IPET Report 3 Vol VIII

geographical region that may be considered uniformly impacted by any given hurricane or over
an extended region where spatial homogeneity of the hurricane loads cannot be assumed. For
flood hazard, return-period analysis is generally easier than risk analysis because hurricane
severity may be ranked using surrogate quantities (such as a rough estimate of maximum surge)
that are much easier to calculate than the flooding conditions themselves.
Since medium to long return periods are of interest, historical methods are discarded. Both JP
and MC methods can handle such return periods. Monte Carlo approaches face the problem of
sorting out the potentially damaging events from large suites of simulated hurricane scenarios.
This is not a trivial problem for the geographically extended and differently vulnerable system
we are considering. For these reasons, the joint probability approach has been selected. This
approach is further described in the following sections.
To implement a joint probability method for hurricane hazard, it is convenient to describe
hurricanes at landfall through the following parameters
• ΔP (mb) = central pressure deficit at landfall
• R max (km) = radius to maximum winds at landfall
• X (km) = longitudinal landfall location relative to downtown New Orleans (positive if
east of New Orleans)
• θ (degrees) = direction of storm motion at landfall, (θ = 0 for tracks pointing north,
increasing clockwise)
• V (m/s) = storm translation speed at landfall
• B = Holland’s radial pressure profile parameter at landfall (Holland 1980)
While the variation of these parameters before and after landfall is also of interest, the primary
characterization of hurricanes required in the risk studies is in terms of their properties at
landfall. Hence the main tasks of the hazard quantification for the initiating events in the risk
analysis is the estimation of the hurricane recurrence rate and the evaluation of the wave and
surge loads over a suitable range of characteristic parameters.
Hurricane Recurrence at Landfall
Information used to estimate recurrence rates includes historical data sets (mainly NOAA’s
HURDAT data for frequency, ΔP , X, θ and V and data on R max from Ho et al. 1987) as well as
published distribution results. The HURDAT data set (Jarvinen et al. 1984, and recent updates)
has been used to extract values of ( ΔP , X, θ, V) at landfall over the stretch of coastline between
longitudes 85W and 95W. For recurrence analysis, only storms of hurricane strength at landfall
have been considered (defined as those having measured or estimated ΔP ≥ 25 mb) since 1890.
Earlier events have been neglected because prior to 1890 the historical record is incomplete and
less accurate. The HURDAT data set has been used also to analyze pre-landfall conditions.
Volume VIII Engineering and Operational Risk and Reliability Analysis VIII-29
This is a preliminary report subject to revision; it does not contain final conclusions of the United States Army Corps of Engineers.