EM 1110-2-1100 (Part II)
30 Apr 02
(c) Consider whether an adjustment to the probability assigned to a high outlier data value can be
justified. Often a high outlier is due to a storm event and extreme hydrodynamic response which are much
more severe than would normally be expected over the length of hydrodynamic record. Meteorological
records generally cover a much longer historical time period than hydrodynamic records. By carefully
analyzing storm probabilities and longer-term records from nearby sites if available, it may be possible to
assign a more realistic (lower) probability to the hydrodynamic outlier. Then a more valid distribution
function fit can be obtained.
(5) Choosing an extremal distribution function. When several candidate distribution functions are under
consideration, usually one is selected as a best fit to the data. The selection criteria can range from visual
inspection of plotted results and simple statistics such as the correlation between data and model (e.g.,
Leenknecht, Szuwalski, and Sherlock 1992) to more elaborate statistical tests (Mathiesen et al. 1994). An
objective approach to selecting a distribution function for significant wave heights is given by Goda and
Kobune (1990).
(6) Confidence intervals. Confidence intervals associated with the chosen distribution function should
also be estimated, preferably with a computer program (Leenknecht, Szuwalski, and Sherlock 1992; Goda
1988; Goda 1990; Mathiesen et al. 1994). They depend on the distribution function and number of data
values. Confidence in computed values can also be influenced by random and systematic errors in the data
and physical site characteristics such as long-term variability of water level and climate, possible extreme
events not represented in the recorded population, and physical limits on extremes (such as the depth-imposed
limit on wave height in shallow water).
f
Return period and encounter probability.
(1) Extreme conditions in coastal engineering are often described in terms of return values and return
periods. The return period is the average time interval between successive events of the design wave being
equalled or exceeded. For example, a 25-year significant wave height is that height that is equalled or
exceeded an average of once during a 25-year time period. Return period is expressed as
t
Tr '
(II-8-1)
^
1 & P (Hs)
where
Tr = return period, in years
t = time interval associated with each data point, in years
^
^
P(H s) = cumulative probability that Hs # H s
^
H s = design significant wave height
(2) A related concept, encounter probability, gives the probability that waves with Hs equal to or greater
^
than H s will occur during the design life or other time period. It is given by
L
t
t
Pe ' 1 & 1 &
(II-8-2)
Tr
II-8-12
Hydrodynamic Analysis and Design Conditions
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