EM 1110-2-1100 (Part II)
30 Apr 02
(a) The duration of storm events is another important component of the wave climate. Storm duration
in this context is usually defined as the length of time Hs persists above some fixed threshold value (Figure II-
8-16). Storms with long duration are likely to be more damaging than storms with short duration, in part
because they are likely to encompass one or more highs in astronomical tide. Storm duration decreases as
threshold increases, but it appears to be fairly independent of storm intensity (Smith 1988). The choice of
threshold value is based on the application. Smith (1987) suggested using an Hs value that is exceeded by
6 percent of the observations, which gave mean durations of about 25 hr at U.S. east and west coast hindcast
locations. Thus these events can be expected to encompass two high tides along the U.S. east coast and one
higher high water along the U.S. west coast.
Persistence of storm waves
(b) Persistence of wave conditions below a threshold Hs can be an important operational concern, since
it provides information on operational windows of low wave activity. It can also be a consideration in
functional design of some coastal projects.
e. Long waves. Wave phenomena with periods between those of swell and tides are collectively termed
long waves. In most coastal engineering applications, they have a limited role in design. Occasionally they
can be a major design concern (e.g. harbor oscillations).
(1) Tsunamis. Tsunamis are briefly discussed in Part II-5. They are sufficiently rare and unpredictable
that they become a concern for design return periods of about 100 years or longer. Since most coastal
engineering works are designed for return periods of 50 years or less, tsunamis can generally be omitted from
(2) Seiche. Seiche is discussed in Part II-7. It can be an important concern in harbor design or
modification. Harbors in areas where energetic, long-period swell can occur are especially prone to seiching
Hydrodynamic Analysis and Design Conditions