Geoscience Reference
In-Depth Information
On June 14, 1990, a storm system dropped 4 inches of rain in less than
2 hours. This produced a 30-foot-high wall of water and killed 26 people.
Damage estimates range anywhere from $6 to $8 million.
4
Dam and Levee Failure Flooding
—Dams and levees are built to hold
back a computed amount of water. However, if the amount of water behind
the dam or levee exceeds the computed amount that the dam can hold,
overtopping occurs. Another type of failure can occur in these systems
when the dam or levee structure fails either due to it being washed out or
possibly a mechanical or maintenance failure. The amount of water released
can cause catastrophic flash floods due to the high velocity of the moving
water.
One of the most historic disasters involving a dam break occurred on May
31, 1889. The dam broke, resulting in a 36-foot to 40-foot wall of water
washing through the town of Johnstown, Pennsylvania (Figure 5.1). A total
of 2200 people lost their lives in this event, making it the worst flood in the
history of the United States.
4
Storm Surge Flooding
—Storm surge is water pushed up onto dry land by
onshore winds. This phenomenon is primarily caused by tropical storms,
hurricanes, and other intense low-pressure systems. Storm surge is the most
dangerous and most destructive part of most hurricanes. The waves and the
velocity of the storms surge often causes great destruction and is known to
be the highest cause of deaths during hurricanes. At times, storm surge can
come during high tide, which causes a storm tide. The height of the waves
associated with this type of flooding can reach more than 25 feet in worst-
case scenarios.
Hurricane Katrina in August 2005 was one of the most devastating hur-
ricanes in the history of the United States. It produced storm surge flooding
of 25 feet to 28 feet above normal tide level along portions of the Mississippi
coast and storm surge flooding of 10 feet to 20 feet above normal tide lev-
els along the southeastern Louisiana coast. Hurricane Katrina produced
Figure 5.1
Damage from Johnstown Flood, May 1, 1889. (National Park Service.)
