Geology Reference
In-Depth Information
STORM WAVES AND
What causes most fatalities during hurri-
canes? Many people think it is strong wind,
but actually, coastal fl ooding caused by
storm waves and heavy rainfall are more
dangerous. Coastal flooding during
hurricanes results when large waves
are driven onshore, and by as much as
60 cm of rain in as little as 24 hours. In
addition, as a hurricane moves over the
ocean, low atmospheric pressure causes
the ocean surface to bulge upward as
much as 0.5 m. When the eye of the
storm reaches shoreline, the bulge, cou-
pled with wind-driven waves, piles up in
a
storm surge
that may rise several me-
ters above normal high tide and inun-
date areas far inland.
In 1900, hurricane-driven waves
surged inland, eventually cover-
ing the entire island that Galveston,
Texas, was built on. Buildings and
other structures near the seashore were
battered to pieces and “great beams and
railway ties were lifted by the [waves] and
driven like battering rams into dwell-
ings and business houses.”* Between
6000 and 8000 people died. In an effort
to protect the city from future storms,
a huge seawall was constructed, and the
entire city was elevated to the level of the
top of the seawall (
Delaware Bay
Potomac River
Chesapeake Bay
Cape Hatteras
◗
Figure 16.16
Submergent Coasts Submergent coasts tend to be extremely
irregular, with estuaries such as Chesapeake and Delaware bays. They formed when
the East Coast of the United States was fl ooded as sea level rose following the
Pleistocene Epoch.
Figure 16.18).
Although Galveston, Texas, has been largely protected
from more recent storm surges, the same is not true for
several other areas. In 1989, Charleston, South Carolina,
and nearby areas were flooded by a storm surge gener-
ated by Hurricane Hugo, which caused 21 deaths and more
than $7 billion in property damages. Bangladesh is even
more susceptible to storm surges; in 1970, 300,000 people
drowned, and in 1991, another 130,000 were lost. Large-
scale coastal fl ooding took place when Hurricane Isabel hit
the Outer Banks of North Carolina in 2003. And in 2004,
four hurricanes hit Florida and parts of the Gulf Coast,
causing widespread wind damage and coastal fl ooding.
The examples just cited were certainly disasters of
great magnitude, but the U.S. Gulf Coast was hardest hit in
2005, first in August by Hurricane Katrina and then again
in September by Hurricane Rita. When Hurricane Katrina
roared ashore on August 29, high winds, a huge storm surge,
and coastal fl ooding destroyed nearly everything in an area
◗
Emergent coasts
are found where the land has risen
with respect to sea level (
Figure 16.17). Emergence takes
place when water is withdrawn from the oceans, as occurred
during the Pleistocene expansion of glaciers. Presently,
coasts are emerging as a result of isostasy or tectonism.
In northeastern Canada and the Scandinavian countries,
for instance, the coasts are irregular because isostatic
rebound is elevating formerly glaciated terrain from beneath
the sea.
Coasts that form in response to tectonism, on the other
hand, tend to be straighter because the seafl oor topography
being exposed by uplift is smooth. The west coasts of North
and South America are rising as a consequence of plate
tectonics. Distinctive features of these coasts are marine
terraces (Figures 16.9b and 16.17), which are wave-cut plat-
forms now elevated above sea level. Uplift in these areas
appears to be episodic rather than continuous, as indicated
by the multiple levels of terraces in some places. In southern
California, several terrace levels are present, each of which
probably represents a period of tectonic stability followed
by uplift. The highest terrace is now about 425 m above sea
level.
◗
*L. W. Bates, Jr., “Galveston—A City Built upon Sand,”
Scientifi c American
,
95 (1906), p. 64.
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