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Independent Levee Hurricane Katrina

Investigation Team May 22, 2006

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CHAPTERFIVE: THE LOWERMISSISSIPPI REGION

AND PLAQUEMINES PARISH

5.1 Overview

Plaquemines Parish is the area where the last portion of the Mississippi River flowsout into the Gulf of Mexico (see Figures 2.6 and 5.1). Extending southeast from NewOrleans, Plaquemines Parish straddles both sides of the lower reaches of the Mississippi Riverfor about 70 miles out to the rivers mouth in the Gulf. This protected strip, with riverlevees fronting the Mississippi River and a second, parallel set of storm levees facing awayfrom the river forming a protected corridor less than a mile wide, serves to protect a numberof small communities as well as utilities and pipelines. This protected corridor also providesprotected access for workers and supplies servicing the large offshore oil fields out in the Gulfof Mexico.

It is an area that is sparsely populated, with a population of only about 27,000 peoplein the entire parish just prior to Hurricane Katrinas arrival (see Plaquemines ParishGovernment Website: http://www.plaqueminesparish.com). Most of these people live insmall, unincorporated towns and villages along the river. Not only are these communitiessubject to potential flooding from the Mississippi River, but they are also vulnerable toflooding from hurricane surges because the parish extends so far out into the Gulf from themainland.

For flood protection from the Mississippi River, large federal project levees wereconstructed along both sides of the river with design crest elevations of approximately +25feet (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 fromhurricane surges coming from the Gulf. These hurricane levees were constructed with lessercrest heights than the river levees, and typically had crest heights on the order of +17 to +18feet (MSL). Thus, many of the homes in these areas are sandwiched between two sets oflevees: one along the river and the other behind the towns.

The Independent Levee Investigation Team was not able to devote significant time todetailed investigations and analyses of the numerous individual levee failures that occurredalong this protected corridor. Accordingly, this chapter will present only a brief overview ofthe 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 areato be severely affected by the large onshore storm surge as Hurricane Katrina approached thesouthern coast in the early morning of August 29, 2005.

Hurricane Katrina devastated many of the Plaquemines Parish communities.Hurricane Katrina was reported to have induced storm surges on the order of up to 20 feet inthis region, as shown in Figure 5.2. In addition, large storm waves atop this surge rose togreater heights. This storm surge, and the waves that accompanied it, overtopped anddamaged many portions of the storm levees. Both the United States Army Corps of

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Engineers (see Figures 2.6 and 5.1) and the Plaquemines Parish Government website reportnumerous 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 thatresulted 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, onthe west side of the Mississippi River, as it appeared on September 25, 2005, one day after thesecond Hurricane (Rita) again inundated this section.

Figure 5.4 shows an aerial photograph of a levee breach of the hurricane (back) leveeon the western side of the Mississippi River near the community of Sunrise. The breachoccurred at a transition between an earthen levee section with a sheetpile-supportedconcrete I-wall, and a plain structural floodwall section. Failures at transitions betweendifferent adjoining sections were relatively common throughout the affected area duringHurricane Katrina.

Figure 5.5 shows an aerial photograph of a breach of the hurricane (back) levee atanother transition near the Hayes Pump Station. This time the failure occurred at asheetpile transition between an earthen embankment and a structural floodwall section, andsheetpile 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 theEmpire High Rise Bridge. As illustrated by this photo, overtopping was quite severe, andlarge 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 eastside of the Mississippi River. Storm surges from the east largely overwhelmed the backlevee, breached it in several places, and inflicted deep flooding and widespread destruction inthis town. Figure 5.7 presents an aerial photograph of one such breach taken on September25, 2005 (from Plaquemines Parish Government Website). Shown in this photograph is atemporary 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 asheetpile cutoff that was undoubtedly intended to be part of an interim, and perhapspermanent 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 breachand loose debris. However, during the installation, team members noted that the contractorwas having difficulty advancing the southern portion of the sheetpiles very far into the groundusing the equipment in use at the time of the teams visit. It is hoped that the pilings ended upbeing 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 communityfrom the east towards the Mississippi River, and even overtopped the Mississippi River levee

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(at least with significant wave splashover) by several feet. Based on debris found on tractorequipment left on the levee crown along the Mississippi River, overflows or splashover of upto 4 feet were estimated. For most of the areas visited by our team, relatively little significantdamage was observed on the Mississippi River levees, possibly because the river sides of thelevees 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 Hachewere commonly founded on cinderblock piers. As a result of the deep flooding and the flowtowards the Mississippi River, homes in Pointe a la Hache were commonly picked up andfloated away from their foundations. Many ended up being deposited on or across theMississippi River Levee as a result of storm surges flowing from the overtopped stormlevees towards the river levees alongside the Mississippi River (see Figures 5.10 through5.12).

