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Levee Certification
Pages 139-158

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From page 139...
... . TIISTORY OF LEVEE CERTIFICATION Congress sought to accomplish two main goals in the National Flood Insurance Act of 1968 and the Flood Disaster Protection Act of 1973.
From page 140...
... , thereby removing mandatory flood insurance purchase regulations (the 1973 Flood Disaster Protection Act requires those who are buying, building, or improving property in special flood hazard areas within NFIP communities to purchase flood insurance as a prerequisite for federal financial assistance (e.g., loan, grant, disaster assistance) when the building or personal property is the subject of or security for such assistance)
From page 141...
... Risk analysis permits the Corps and FEMA to address problems resulting from uneven safety levels and from expenses inherent in the criterion of having the levee sized to the 100-year flood plus 3 feet of freeboard. As shown in Column 2 of Table 7.1, the 3-feet-of-freeboard criterion resulted in very different levels of protection in different communities.
From page 142...
... 142 Risk Analysis and Uncertainty In Flood Damage Reduction Studies :: ~ >: :} i ~ ~ ~ :: ~ ~ ~ ~ ~: ~ ~ ~ ~ ~ ~ :,: t,, ,_:f ~ :: ·: r~ha~si:: - : ~ r: :~. .~; | I :i ~is central :to the Nat~on ~: ~: ~1 .~ 1~ ~ ~ ~ ~ < ~of ~axpayer clollars~ hav: ~ :': ~,oo yeaF :floo~plair~ ~ an~d s~ ~ ~ ~ ~ ~ I 1~: 1ions ~ ;~ ~ a ~if~ theinall°ns A '- ~ .~: i~ s~a~jSt'Cal ~and~pu Ic I empl ed In m Pr I ~ ~ naly:ze:the~hydroJogica,~ ;~ ~: , 1 : ~ niversity~ professors:: ~a ~ ~ ~ ~ ~ ~ ~ f ~hoo~i:~ :Given~ the eco ~ ~ ~ .~ ~n ~; ~1 m li ions he OO- a ~ ~ lti assume that~the select~o ~: c~a mpo ance ihese onew (~eff~nint 'hydrolog~~cat e~ rd is bape¢i Oh ~sou:nd 1:OO-year flood as:~a :th ~ ~' ~ ~ statisi:iGal ~un~a:~ns~ ~ ~ ~ ~ ~ ~ ~ ~ ~ raphy a~ ~he uni~versit ~ !
From page 143...
... In its flood damage reduction studies, the
From page 144...
... 144 cn ~5 o ._ > Q a .
From page 146...
... this gave rise in 1996 to a subsequent Corps-FEMA proposal for levee certification, which combined the 3-feet-of-freeboard requirement when risk analysis was not performed with a probabilistic estimate of protection using annual exceedance probability (AEP) when risk analysis was performed.
From page 147...
... If the Corps had accounted for the various sources of uncertainty, as well as quantifying reasons for levee failure other than overtopping, the levels of freeboard and probability of containing a base flood would have been greater. In the committee's judgement, the major reasons for the difficulties with this 1 percent expected annual exceedance probability criterion were the Corps's method, the procedures that were followed to implement it, and the Corps's lack of experience (at that time)
From page 148...
... at the old criterion's elevation, if the old criterion resulted in a conditional nonexceedance probability between 90 percent and 95 percent. Figure 7.1 illustrates the levee certification decision tree.
From page 149...
... The discrepancy between these values and the traditional three feet of freeboard led the Corps and FEMA in 1996 to abandon the mean 1 percent expected annual exceedance probability criterion. Columns (9)
From page 150...
... Levees designed to the 90 percent conditional nonexceedance probability level have approximately the same median level of safety as the traditional standard of 3 feet of freeboard.2 The range in the freeboard required for the Corps-FEMA combined criterion (90%-3 ft-95%) compared to the FEMA standard 3-feet cnter~on 2 The risk analysis method adopted by the Corps involves a process that is considerably more complicated and requires much more data and analysis than the former procedure of adding 3 feet of freeboard.
From page 151...
... Figure 7.3 compares the elevation required for the National Economic Development plan with that for the three levee certification criteria for 11 of the 13 levee projects in Table 7.1 for which an National Economic Development plan elevation exists. On average, the National Economic Development plan provides approximately 0.5 feet of additional freeboard beyond the Corps-FEMA 90%-3ft-95% criterion, but in 4 of the 11 projects (Portage, Wisconsin; Pender, Nebraska; Guadalupe River, Texas; White River, Indiana)
From page 152...
... The federal government will, consequently, have little choice but to offer grants and low-interest loans, which is precisely what Congress wanted to eliminate by establishing the National Flood Insurance Program. There are two consequences of using an annual exceedance probability of 1/230 rather than 1/100 as the certification criterion.
From page 153...
... have to buy flood insurance that they need not have purchased. The data for the 13 flood damage reduction projects in Table 7.1 are plotted in Figure 7.4 to illustrate the relationship between levee freeboard and the expected level of protection for a number of levee sizing criteria.
From page 154...
... Assuming that the Corps's risk analysis was correct, the Corps was correct in resisting so expensive a project that went beyond the levee called for in the National Economic Development alternative. All Corps flood damage reduction studies prepared by Corps offices across the country are reviewed at Corps Headquarters in Washington, D.C.
From page 155...
... In this interim period, the committee therefore recommends that the Corps and FEMA adopt a single conditional nonexceedance probability for use in their joint levee certification program. A sample of 13 projects is of limited size for drawing general conclusions about the equivalence of risk measures among different levee sizing criteria.
From page 156...
... In reviewing the current certification procedure, the committee identified four technical issues that should be addressed in the near term to improve the risk analysis method. First, the procedure does not use straightforward probabilistic measures, such as annual probabilities of flooding, that are easily interpreted and easily compared to program mandates.
From page 157...
... In the committee's judgment, the criterion should aim to provide the level of protection provided to the most people in the past—the median level historically provided. Based on a small sample of Corps flood damage reduction projects, that median annual exceedance probability is roughly 1/230.
From page 158...
... Although the committee judges the latter to be what Congress intended in the National Flood Insurance Act of 1968 and its later amendments, we recognize that it has not been the criterion used in federal regulations. It is therefore recognized that review and discussion will likely be required before implementing the alternative criterion.


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