The entire system is re-designed and new closure structures are included to protect this city against hurricane threats in the future. Right now, the system is designed to protect the flood prone areas against a 1% hurricane event, better known as 100-year level of protection. This system-wide level of protection has to be effective in 2011 and is considered as a first step towards a better flood protection of the low-lying areas in the Mississippi river delta region. Ongoing planning efforts from US Army Corps of Engineers (USACE) and the State of Louisiana evaluate the need and possible measures for higher safety levels to protect populated areas.

The FEMA guidance for levee certification for coastal levees says that “the freeboard must be established at one foot above the height of the 1% wave or the maximum wave runup (whichever is greater) associated with the 100-year still water surge elevation at the site” (FEMA, 2007). The extra 1ft is added to account for uncertainties in the design variables. A disadvantage of this deterministic approach is that the uncertainty in the various hydraulic variables is not taken into account explicitly. The Corps guidance on certification is for a probabilistic approach to be followed similar to the approach used in Corps planning studies; yet, the procedure for coastal levees and floodwalls has not been well defined.



Haskoning Inc. has developed and implemented an innovative probabilistic method to determine the levee and floodwall elevations. This method takes into account the uncertainties in the various hydraulic input parameters. The main advantage of this method is that this method does not include a (arbitrary) freeboard as it is the case in the current FEMA certification guideline. Another advantage is that the uncertainty in the water levels and the waves may differ from one area to another. This procedure enables the end-user to take this spatial variability into account in the hydraulic design process of flood protection systems.

The probabilistic approach essentially differs from a deterministic approach that has been applied before. A deterministic approach uses one (pre-selected) input combination and results in one output. No formal insight is obtained in the probability of these output results in a sensitivity analysis. Using the MCS approach, hundreds (or thousands) of outputs are obtained and this gives insight how the input uncertainty is reflected in the output uncertainty. The development and implementation of this probabilistic method for the HSDRRS around New Orleans is considered to be a necessary step forward toward a more rational way of including uncertainties into the design procedures of flood protection systems.

Background information:
- Van Ledden et al.: Probabilistic design method of levee and floodwall heights for the hurricane protection system in the New Orleans area, International workshop on wave hindcasting and forecasting and coastal hazards Symposium 2007.

- Figure: 100-year Ribbons of protection (image credits: Times Picayune, staff graphic by emmet mayer III. Source: Army Corps of Engineers)