The story linked here details the costs (at least $29 million) and delays (at least 20 months) imposed upon a major highway and toll road project in Dallas, Texas by the levee that is to protect it. It seems to have been taken for granted that the levee along the Trinity River would be able to protect this major new piece of infrastructure that is to be placed behind it. However, the city has been instructed by the Army Corps of Engineers to verify the structural integrity of the levee before potentially risking the lives of thousands of drivers, especially in light of new and more stringent regulations in place after Hurricane Katrina. This appears to be a failure in planning, but it also calls to mind similar issues caused by levees.
FEMA requires that a levee have at least 3 feet of freeboard in order to remove the land behind it from the regulatory floodplain. The levee may have met this standard at the time of construction, but a new study may revise the floodplain elevation upward so that the levee no longer has the required freeboard. At this point, FEMA would have the Flood Insurance Rate Maps (FIRMs) changed so that the land once considered protected behind the floodplain is now in the regulatory floodplain. The effect of this is that mortgaged properties in this area will likely be required to obtain flood insurance. This may be costly and would likely both inhibit development and depress property values. The alternative is to upgrade the levee so that it, once again, has the freeboard required by FEMA.
The unfortunate part of the above situation is that FEMA has an all or nothing policy that does not recognize that the levee, assuming it is structurally sound, does still offer protection from events lesser than the regulatory 100-year flood. It assumes that the land behind this de-listed levee has the same flood risk as if the levee did not exist. A number of communities, many of which sit behind such levees that predate FEMA’s National Flood Insurance Program (NFIP), have become quite vocal about this policy and have pressured their Congressional representatives to change it.
The last thing I will mention here is the implications of a properly designed and maintained levee. There are costs here that are completely ignored. Specifically, though the event may be rare, a lot of damage will occur the moment the levee is breached. If the levee is 20 feet tall, it is likely that structures behind the levee would be flooded by at least 20 feet, which causes a lot of damage. Risk here is damage multiplied by frequency and results in the annual expected loss in dollars. The large amount of damage may offset the low frequency to result in a significant risk. Failure to account for this, along with the failure to account for structural deficiencies and maintenance issues, has contributed to the insolvency of the NFIP. Witness New Orleans after Hurricane Katrina.
Tags: Army Corps of Engineers, civil engineering, Dallas Morning News, FEMA, floodplain, infrastructure, levees
According to the New York Times, the White House Council on Environmental Quality (CEQ) is drafting new federal floodplain rules to be mandated by Executive Order. These rules are to be followed by the federal government and likely by anyone using federal money for a project. It seems that the federal government, like state departments of transportation, has been having trouble following the rules put forth by the FEMA National Flood Insurance Program (NFIP) and the White House would like have it lead by example. For this reason, these proposed rules are likely to exceed the minimums mandated by the NFIP. For example, critical infrastructure is not to be located in the 500-year floodplain unless no alternative is available. If development does occur in the 100-year floodplain, it is to occur in consultation with state and local officials; thus making it follow their rules should they prove more strict.
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