EM 1110-2-1100 (Part II)
(Change 1) 31 July 2003
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Pacific coast.
(1) Table II-2-5 provides information for the Pacific coast that is comparable to that presented in
Tables II-3-3 and II-2-4 for the Atlantic and Gulf of Mexico coasts, respectively. The Pacific coast is very
different from the east coast in that wave-producing storms within the Pacific Ocean are travelling toward
this coast. This means that the west coast typically has a much richer source of swell waves tha do other
U.S. coastal areas. As can be seen by comparison to the Atlantic coast results (Table II-2-3), this results in
higher wave conditions along the Pacific coast, with mean wave heights ranging from 2.5 m near the Mexican
border to 3.2 m near the Canadian border. This difference is also reflected in the mean periods along these
coasts, which vary from 9.6 to 12.1 sec. During (Northern Hemisphere) summer months, storm tracks usually
move far to the north and storms are less intense. Consequently, swell from mid-latitude storms in the
Northern Hemisphere diminish in size and frequency, allowing swell from tropical storms spawned off the
west coast of Mexico and from large winter storms in the Southern Hemisphere to become important elements
in the summer wave climate.
(2) Typical winter storm tracks move storm centers inland in the region from northern California to the
Canadian border. Hence, large waves in these regions frequently come in the form of local seas. South of
San Francisco, local storms strike the coast with less frequency; thus, many of the large waves in this area
arrive in the form of swell. Many notable exceptions to this general rule of thumb can be found in the late
1970's and 1980's, however. In particular, the storm of January 1989 moved across the California coast in
the vicinity of Los Angeles and caused much damage to southern California coastal areas.
(3) The 90th percentile wave heights along the Pacific coast are about twice their Atlantic coast
counterparts. In the southern California region, these values are typically in the 3.9- to 4.2-m range. As one
moves northward, the 90th percentile wave height increases to a maximum of about 5.4 m along the coast of
Washington. Periods associated with these waves tend to be quite long, ranging between 11 and 14 sec.
(4) The 5-year wave heights in the southern California region are comparable to those found along the
New England coast on the Atlantic (6.8-6.9 m compared to 6.7 m). However, associated periods are
considerably longer (16.8 sec compared to 12-13 sec). As one moves northward, these wave heights increase
to levels greater than 10 m along much of the coast north of the California-Oregon border. Periods of these
large waves tend to fall in the 14- to 16-sec range.
(5) Although many studies have dismissed the importance of tropical storms to the extreme wave
climate along the Pacific coast, at least one tropical storm has moved into the Los Angeles basin during this
century, suggesting that this threat is not negligible. Given the curvature of the coast and the water
temperatures north of Point Conception, it is unlikely that tropical storms can produce a significant threat at
coastal sites north of this point; however, south of this point it is important to include tropical storms in any
design and planning considerations.
e.
Great Lakes.
(1) Table II-2-6 provides comparable information for the Great Lakes as provided for previous coastal
areas in Tables II-2-3 through II-2-5. Wave conditions within the Great Lakes are strongly influenced by
fetches aligned with the dominant directions of storm winds. These winds are mainly produced by various
extratropical storms moving across the Great Lakes region. Table II-2-6 compares the largest 50-year (return
period) wave heights for each lake. Since strong storms are not very frequent in late spring through early
autumn, this interval is usually relatively calm along most shorelines. During the period from mid-autumn
until ice effects on the lakes reduce the wave generation potential, the largest waves are generated. Again
II-2-58
Meteorology and Wave Climate
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