! 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 *H*s 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 *T*p is usually an intermediate period between the limits of mild local sea and long

swell periods. At locations exposed to large swell, a design *T*p 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 *T*p can be associated with the largest values of *H*s. At many locations, it may

be reasonable to estimate design *T*p with a scatter plot of peak storm *H*s and associated *T*p values. A regression

line relating *H*s and *T*p is computed and then used to estimate *T*p for any given *H*s (e.g., Goda (1990)).

(c) In using a design *T*p, 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 *H*s, *T*p, and wave direction in intermediate-depth water can be analyzed for design using

the same procedures as for deepwater waves. The *H*s and wave direction values are modified from the deep-

water values because of nearshore bottom effects. Values of *T*p are usually considered to be unchanged from

the deepwater values by the transformation process. However, some spectral transformation techniques can

predict changes in *T*p. 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

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