Environmental Engineering Reference
In-Depth Information
Seiches
Description
Seiches are caused when ground motion starts water oscillating from one side to the other
of a closed or partly closed water body, such as a lake, bay, or channel.
Occurrence
Large seiches are formed when the period of the arrivals of various shocks coincides with
the natural period of the water body, which is a function of its depth, and sets up reso-
nance. During the 1959 earthquake at Hebgen Lake, Montana, a witness standing on
Hebgen Dam saw the water in the reservoir disappear from sight in the darkness, then
return with a roar to flow over the dam. The fluctuation continued appreciably for 11 h
with a period of about 17 min. The first four oscillations poured water over the dam. The
Lisbon event of 1977 (
M
8.7) set up seiches all over Europe with the most distant ones
reported from Scandinavia, 3500 km away.
11.3.6
The Volcano Hazard
Eruptions
Eruptions, lava flows, and particles thrown into the atmosphere present the hazardous
aspects of volcanic activity. In the last 2000 years, there have been relatively few tremen-
dous and disastrous eruptions. Mt. Vesuvius erupted in 79
A
.
D
. and destroyed the ancient
city of Pompeii. Mont Pele in Martinique erupted violently in 1902, destroying the city of
St. Pierre and leaving but two survivors. Krakatoa in Indonesia literally “blew up” in 1883
in what was probably the largest natural explosion in recorded history.
Two modern significant events were Mt. St. Helens in the United States and Mt.
Pinatubo, in the Phillipines. On March 27, 1980, after a week of intermittent Earth shaking,
Mt. St. Helens, 50 miles northeast of Portland, Oregon, started ejecting steam, ash, and gas.
Finally on May 28, 1980, the mountain top exploded, sending ash and debris some 15 mi
into the air. Avalanches, debris flows, and huge mudflows, in addition to the blast forces,
caused widespread devastation and flooding, and about 60 deaths. A thick layer of ash
was deposited over thousands of square miles (see
Section 7.5.4).
Mt. Pinatubo, in June
1991, was one of the largest eruptions of the 20th century. It destroyed Clark Airbase and
displaced thousands of Filipino citizens. Enormous amounts of ash were released, which
blanketed large areas.
Flowing Lava
Flowing lava is perhaps the most common cause of destruction. An example is Iceland's
first geothermal power plant (ENR, 1976). As work on the $45 million plant was nearing
completion, the site was shaken for about a year with tremors, and then fissures opened
about 1.6 km away and lava was spewed, threatening the installation. Apparently, the lava
was contained by dikes erected around the plant area. Many of the 11 deep wells drilled
to capture the geothermal energy were severely damaged by subsurface movements.
Between 1983 and 1990, Kilauea volcano on Hawaii erupted with a series of lava flows that
eventually destroyed over 180 homes in Kalapana.
Lahars
Lahars have received much attention in recent years as an important geologic hazard. The
term lahar describes a hot or cold mixture of water and rock fragments flowing down
