EM 1110-2-1100 (Part II)
(Change 1) 31 July 2003
(2) These results appear consistent with the mean storminess expected in these Atlantic coastal regions.
In the northern portion of the Atlantic coast, the primary source of large waves is migratory extratropical
cyclones. Between storm intervals in this region, waves come primarily from swell propagating from storms
moving away from the coast. Due to this direction of storm movement, the swell from these storms is usually
not very large (less than 2 m). As one moves southward past Cape Hatteras, waves from high-pressure
systems (both migratory and semipermanent) begin to become dominant in the wave population. Once south
of Jacksonville, the wave climate is typically dominated by easterly winds from high pressure systems, with
a secondary source of swell from northeasters. Farther south, as one approaches Miami, the Bahamas provide
considerable shelter for waves approaching from the east. In coastal areas without significant swell, sea
breeze winds can play a significant role in producing coastal waves during afternoon periods. This situation
occurs over much of the U.S. east coast during intervals of the year.
(3) The 90th percentile wave heights can be considered as representative of typical large wave
conditions. As can be seen here, this wave height varies from 1.9 to 2.4 m along the New England region
down to 1.4 to 1.9 m along the Florida coast. As was seen in the distribution of mean wave heights, the
overall pattern appears to have maxima at Cape Cod (2.4 m), Cape Hatteras (2.1 m), and Cape Canaveral
(1.9 m). The associated periods are very consistent along most of the Atlantic coast (8.5 to 9.9 sec) except
for the southern half of Florida, where the periods are somewhat lower (6.2 to 7.7 sec). Directions of the
90th-percentile wave reflect the general coastal orientation.
(4) Extreme waves along the Atlantic coast are often produced by both intense extratropical storms and
tropical storms. Table II-2-3 does not provide any information that extends into the return period domain
dominated by tropical storms; consequently, this table can be regarded as actually providing information only
on extratropical storms. Since this table is not intended to be used directly for any coastal design
considerations, information on large-return-period storms is specifically excluded.
(5) The 5-year wave heights presented in Table II-2-3 can be considered as representing typical large
storms that might affect short-term projects (beach nourishment, dredging operations, sand bypassing, etc.).
Values of the 5-year wave height range from generally greater than 6 m north of Long Island to only 4.2 m
in the Florida Keys. Again, north to south decreasing maxima appear in the regions of Cape Cod (6.7 m),
Cape Hatteras (5.9 m), and Cape Canaveral (4.9 m). Associated wave periods are generally in the range of
11 to 13 sec, except for the Florida Keys site, where this period is only 9.5 sec.
(6) Various types of extratropical storms have wreaked havoc along different coastal areas. These
storms range from "bombs" (small, intense, rapidly developing storms) to large almost-stationary storms
(developing typically after a change in the large-scale global circulation). Bombs produce higher wind speeds
(sustained winds can exceed 70 knots) but due to fetch and duration considerations, the larger, slower-moving
storms produce larger wave heights (a measured Hm0 greater than 17 m south of Nova Scotia in the Halloween
Storm). Other examples of classic storms along the U.S. east coast include the Ash Wednesday Storm of
1962 (affecting mainly the mid-Atlantic region), the Blizzard of 1978 (affecting mainly the northeastern
states), and the Storm of March 1993, which affected most of the U.S east coast. This last storm has been
called the "Storm of the Century" by some; however, it is by no means the worst storm in terms of waves for
most areas along the east coast in this century. In fact, along much of the Atlantic coast, the wind direction
was toward offshore; consequently, there was almost no wave action at the coast in many locations. Farther
offshore the situation was considerably different and many ships and boats were lost.
(7) Hurricanes can also produce extreme wave conditions along the coast. Particularly at the coast itself
where storm surges of 10 ft or more can accompany waves, hurricane waves represent an extreme threat to
both life and property. An excellent source of hurricane information is the HURDAT file at the National
Climatic Center in Asheville, NC. This file (available on magnetic tape or PC diskette format) contains storm
tracks, maximum wind speeds, central pressures, and other parameters of interest for all hurricanes affecting
Meteorology and Wave Climate
II-2-55
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