sets of levees: one along the river and the other behind the towns. The Investigation Team was not able to devote significant time to detailed investigations and analyses of the numerous individual levee failures that occurred along this protected corridor. Accordingly, this chapter will present only a brief overview of the performance of the flood defenses in this parish during Hurricanes Katrina and Rita. As described previously in Chapter 2, Plaquemines Parish was the first developed area to be severely affected by the large onshore storm surge as approached the southern coast in the early morning of August 29, 2005. devastated many of the Plaquemines Parish communities. was reported to have induced storm surges on the order of up to 20 feet in this region, as shown in Figure 5.2. In addition, large storm waves atop this surge rose to greater heights. This storm surge, and the waves that accompanied it, overtopped and damaged many portions of the storm levees. Both the United States Army Corps of 5-1
Engineers (see Figures 2.6 and 5.1) and the Plaquemines Parish Government website report numerous breaches of the storm levees and widespread deep flooding and destruction. Figures 5.3 through 5.12 show examples of the types of damage and flooding that resulted from the overtopping and breaching of the protective hurricane levees. Figure 5.3 shows an aerial view of the inundation of the hamlet of Myrtle Grove, on the west side of the Mississippi River, as it appeared on September 25, 2005, one day after the second Hurricane (Rita) again inundated this section. Figure 5.4 shows an aerial photograph of a levee breach of the hurricane (back) levee on the western side of the Mississippi River near the community of Sunrise. The breach occurred at a transition between an earthen levee section with a sheetpile-supported concrete I-wall, and a plain structural floodwall section. Failures at transitions between different adjoining sections were relatively common throughout the affected area during. Figure 5.5 shows an aerial photograph of a breach of the hurricane (back) levee at another transition near the Hayes Pump Station. This time the failure occurred at a sheetpile transition between an earthen embankment and a structural floodwall section, and sheetpile to earthen embankment connection appears to have been the weak link. Figure 5.6 shows a pair of large shrimp boats on Highway 23, near the foot of the Empire High Rise Bridge. As illustrated by this photo, overtopping was quite severe, and large objects were floated up onto, and sometimes over, the levees. 5.2 Point a la Hache Point a la Hache is the parish seat for Plaquemines Parish and is located along the east side of the Mississippi River. Storm surges from the east largely overwhelmed the back levee, breached it in several places, and inflicted deep flooding and widespread destruction in this town. Figure 5.7 presents an aerial photograph of one such breach taken on September 25, 2005 (from Plaquemines Parish Government Website). Shown in this photograph is a temporary road constructed across the interim breach repair to facilitate access and repairs. Figure 5.8 shows this same levee breach a few weeks later during the installation of a sheetpile cutoff that was undoubtedly intended to be part of an interim, and perhaps permanent repair. The team members viewing the installation believed that the sheetpile wall was a good concept to affect a positive cutoff of seepage through the deeply scoured breach and loose debris. However, during the installation, team members noted that the contractor was having difficulty advancing the southern portion of the sheetpiles very far into the ground using the equipment in use at the time of the team s visit. It is hoped that the pilings ended up being driven to their needed depths. Residences in Pointe a la Hache were commonly inundated to depths of 12 to 18 feet (see Figure 5.9). Inundation flooding was so great that water flowed across the community from the east towards the Mississippi River, and even overtopped the Mississippi River levee 5-2
(at least with significant wave splashover) by several feet. Based on debris found on tractor equipment left on the levee crown along the Mississippi River, overflows or splashover of up to 4 feet were estimated. For most of the areas visited by our team, relatively little significant damage was observed on the Mississippi River levees, possibly because the river sides of the levees viewed by the team were paved with concrete slope protection (see previous Figure 2.17). Damage to the storm levees was significant at many locations, however, Like many New Orleans residences, the small wooden homes in Pointe a la Hache were commonly founded on cinderblock piers. As a result of the deep flooding and the flow towards the Mississippi River, homes in Pointe a la Hache were commonly picked up and floated away from their foundations. Many ended up being deposited on or across the Mississippi River Levee as a result of storm surges flowing from the overtopped storm levees towards the river levees alongside the Mississippi River (see Figures 5.10 through 5.12). 