IPET
The U.S. Army Corps of Engineers established the Interagency Performance Evaluation Task Force (IPET) to provide independent findings on how the New Orleans Hurricane and Storm Damage Reduction System (HSDRS) performed during Hurricane Katrina. IPET included more than 150 nationally recognized experts from over 50 different organizations (federal, state and local government agencies; academic institutions and the private sector) with two levels of outside peer review. They gathered and analyzed data to answer five basic questions:
- System (what was the status of the protection system on August 29, 2005?)
- Storm (what exact forces did Katrina put on the system?)
- Performance (how did the system respond?)
- Consequences (understanding the flooding and the losses – both economic and loss of life?)
- Risk (what is the risk and reliability of the system?)
Risk Analysis
The risk analysis modeling employs physics, mathematics, engineering, hydrology, geology and meteorology, and incorporates three main factors when determining risk:
- Hazard (probability of storms, their surge and waves)
- Protection System (performance of levees, floodwalls, other structures)
- Consequences (loss of life, property, etc.).
Storm Types
The probability of a storm occurring is typically communicated in one of two ways; as a percentage, or as a #-year storm. Here are some examples:
- a “50-year storm” = a storm that has a 2% chance of occurring in any given year
- a “100-year storm” = a storm that has a 1% chance of occurring in any given year
- a “5,000-year storm” = a storm that has a .02% chance of occurring in any given year
(Katrina was a 400-year storm = .25% chance of occurring in any given year) (a quarter of 1%)
Incorporating Hazard
Incorporating hazard into risk involves modeling the chances of possible future hurricanes. IPET factored the chances by using a suite of 152 different possible future storms, ranging in severity from a 50-year storm (2% annual chance of occurring) to a 5000-year storm (.02% annual chance of occurring).
The hurricane hazard for probable future storms included central barometric pressure; maximum winds; size of the storm (diameter of maximum winds); speed of the storm; and direction of the storm. Modeling this complexity required IPET, in cooperation with FEMA, NOAA, Corps of Engineers, universities and private industry, to develop a new, advanced hurricane modeling method that will be used by all organizations in future coastal work.
One of the critical pieces of information IPET had to determine was the water levels from surge and waves from this broad range of storms (around the complex Louisiana coast), which varied at hundreds of different locations around the 350-mile protection system. Using this new hurricane modeling method, IPET ran 152 possible future storms from the range of hurricanes discussed earlier on supercomputers to determine the expected storm surge and waves. IPET also added expected rainfall from the hurricanes. This model provided the critical water levels needed for the Risk analysis.
Factoring in the Protection System
Extremely complex, the Hurricane Protection System spans 350 miles of protection structures, that were added, repaired, or are currently being improved. IPET evaluated areas that breached and areas that did not breach.
IPET identified 134 levee and floodwall reaches around the New Orleans area (each reach ranged from hundreds of feet to a couple of miles) that were considered uniform and representative of the whole system and 350 specific structures (gates, floodwalls, transition points, etc.). IPET considered the following factors in identifying these representative reaches and structures: elevation, design, maintenance, construction, soil foundation characteristics, erosion values and other inputs.
IPET used the water levels computed earlier to drive the analysis of the reliability of the hurricane protection system. The water levels of each storm were applied at many different locations to estimate how the individual reaches and structures would likely perform. This calculated the probable reliability of the system and considered, by geographic area, the probability of overtopping and the probability of overtopping and erosion that could cause breaches, foundation failures, etc.
The results of the analysis were broken-down by sub-basin. Results show the probability of flooding (to various depths) or the potential volume of flood water inside the protected areas. Numerous results are calculated for each water level range produced by the various storms, as well as the vulnerability of each component (structure or reach) of the system. The results from the Hazard and the System analyses are then applied to the Consequences model.
Determining Consequences
Risk is calculated for probable loss of life and property. To realistically show how changes in the system affected consequences, all risk analysis scenarios were run with the pre-Katrina population and property information so that the results are not skewed by current lower population densities and/or varying rebuilding efforts.
Modeling Products
Risk modeling products will include a variety of maps, graphs, and other information. IPET will show (by sub-basin or region):
- probability of inundation (getting your feet wet, to deep water flooding)
- risk to population or property
- relative risk by parish
- principal sources of risk (by parish, sub-basin, reach (risk from breaching, overtopping, flooding from a transition point or special feature, etc.).
- specific sources of risk (i.e., a gate or structure).
The first set of Risk maps that will be publicly released will be DRAFTS that need both detailed technical review and public comment. The review panels (American Society of Civil Engineers and National Academies National Research Council) will provide technical reviews as they have for all IPET analyses.
Conclusion
Risk products will profile pre-Katrina and current protection system conditions. This will show the dynamics of risk and the effects of system improvements, current and future, on risk and vulnerability. This information will be useful for officials and the public to make their own informed decisions. The Risk products for the 100-year elevation levels of the protection system will be done later this year by the New Orleans District using the IPET Risk models.
This is the first systematic look at risk for the entire protection system. This complicated and new risk analysis process is the prototype. As we do more risk analysis work, it will become easier, and the tools will be modified and simplified for widespread future use in Louisiana and in other areas.