As a coastal hydrologist who has been working on Southeast Louisiana hurricane surge hazard since 2004, I have been regularly asked by reporters, officials, and interested citizens: Are the upgraded New Orleans surge levees adequate to protect the city from the next Big One? I’ve responded that there’s a much more important question which should be directed to our state coastal and emergency response officials: Are hurricane evacuation plans adequate to prevent another catastrophic loss of life?
Over the past 10 years there’s been a drive to build stronger surge protection and eliminate the kinds of gross defects that contributed to the 1,400-plus deaths during Hurricane Katrina. But unfortunately this emphasis has obscured fundamental limitations with surge protection systems and the resulting need for critical measures to secure the lives of our coastal residents.
As The Lens has accurately reported, the US Army Corps of Engineers (per Congress) reconstructed the New Orleans post-Katrina levees for the National Flood Insurance Program (NFIP). The NFIP insures residential property, assessing premium rates based on statistical analyses of flooding. In particular, the NFIP relies on something called the 100-year flood hazard.
At any location, the flooding that is estimated to be reached or exceeded on average 10 times during a 1,000-year period (or 100 times in 10,000 years, etc.) has a one percent annual probability, or what scientists call an average “return period” of 100 years. A 100-year flood for a location is worse than a 50-year flood but not as bad as a 500-year flood.
NFIP statistical flood estimates only have to be good enough for NFIP purposes. They only have to support an actuarial management of residential property risks. The NFIP is mostly concerned with aggregate financial exposure and doesn’t focus intently on the local accuracy of flood statistics. Moreover, the NFIP flood hazard estimates are often biased low due to institutional and political constraints, as well as lack of frequent updating. NFIP flood hazard estimates, despite their multi-million-dollar price tag, are not sufficiently rigorous to manage risks to crucial economic and cultural assets — the Nuclear Regulatory Commission does not rely on NFIP estimates — much less when lives are directly at stake.
Nonetheless, because the NFIP premium rates affect property values in flood-prone areas, a wide range of federal, state, and local projects are designed to reduce the NFIP 100-year flood damage. The New Orleans post-Katrina levees were designed based on a 2008 NFIP study to minimize 100-year surge overtopping. While the new system is higher and stronger than before — and has revitalized interior property values — the NFIP surge overtopping estimates are not reliable for purposes of protecting lives and crucial economic and cultural assets.
Even the most rigorous scientific estimates of local surge hazard have tremendous uncertainties. Given the relatively short record of storms and surges, the best estimates of surge return periods could easily be off by 50 percent. A surge height of 10 feet at a location might be estimated to have a 100-year return, but could really have a 50-yr return; and a 13-foot 500-year surge estimate could really be a 250-year event. Surge hazards, including levee-overtopping statistics, should therefore be regarded as scientific guesstimates.
NFIP purposes do not require that a reasonably conservative “factor of safety” be applied to an estimate of the 100-year surge and overtopping to address all the crucial uncertainties and provide a cushion for protecting lives and community resources. Applied to an NFIP 100-year surge estimate of 10 feet, a reasonably conservative factor of safety might be 40 percent — increasing the value to 14 feet. Thus, a 100-foot surge estimate adjusted for uncertainty may prove higher than an unadjusted 500-year estimate!
A further problem is that, as expected, many raised levee segments in the New Orleans system are experiencing high rates of consolidation and settlement. Without timely addition of lifts, these segments have higher chances of overtopping to the point of breaching and collapse. Over the coming decades, geologic subsidence, sea level rise, and coastal erosion will further increase levee overtopping and breaching hazard. (Overtopping and breaching risks are lower for segments with new, overbuilt floodwalls.) Funding adequate maintenance of the system is going to be a continuing challenge for state and local governments.
The Corps’ plan for New Orleans earthen levees does include additional armoring — not required under the NFIP — to provide some resiliency against breaching. The level of armoring, which can include enhanced grass seeding and fertilization, plastic mesh turf reinforcement, or concrete, depends on local overtopping during an NFIP estimated 500-year surge. Such armoring is a cost-effective way to further reduce property flood risk. However, when a reasonably conservative uncertainty factor is applied, the estimated 500-year overtopping increases up to 10 times. Such armoring plans, therefore, should not be relied upon to protect lives, as the Corps itself acknowledges.