5.3 Erosion Studies Although overtopping caused numerous breaches in the storm levees facing away from the Mississippi River, less erosion was observed along most of the Federal river levees. This may have been due in part to the fact that the river-side levee embankments slope faces were paved with concrete slope face protection (as shown previously in Figure 2.17, which clearly shows this river-side slope face protection.) It may also have been due in part to the fact that the backsides of these river levees, which had no formal slope face protection, were at least partially protected from the full energy of the storm surge and the wind driven waves by the obstacles presented by the hurricane levees, and by other obstructions including buildings and trees, etc. Nonetheless, it is a noteworthy performance on the part of these levee embankments, and it merits further study. It is hoped that with further testing trends will emerge showing that soil type and character, as well as placement and compaction conditions, can be used as a relatively reliable basis for prediction of the level of vulnerability of levee embankment soils to erosion and scour. Issues associated with erosion are discussed in more detail in Chapter 9. 5.4 Summary Plaquemines Parish is the most obviously exposed populated and flood protected area in the region. It juts out into the Gulf of Mexico much like a boxer s chin, almost daring a knockout blow. Because Plaquemines Parish is so obviously exposed, the evacuation of the Parish was unusually comprehensive prior to Katrina s arrival. That was a good thing, as most of the lower reaches of the Parish were catastrophically flooded. Massive damage was done to homes and businesses in the many small and generally unincorporated townships, and there was at least one major rupture in an oil transmission line. The best information available to this investigation team at this time is that approximately 60 lives were lost in Plaquemines Parish during hurricane Katrina. 5-3
The merits of expending Federal dollars to attempt to defend the full Parish, or even large portions of it, in the face of ongoing regional subsidence, sea level rise, and increasing projected hurricane intensity due to rising Gulf water temperatures, warrant further study. Recent requests for up to $3 billion in Federal funds to repair and upgrade the levees for a narrow strip of land into which less than 15,000 to 20,000 people are currently expected to return would represent an expenditure of approximately $150,000 to $200,000 per capita. In the mean time, large amounts of Federal funds are currently being expended to repair the damaged levees in this Parish. 5.5 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, March 10, 2006. Plaquemines Parish Website, (2006), http://www.plaqueminesparish.com 5-4
Source: Modified after USACE Figure 5.1: Map showing the levee protected areas along the lower reaches of the Mississippi River (in the Plaquemines Parish Area). 5-5
New Orleans Levee Systems July 31, 2006 Investigation Team Source: IPET (2006) Figure 5.2: Aggregated maximum storm surge elevations (maximum among all times). 5-6
Source: http://www.plaqueminesparish.com/ Figure 5.3: Aerial photograph of inundated portion of Myrtle Grove along western side of the Mississippi River. [September 25, 2005] Sunrise: 29 21.62N 89 33.67W Source: http://www.plaqueminesparish.com/ Figure 5.4: Aerial photograph of levee breach of storm (back) levee along western side of the Mississippi River near the community of Sunrise. [September 25, 2005] 5-7
Source: http://www.plaqueminesparish.com/ Figure 5.5: Aerial photograph of levee breach of storm (back) levee at levee-to-wall transition near Hayes Pump Station. [September 25, 2005] Source: http://www.plaqueminesparish.com/ Figure 5.6: Aerial view of two large shrimp boats deposited on Highway 23 at the foot of the Empire High Rise Bridge. 5-8
East Pointe a La Hache: 29 35.68N 89 Source: http://www.plaqueminesparish.com/ Figure 5.7: Aerial photograph of levee breach of storm (back) levee East of Pointe a la Hache. [September 25, 2005] Photograph by Les Harder Figure 5.8: Photograph of Sheetpile Cutoff Being Placed into Levee Breach of Storm (Back) Levee East of Pointe a la Hache. [October 12, 2005] 5-9
Photograph by Les Harder Figure 5.9: Photograph of flood elevation on trees landward of hurricane levee East of Pointe a la Hache illustrating that flood waters remained to large depths for extended periods. [October 12, 2005] Photograph by Les Harder Figure 5.10: Photograph of Pointe a la Hache home deposited on Mississippi River levee crown after storm surges overtopped the storm levee from the East (left) towards the River which is to the right in this photograph. 5-10
Photograph by Les Harder Figure 5.11: Photograph of Pointe a la Hache homes deposited on Mississippi River Levee after storm surges overtopped the levee from the East (left) towards the River (right). [October 12, 2005] Photograph by Les Harder Figure 5.12: Photograph of Pointe a la Hache home site where a wood home was floated off of its cinderblock piers. [October 12, 2005] 5-11