GLG110 Geologic Disasters and the Environment

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GLG110 Geologic Disasters and the Environment
Today Chapter 7Rivers and flooding
Instructor Professor Ramon Arrowsmith Email
ramon.arrowsmith_at_asu.edu Office PSF-640
480-965-3541
TA Tom Foltz Email Thomas.foltz_at_asu.edu Office
PSH-574
Course Website http//glg110.asu.edu
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Announcement

• Guest speaker on Thursday on Tornadoes! Dont
  miss it!

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Outline

• River forms
• Meander movie
• Flooding
• Flood movie
• Phoenix area flooding 93 floods
• Flood frequency
• Effects of urbanization
• Adjustments

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Keller, 2002
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(No Transcript)
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• Flooding true or false?
• Since we recently had a 100-year flood, I will be
  safe from flooding for another 100 years
• If my house or property was not flooded in the
  last flood, then it is not in a floodplain and I
  don't need to be concerned about flooding
• If I locate my home outside the floodplain I will
  never have any flood damages
• If it is not raining in my neighborhood, then the
  wash next to my house will not flow
• Placing branches along the embankment of a wash
  is an effective method of flood protection
• If a roadway dip section looks like it has only
  6-8 inches of water running over it, then it is
  safe for me to cross
• I can make any drainage improvement I need to on
  my own private land

http//www.fcd.maricopa.gov/Flooding/Myths.asp
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Photos by DM Burt
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Photos by DM Burt
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Photos by DM Burt
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Photos by DM Burt
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Photos by DM Burt
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Maricopa County flooding history
http//www.fcd.maricopa.gov/Flooding/History.asp
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October 2000
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Arizona Flooding Potential and Precipitation 2
main periods in each year Winter (November to
March ) precipitation is greatest when the middle
latitude storm track is unusually far south so
storms enter Arizona directly from the west or
southwest after picking up considerable moisture
from the Pacific Ocean. These storms are
generally more regional in nature and can affect
one or more of the large river systems during the
same period of time. The second rainfall season
occurs in July, August and most of September when
the area experiences widespread thunderstorm
activity associated with moist air moving into
Maricopa County from the south and southeast.
These thunderstorms are extremely variable in
intensity and location, and some of the heaviest
amounts of precipitation in a short period occur
during these months. The flooding that results is
also more localized and of a shorter duration.
However, the damages resulting from a flood of
this nature can be just as devastating to the
area where they occur.
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Disaster of the day 1993 Mississippi floods
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The river scenes are from central Missouri near
Gasconade, Rocheport, and the confluence of the
Grand River. The first in each pair was in
September, 1992, the second in September of 1993.
The data uses LandSat TM bands 5, 4, and 2 mapped
to red, green, and blue respectively. Bare soil
and plowed land appears red, vegetation (which
reflects very strongly in the near infrared)
appears green, and water is dark blue.
http//svs.gsfc.nasa.gov/vis/a000000/a000900/a0009
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July 1993 Extent of flooding (light blue)
versus normal flow (dark blue) From top Illinois
R., Mississippi R., and Missouri R.
See Fig. 7.2 Keller, 2002
http//rsd.gsfc.nasa.gov/rsd/images/Flood_cp.html
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August 1993 Extent of flooding (light blue)
versus normal flow (dark blue) From top Illinois
R., Mississippi R., and Missouri R.
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Disaster of the day 1993 Mississippi floods
The floods were caused by events which developed
as early as the Fall of 1992. At that time, soil
moisture levels in the central United States were
already high. Winter rain and snow contributed to
the nearly saturated soil conditions, so that
spring precipitation and snowmelt - normally able
to soak into the ground - could only run off into
streams and rivers. By June 1st of 1993,
rivers were running high as a persistent upper
level atmospheric pattern developed over the
central US. Moist air combined with upper level
atmospheric disturbances spawned drenching
thunderstorms. Wave after wave of these storms
rumbled across the already soaked Mississippi
River basin from June through August. By the end
of summer, some locations had received over 30
inches of rain - nearly 200 of normal.
Minnesota, Iowa, Illinois, and Missouri were
hardest hit. At St. Louis, the river crested at
49.6 feet - over 19 feet above flood stage, and
more than six feet above the old record set in
1973. The Mississippi remained over flood stage
at St. Louis for over two months.
Transportation and industry along the Mississippi
was disrupted for months. Damages to surface and
river transportation in the region were the worst
ever incurred in the United States. Over 1,000
of the 1,300 levees designed to hold back flood
water failed, though major cities along the
rivers, like St. Louis, were protected from
flooding by massive flood walls. Over 70,000
people were displaced by the floods. Nearly
50,000 homes were damaged or destroyed and 52
people died. Over 12,000 square miles of
productive farmland were rendered useless. Damage
was estimated between 15-20 billion.

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Flood Frequency

• Return of what sized flood?
• High magnitude low frequency

• R (N1)/M
• R is recurrence interval in years
• N is number of years of record
• M is rank of flood

Keller, 2002
R (9 1)/1 10
10 year flood
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Effects of urbanization

• Increasing impervious surface cover causes more
  common high discharge and shorter lags between
  peak of rain and peak of flow

Keller, 2002
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Keller, 2002
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Adjustments to Flood hazards

• Structural approach
• Physical barriers, levees, Channelization
• Alternative approach
• Channel restoration use channels natural form
  and vegetation to manage runoff

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Keller, 2002
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Concrete channel in Los Angeles Rivers system
Channel restoration in North Carolina
Keller, 2002
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Channel restoration for urban streams using
variable channel shape to induce scour and
deposition (pools and riffles) at desired
locations
Keller, 2002
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Flood insurance U.S. National Flood insurance
program. Must satisfy minimum standards of land
use regulation within the 100 yr flood plain as
mapped by FEMA.
Keller, 2002
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Flood Plain Mapping
Typical zoning map before and after flood
regulation
Keller, 2002
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Maricopa County Flood Control District 100 year
flood plain maps
http//www.fcd.maricopa.gov