Recall that the NFIP flood statistics I’ve been discussing are referenced, like the 100-year surge hazard, to a specific location. The 10-foot NFIP 100-year surge hazard noted above might be for a specific segment of the lakefront levee. When surge probabilities are considered at a regional scale — say, around the entire New Orleans surge system — we must take into account multiple independent exposures. (The more people who flip a coin, the more the odds go up that someone will get tails.) Independent exposures around the New Orleans system mean that a 100-year surge event will occur somewhere around the system with a return period of less than 40 years; and a 500-year event with less than a 200-year regional return.
Let’s consider all four issues with New Orleans surge protection together: the rigor of NFIP surge hazard estimates, their inherent uncertainty, levee maintenance, and greater regional exposure. In this case the allowable overtopping under the system design is likely to occur somewhere around the New Orleans surge system at a regional return period of 10 to 20 years. Furthermore, a significant overtopping event with potential breach implications in spite of armoring is likely to occur somewhere at a regional return period of 50 to 100 years. Keep in mind that in addition to Katrina, extreme hurricanes struck the New Orleans area in 1893, 1915, and 1965 (Betsy).
Given that prospects are remote for raising the NFIP levee system, we might hope that coastal restoration can substantially lower the levee overtopping risks for New Orleans. But the surge science and realistic scope of restoration efforts are not comforting. While some exterior damage from lesser storms may be mitigated, dollars rightly dedicated to coastal ecosystem productivity are dollars not available to build the kinds of massive, robust features required to reduce extreme surge hazard.
Across Southeast Louisiana, NFIP surge protection systems also protect areas in Lafourche and Plaquemines parishes, and one is under construction in Terrebonne Parish. Additional NFIP levees are being contemplated for portions of St. Charles, St. John, St. James, and St. Tammany parishes. While officials can and should tout the benefits of NFIP surge protection, they must realistically face, and remind the public to appreciate, the remaining vulnerabilities. The mantra should be this: An NFIP levee is to surge what firefighting is to wildfires — no more than a complement to evacuation and property insurance.
It is OK to build coastal communities behind NFIP surge levees only if we are capable of carrying out comprehensive evacuation in the event of any storm threatening a 100-year surge. And experience and surge science have both shown that it doesn’t take a Betsy or Katrina to produce extreme surges over the long, shallow fetches of Louisiana’s coastal bays and lakes. A slow-moving, low-intensity storm like Isaac can be just as bad.
Southeast Louisiana experiences hurricane evacuation orders at a return period of less than five years. We should expect a hurricane requiring evacuation of one or more Southeast Louisiana NFIP-leveed areas to recur at a return period of between five and 10 years. And we should expect to evacuate New Orleans at a return period of between 10 and 20 years! (Note that these are average returns. Evacuations would occur more often during active hurricane periods in the Gulf of Mexico.)
Protecting lives is the No. 1 mission of state and local government. Therefore, effective hurricane evacuation planning must be the top coastal-protection priority. The lives of hundreds of thousands of Southeast Louisiana residents depend on safe, adequate hurricane evacuation routes. Parish and state official must ensure that the network of coastal roads, major highways, and interstates is always in good condition. “Contra-flow” plans that turn all lanes of major roadways into same-way escape routes must be fine-tuned. And crucially, officials must have proven pick-up, transportation, and sheltering contingencies in place to meet the needs of thousands of residents with health, financial, and logistical challenges to self-evacuation.
Our next Governor must ensure effective evacuation readiness for our entire coastal zone. As an important step, the 2017 Coastal Master Plan must provide for a detailed assessment of hurricane evacuation needs, developed in conjunction with the full complement of hurricane emergency response agencies. Ironically, evacuation was not even mentioned in the state’s 2012 Coastal Master Plan. With regard to the recent controversy over funding improvements to LA Highway 1, maintaining and upgrading this and other key parts of our coastal road network clearly are logical components of any plan to protect coastal lives — in addition to our coastal economy. They deserve very careful consideration in the 2017 Coastal Master Plan.
Bob Jacobsen, PE (www. bobjacobsenpe.com) has served as a surge hazard consultant to both the Southeast Louisiana Flood Protection Authority, East and the Louisiana Coastal Protection and Restoration Authority. He is the author of a 2013 State-of-the-Practice report. He was the President of the ASCE Louisiana Section in 2013-14. He recently authored the two-part article “Managing Hurricane Surge Risks in the Supercomputing Era.” He grew up in Metairie (wade fishing in Lake Pontchartrain) and his mother and sister still live in the home the family moved into two weeks before Hurricane Betsy.