Geoscience Reference
In-Depth Information
weather conditions from year to year, the national 30-year average (1977-2006)
for flood deaths is 99. That compares with a 30-year average of 58 deaths for light-
ning, 54 for tornadoes, and 49 for hurricanes.
1
There are different types of flooding
that can occur. It is important to discern between the different types of flooding
for emergency planning purposes. The different types of flooding that can occur
include
Riverine Flooding
—Typically caused by overbanking of water in rivers,
creeks, or streams. This is the most common type of flood event. Typically,
these types of floods are a result of large-scale weather systems that generate
prolonged rainfall over wide geographic areas. This type of flooding can
also be caused by upstream snow melts, alluvial fan floods, and ice jam
floods.
In 1993, the Midwestern United States experienced extensive rainfall for
four months. This rainfall caused the Mississippi River and many of its tribu-
taries to flood. Flooding was experienced in North Dakota, South Dakota,
Nebraska, Kansas, Minnesota, Iowa, Missouri, Wisconsin, and Illinois. In
this area, hundreds of levees that were built to protect agricultural interests
and towns were breached due to the excessive amounts of rainfall. Damages
for this event were estimated at approximately $15 billion. Approximately
10,000 homes were destroyed, and 50 people lost their lives in this event.
2
Alluvial Fan Flooding
—Alluvial fans are fan-shaped deposits where a fast
moving stream flattens out and spreads over a flatter plain. Typically, this is
seen from a stream moving from a mountainside into a canyon or a plain.
Risks associated with these floods include fast, widespread flooding since
there are no banks to control the stream flow. Also, due to the velocity of the
water, large debris can be carried in the floodwaters.
In December 1999, debris flows and flash floods on alluvial fans inun-
dated coastal communities in Cordillera de la Costa and Vargas, Venezuela.
These floods caused severe property destruction, and resulted in a death toll
estimated at 19,000 people. Because most of the coastal zone in Vargas con-
sists of steep mountain fronts that rise abruptly from the Caribbean Sea, the
alluvial fans are the only areas that have slopes that can be built upon.
3
Ice Jam Flooding
—Caused typically when a frozen river or stream begins to
melt and break up. The masses of ice then block normal downstream drain-
age paths, forming an ice dam and causing a backup of water. Secondarily,
once the ice dams break, the velocity of the water rushing downstream can
cause flash flooding. Ice jams on the Yukon River in Alaska contributed to
severe flooding during the spring breakup of 1992.
Flash Flooding
—Can occur within seconds, or may occur over a period of a
few hours. These floods typically cause a rapid rise in water flow and velocity.
Several factors contribute to flash flooding. These include surface conditions,
rainfall duration, rainfall intensity, and topography.
