TIDE1220  New Orleans & Hurricanes 
Tulane University  Prof. Stephen A. Nelson 
The Risk to New Orleans  Present and Future 
What is the 100  Year Flood/100 Year Storm?
For river flooding, the statistical data is the peak annual discharge over a long period of time.
R = (n+1)/m Graph of Peak Dischage vs. Recurrence Interval, then can be made to determine the discharge of the 100 year flood, 50 year flood, 10 year flood, etc. (see Figure 1).

Figure 1 
The probability, Pe, of a certain discharge can be calculated using the inverse of the Weibull equation:
P_{e} = m/(n+1) The value, P_{e}, is called the annual exceedence probability. For example, a discharge equal to that of a 10year flood would have an annual exceedence probability of 1/10 = 0.1 or 10%. This would say that in any given year, the probability that a flood with a discharge equal to or greater than that of a 10 year flood would be 0.1 or 10%. Similarly, the
probability of a flood with discharge exceeding the 100 year flood in
any given year would be 1/100 = 0.01, or 1%. Note that such probabilities are the same for every year. Discharge (volume/sec) can be converted to Stage (elevation), then maps can be made of the area expected to be flooded by floods with various recurrence intervals (see figure. 2) 
Figure 2 
Despite the fact that the 100 year flood has only a 1% chance of occurring each year, the probabilities do accumulate over time.
The probability of a certainsize flood occurring during any period can be calculated using the following equation: P_{t} = 1(1P_{e})^{n}
and P_{e} is the probability of occurrence in any year.

We can use this equation to calculate how the probabilities change over time. The result is depicted in the graph below for P_{e} = 0.01 (100 year flood) (see Figure 3) 
Figure 3 
2 important points emerge:

Figure 4 
To figure out the volume of water expected from hurricanes is more complicated. The complications include:
The amount of rainfall and the height of the storm surge and waves, also depends on many factors, including:
For an area like New Orleans which is surrounded by levees and is composed of many subbasins that need to be pumped out, the level of flooding is further complicated by such factors as
Risk to New Orleans from Hurricanes The Interagency Performance Evaluation Task Force (IPET) formed after Hurricane Katrina, undertook a risk analysis study that can be found in several different forms on the internet. The full Risk analysis can be found at: http://biotech.law.lsu.edu/katrina/ipet/ipet.html As Volume VIII of the IPET Final Report. A general description of the results of this study can also be found at this site (direct link below). http://biotech.law.lsu.edu/katrina/ipet/SimpleRisk%20FINAL%2023jun09%20mh.pdf Flood risk maps can be found at all of the above, and at: http://nolarisk.usace.army.mil/#map
Summary of Findings
Vulnerability to flooding and risk are functions of three things:
The IPET Risk Assessment Model is shown in Figure 5. 
IPET Risk Assessment Model (see references above) Figure 5 
The Hazard
The System
The Vulnerability
Consequences
Risk
