EM 1110-2-1100 (Part II)
30 Apr 02
Averaging Time Adjustments for Extreme Wind Speeds1
Ut,find = (factor from table) Ut,given
c. Extreme waves. Part II-1 and II-4 discuss some aspects of extreme waves. Further information is
provided in this chapter.
(1) Design importance. Extreme wave conditions are almost always a major design concern in coastal
engineering projects. Extreme significant wave heights are usually the most critical concern, but wave period,
wave direction, and spectral shape (both frequency and direction) can be important as well. Possible
secondary wave systems, an integral part of wave climate at exposed ocean sites, are generally not important
in conjunction with an extreme event. Energetic extreme waves are usually a key factor causing coastal
structure damage, vessel damage, beach erosion, channel sedimentation, and coastal flooding. Extreme waves
may be generated by a winter storm, a tropical storm, or some combination of storms (see Part II-8-7).
(2) Deep water.
(a) Extreme values of Hs and associated Tp are usually obtained from measurements or hindcasts. Values
of Hs, typically in the form of a partial duration series (see Part II-8-5-e-(1)), can be expected to give a
reasonable fit to the FT-I, Weibull, FT-II, or log-normal distribution function (Figure II-8-2), from which
design values of Hs can be estimated. FT-I or a form of the Weibull distribution function is generally
preferred. Some of the following considerations may also apply:
! If extreme values are generated by more than one population of extreme events (e.g. winter storms
and hurricanes) and if sufficient data are available from each population, it may be desirable to fit
each population separately.
! If the largest value or several values deviate significantly from the general trend of the data, these
outliers should be given special consideration as discussed in Part II-8-5-e-(2).
! Maximum Hs might be limited by sheltering or fetch constraints due to geography or consistent storm
! Extreme wave measurements, observations, and hindcasts are often subject to larger errors than
would normally be expected. For example, gauges can be affected by large, steep waves, breaking
waves, severe winds, and ice accumulation. A floating accelerometer buoy gauge may tilt severely,
stretch the mooring lines, or even break loose from the mooring. Wave height estimates may be
modified in ways that are difficult to predict. Data loss from gauges, due to equipment damage or
loss of power, is more likely during severe storms than during normal conditions.
Hydrodynamic Analysis and Design Conditions