Final Report from the NRC Committee on the Review of the Louisiana Coastal Protection and Restoration (LACPR) Program (2009) / Chapter Skim
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3 Key Scientific, Engineering, and Other Technical Topics
3 Key Scientific, Engineering, and Other Technical Topics
Pages 13-32
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From page 13... ...
. Any program or assessment regarding the future sustainability of Louisiana's coastline must consider historical and future trends in erosion and sea level rise.
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From page 14... ...
Treatment of Coastal Erosion Issues in the LACPR Report An assumption that "extensive coastal landscapes in Louisiana can be constructed and maintained" (USACE, 2009; main report, p.
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From page 15... ...
FIGURE 2: 100+ years of land change for coastal Louisiana.
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From page 16... ...
The LACPR draft final technical report suggests, but does not convincingly demonstrate, that wetlands losses can be countered and the current Louisiana coastline sustained. The LACPR draft final technical report proposes a dredging program with six dredges working around the clock, 365 days per year (USACE, 2009, p.
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From page 17... ...
, Louisiana -- Ecosystem Restoration Study. That NRC report concluded that it is not feasible to maintain coastal Louisiana in its current form and that the Corps should modify its plans and educate stakeholders about other approaches that will need to be taken: The Mississippi River delta is inherently dynamic and large.
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From page 18... ...
Rather than focusing energy and resources into trying to maintain the current configuration of southern Louisiana's eroding coastline, the LACPR team is encouraged to focus its protection and restoration plans on high-priority projects. ADDRESSING SCIENTIFIC UNCERTAINTIES IN RESTORATION Restoration efforts in ecosystems as large and complex as those in coastal Louisiana entail many scientific uncertainties and unknowns.
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From page 19... ...
The ADCIRC model was used to calculate water level associated with storm surge and the WAM and STWAVE models were used to calculate wave heights. The LACPR study does not consider the frictional dissipation due to bottom and vegetation interaction in STWAVE simulations.
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From page 20... ...
Third, the total amount of river water diversion -- 525,000 cubic feet/second -- is over 40 percent of the total channel flow. Because of the obviously large impact on main channel flow, the extent to which upstream diversions impact downstream diversions and promote sedimentation within the main channel should be considered; that is, the presence of upstream diversion creates greater uncertainty regarding the potential success of downstream diversions and may affect river sedimentation.
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From page 21... ...
Repairs and strengthening of the system are being carried out by the Corps of Engineers' "Task Force Hope" team with a goal to provide protection against hurricane storm surge with a 100-year recurrence interval, or the surge associated with a hurricane expected to occur on average once in 100 years, for New Orleans by 2011. The term "100-year recurrence interval" is a frequently used term to describe an event that has a one per cent chance of occurring in any given year.
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From page 22... ...
. In addition to the ASFPM report, the National Research Council recently issued a report on the New Orleans hurricane protection system, including the presentation of key lessons learned during Hurricane Katrina (NRC, 2009)
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From page 23... ...
, this committee noted that the LACPR draft technical reports defined ‘Category 5' protection as falling in the range of a 400-year to a 1,000-year event, a point that the LACPR reiterated in its 2009 draft technical report: USACE policy guidance memorandums directed that a set of measures be presented that could reduce risk across a range of storm surge events including 100-year risk reduction, a "low Category 5" event or Hurricane Katrina-like event (estimated as a 400-year surge event) and a "high Category 5" event (estimated as a 1000-year event)
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From page 24... ...
The flooding of New Orleans after Hurricane Katrina -- via waters from Lake Pontchartrain that flowed through breaches in floodwalls that line the city's outfall and navigation canals -- is a poignant example of this and a sobering reminder that failure of the hurricane protection system is a key component of the residual risks that attend hurricane and flood protection systems. The LACPR draft final technical report does not consider the possibility of failure when evaluating water levels and flooding associated with storm surge.
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From page 25... ...
In assessing flood risks for coastal Louisiana, the LACPR draft final technical report does not incorporate the probability of system failure as part of the risk assessment. The current approach highlights the use of a "qualitative" assessment of failure potential based on the total levee length, number and size of hydraulic structures, and overtopping.
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From page 26... ...
The alternative plans fall into five categories: • No-action alternatives, which assume continued loss of coastal lands. FIGURE 4: LACPR planning area and planning units.
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From page 27... ...
Plans assume protection from storm surges having recurrence intervals of 100 years, 400 years, or 1,000 years. As the level of protection rises, implementation costs increase but expected residual damages decrease.
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From page 28... ...
The Corps subsequently employed, in a single set of workshops, a swing weighting approach that produced somewhat better results. Swing weighting is considered preferable to direct elicitation because it provides respondents with an understanding of the possible range of each of the metrics.
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From page 29... ...
This committee is more skeptical, believing that substantial redesign of the MCDA process is needed, including extensive use of focus groups to redefine and refine metrics, not just additional iterations of the existing process. Nevertheless, the LACPR team attempted to make use of the MCDA results by combining them with other scoring and ranking approaches.
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From page 30... ...
Furthermore, they do not support the rankings of alternative plans as presented in the LACPR report. Nonstructural Measures The draft final technical report lacks specificity with regard to a number of the alternatives it proposes for further consideration.
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From page 31... ...
. The LACPR draft final technical report acknowledges many of the recommendations offered in this committee's previous, 2008 report -- such as the need for measures to counter induced development behind levees and to prevent new and more intensive development from occurring in the future in high hazard areas whether or not protected by levees.
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From page 32... ...
Implementation of a variety of nonstructural measures will be essential in better managing and reducing flood risks in southern Louisiana. The LACPR team and the Corps of Engineers should take a more aggressive leadership role in a variety of nonstructural measures that are important to reducing flood risks in coastal Louisiana.
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