EM 1110-2-1100 (Part II)
30 Apr 02
! The effect of errors on extreme wave height estimates can be significant. Errors can be expected to
increase the width of confidence intervals and induce a systematic, artificial increase in Hs values at
return periods of interest (Earle and Baer 1982). Errors can be as important as the finite number of
years of record in limiting the reliability of extreme wave height estimates (Le Mhaut and Wang
1984).
(b) Design wave period is a period representative of extreme wave conditions. Along coasts exposed
to the ocean, the design Tp is usually an intermediate period between the limits of mild local sea and long
swell periods. At locations exposed to large swell, a design Tp representative of long-period swell conditions
may be required. At sites sheltered from ocean swell or enclosed water bodies the size of the Great Lakes
or smaller, the largest values of Tp can be associated with the largest values of Hs. At many locations, it may
be reasonable to estimate design Tp with a scatter plot of peak storm Hs and associated Tp values. A regression
line relating Hs and Tp is computed and then used to estimate Tp for any given Hs (e.g., Goda (1990)).
(c) In using a design Tp, it is assumed that this wave period is representative of the irregular or regular
wave period needed in follow-on design calculations. Often this assumption is realistic, as high-energy wave
events tend to be dominated by a single spectral peak. However, it may be preferable in some applications
to consider more than one spectral peak or even a full design spectrum if follow-on calculations can make
use of the information.
(d) Design wave direction is estimated based on measurements, hindcasts, and/or knowledge of extreme
storm characteristics.
(3) Intermediate-depth water.
(a) When a coastal project is in intermediate water depth (that is, waves are affected by the bottom but
depth-induced breaking has not begun), nearshore processes such as refraction and shoaling must be
considered to transform from the measurement or hindcast site to the project site (Part II-3). Figure II-3-6
provides the simplest methodology. A more comprehensive approach would be to represent each wave
condition as a TMA spectrum with appropriate energy, peak period, and direction, and compute
transformation over straight, parallel bottom contours. More typically, a full numerical model representation
of bathymetry and wave conditions is used, as discussed in Part II-3.
(b) Values of Hs, Tp, and wave direction in intermediate-depth water can be analyzed for design using
the same procedures as for deepwater waves. The Hs and wave direction values are modified from the deep-
water values because of nearshore bottom effects. Values of Tp are usually considered to be unchanged from
the deepwater values by the transformation process. However, some spectral transformation techniques can
predict changes in Tp. These changes are usually quite small.
(4) Shallow water (depth-limited).
(a) Extreme wave heights in coastal engineering applications are often limited by shallow-water depths.
Thus, depending on the local water depth and wave climate, the distribution for significant wave heights can
be expected to follow one of the appropriate functional forms in Figure II-8-2 up to a significant height of
about 0.6 times the water depth (Equation II-4-10) and then increase more slowly beyond that point
(Figure II-8-11). The probability at which the curve flattens depends on the local water depth and wave
climate. The flattened curve can be expected to continue rising slowly, but in this region increases in
significant height depend on other parameters such as wave steepness and water level rather than incident
significant height. Water level can be expected to be the main controlling factor. The probability distribution
for significant heights in this region may be essentially equivalent to the probability distribution of local water
levels.
Hydrodynamic Analysis and Design Conditions
II-8-25
Previous Page
