Geology Reference
In-Depth Information
Tsunami Wave Heights
There are two different common origins for tsunamis that
can strike the coast of the Pacific Northwest. The first are
tsunamis that are generated by distant geologic events, such
as the Alaska earthquake of 1964. The second are tsunamis
that are generated by local subduction zone earthquakes.
After the 1964 Alaskan earthquake, tsunami waves in
this area reached 9.7 feet above the water level of the tides at
Ocean Shores, and 14.7 feet above tide at Wreck Creek,
which is about 20 miles north of Ocean Shores.
PART D. STORMS AND BARRIER ISLANDS
Storms
The most dramatic changes along coastlines take place
during storms. Although hurricanes are dramatic in
their power, other storms can also cause much dam-
age. These include storms such as the "pineapple
express" of coastal Oregon and Washington and
"northeasters" along the East Coast. In both hurri-
canes and these other storms, strong winds can do
much damage to structures. The combination of waves
and a rise in water level known as storm surge also has
major impacts along the coast. Storm surges during
Hurricane Katrina reached nearly 30 feet; when com-
bined with high tides, the runup of water can be even
higher. The amount of storm surge is dependent upon
air pressure, wind speed, and the near-shore shape of
the ocean bottom. Low air pressure and high winds let
the water build up higher. Where the sea bottom has a
gentle slope, storm surges can build up quite high.
Where the sea bottom has an abrupt drop-off, the
surges are not likely to be as great.
6. Study the 1994 topographic map (Figure 11.11) to deter-
mine areas that would be impacted by a 10-feet rise in
water. Use a colored pencil and mark these areas on the
map. Assume that the 10-foot contour is approximately
halfway between the edge of the water and the 20-foot
contour.
7. How much more land would be impacted by a 20-foot rise
in water? Use a different color, and mark these areas on the
map as well.
Think About It
8. After the Sumatra earthquake, tsunami wave heights
reached 30 m (100 feet). Recent studies suggest that a local
magnitude 9+ earthquake could generate a 20 m (65 foot)
wave height at the Ocean Shores, Washington, area of the
coast. Use a third colored pencil, and indicate on the map
areas that would not be inundated by a 20 m wave.
The force of moving water is tremendous. One gallon of
water weighs about 8.35 pounds. There are about 7.48
gallons in a cubic foot. This means that each cubic foot of
water weighs about 62.5 pounds. A cubic yard of water
weighs just slightly less than 1,700 pounds. This means
that a wave that is 3 feet high by 3 feet wide, and 100
yards long, will weigh 170,000 pounds. Add the force of
motion, and it is easy to see how even smaller waves can
pack the punch to knock down houses and easily erode
beaches.
Escape?
9. Use the 1994 topographic map (Figure 11.11) and suggest a
route for escape from the peninsula that includes Ocean
Shores. Sketch your route below or clearly indicate it on the
map.
Figure 11.12 shows a typical low-lying coast with
barrier islands that lie slightly offshore. Although this
example is based on Texas, the same processes can
occur along other low-lying coasts, such as much of
the Gulf of Mexico and Atlantic coasts of the United
States.
It is important to note that storms such as hurri-
canes have winds that rotate in a counterclockwise
direction. This means that as a storm approaches a
coast, winds in the 12 o'clock to 3 o'clock quadrant of
the storm will blow toward the shore, while winds in
the 6 o'clock to 9 o'clock quadrant will be blowing off-
shore. The onshore winds can pile up water into a
storm surge, while the offshore winds will relatively
lower the water level. So the same storm, where sites
are even only a few miles or tens of miles apart, can
have dramatically different impacts from surge, waves,
and coastal flooding. Study this figure carefully, as it
provides the context for the questions that follow.
Hurricanes are rated on the Saffir-Simpson scale.
Table 11.3 shows the wind velocities and typical storm
surges associated with each magnitude of storm.
10. If a distant earthquake generates the tsunami, there may
be several hours before the tsunami hits. Will your suggested
escape route likely work with several hours of advance
warning?
11. If a local major earthquake occurs, it may be less than 30
minutes before a tsunami hits. Will your suggested escape
route likely work with less than an hour of advance warning?
12. Given your analyses in Questions 10 and 11 above, are
alternative escape routes needed and if so, what alternatives
do you suggest for people living, visiting, or working in the
Ocean Shores area?
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