5.3 Erosion Studies

Although overtopping caused numerous breaches in the storm levees facing awayfrom the Mississippi River, less erosion was observed along most of the Federal riverlevees. This may have been due in part to the fact that the river-side levee embankmentsslope faces were paved with concrete slope face protection (as shown previously in Figure2.17, which clearly shows this river-side slope face protection.) It may also have been due inpart to the fact that the backsides of these river levees, which had no formal slope faceprotection, were at least partially protected from the full energy of the storm surge and thewind driven waves by the obstacles presented by the hurricane levees, and by otherobstructions 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 showingthat soil type and character, as well as placement and compaction conditions, can be used as arelatively reliable basis for prediction of the level of vulnerability of levee embankment soilsto erosion and scour. Issues associated with erosion are discussed in more detail in Chapter 9.

5.4Summary

Plaquemines Parish is the most obviously exposed populated and flood protected areain the region. It juts out into the Gulf of Mexico much like a boxers chin, almost daring aknockout blow.

Because Plaquemines Parish is so obviously exposed, the evacuation of the Parish wasunusually comprehensive prior to Katrinas arrival. That was a good thing, as most of thelower reaches of the Parish were catastrophically flooded. Massive damage was done tohomes and businesses in the many small and generally unincorporated townships, and therewas at least one major rupture in an oil transmission line. The best information available tothis investigation team at this time is that approximately 60 lives were lost in PlaqueminesParish during hurricane Katrina.

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The merits of expending Federal dollars to attempt to defend the full Parish, or evenlarge portions of it, in the face of ongoing regional subsidence, sea level rise, and increasingprojected 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 anarrow 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. Inthe mean time, large amounts of Federal funds are currently being expended to repair thedamaged levees in this Parish.

5.5 References

Interagency Performance Evaluation Task Force, (2006), Performance Evaluation, Statusand Interim Results, Report 2 of a Series, Performance Evaluation of the New Orleansand Southeast Louisiana Hurricane Protection System, March 10, 2006.

Plaquemines Parish Website, (2006), http://www.plaqueminesparish.com

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Source: Modified after USACE

Figure 5.1: Map showing the levee protected areas along the lower reaches of theMississi i River in the Pla uemines Parish Area .

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Source: IPET (2006)

Figure 5.2: Aggregated maximum storm surge elevations (maximum among all times).

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Source: http://www.plaqueminesparish.com/

Figure 5.3: Aerial photograph of inundated portion of Myrtle Grove along westernside of the Mississippi River. [September 25, 2005]


Figure 5.4: Aerial photograph of levee breach of storm (back) levee along western side of theMississippi River near the community of Sunrise. [September 25, 2005]

Sunrise: 29 21.62N 89 33.67W

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Figure 5.5: Aerial photograph of levee breach of storm (back) levee at levee-to-walltransition 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.

Hayes Pump Station: 29 30.79N 89 48.75W

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Figure 5.7: Aerial photograph of levee breach of storm (back) levee East of Pointea la Hache. [September 25, 2005]

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]

East Pointe a La Hache: 29 35.68N 89


Photograph by Les Harder

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Photograph by Les HarderFigure 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 depthsfor extended periods. [October 12, 2005]


Figure 5.10: Photograph of Pointe a la Hache home deposited on Mississippi River leveecrown after storm surges overtopped the storm levee from the East (left)towards the River which is to the right in this photograph.

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Figure 5.11: Photograph of Pointe a la Hache homes deposited on Mississippi River Leveeafter storm surges overtopped the levee from the East (left) towards the River(right). [October 12, 2005]


Figure 5.12: Photograph of Pointe a la Hache home site where a wood home was floated offof its cinderblock piers. [October 12, 2005]

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