Lecture Outlines Natural Disasters, 5th edition

1
Regional Floods

• Mississippi River System
• Carries sediment to sea and deposits in Birdfoot
  delta lobe in Gulf of Mexico

Figure 14.21
2
Regional Floods

• Mississippi River System
• River builds up channel bottom over time, until
  channel bottom is higher than surrounding
  floodplain
• Avulsion in next major flood river adopts new,
  lower elevation channel, and abandons old channel
• Lobes of Mississippi River delta represent
  different avulsions
• Mississippi River overdue for avulsion current
  channel unstably high (above downtown New
  Orleans)
• Should undergo avulsion to channel of Atchafalaya
  River
• U.S. Army Corps of Engineers (instructed by
  Congress) allows 30 water down Atchafalaya, 70
  water down Mississippi

3
Regional Floods

• Mississippi River System Some Historic Floods
• New Orleans first large flood in year of
  founding, 1717
• Built levees to prevent future flood same
  response in place today
• Continuous efforts to build levees to prevent
  flooding result in more destruction in next flood
  when levees fail
• 1927 floods breached levees in 225 places, killed
  183 people
• 1973 floods extended along 1,930 km of river,
  inundating 50,000 km2

4
Regional Floods

• Mississippi River System The Great Midwestern
  Flood of 1993
• Biggest flood in 140 years more than 20 million
  acres
• Wet winter, spring ? even wetter summer, caused
  by low pressure from bend in jet stream
• Record flood levels on lower Missouri and upper
  Mississippi Rivers from April to August
• More than 160 consecutive days of flooding in
  some towns
• Did not significantly affect lower Mississippi
  River low flow from Ohio River

5
Regional Floods
Mississippi River System The Great Midwestern
Flood of 1993
Figure 14.22
Figure 14.23
6
Regional Floods

• Mississippi River System Weather Conditions
• Biggest floods in 1927, 1973, 1993
• Each case wet preceding autumn and winter,
  saturated ground for spring, followed by wet
  summer, caused by low pressure from bend in jet
  stream
• Reasonably common occurrence

Figure 14.24
7
Regional Floods

• Mississippi River System Role of Levees
• Long use of levees transformed Mississippi into
  restricted ribbons of water, cutting off
  floodplains
• Flooded channels can not spread laterally
  forced to rise vertically until levees are
  overtopped
• St. Louis flood would have crested 4 m lower
  without levees
• Apparent protection of levees encourages more
  development on floodplains

8
Regional Floods

• Mississippi River System Role of Levees
• Saturated levees can be compromised by wave
  attack, erosion by overtopping, failing by
  slumping, undermining by piping

Figure 14.25
9
Regional Floods

• Mississippi River System Role of Levees
• 1993 flood
• 1,083 of 1,576 levees were overtopped or damaged
• Floodwaters reoccupied more than 20 million acres
• Entire state of Iowa and sections of North
  Dakota, South Dakota, Minnesota, Wisconsin,
  Illinois, Missouri, Nebraska, Kansas declared
  federal disaster area
• 48 people killed
• 75 towns completely submerged
• 50,000 homes destroyed or damaged
• 12 commercial airports closed
• 4 interstate highways closed
• 12 billion damage

10
Regional Floods

• China
• Attempts to control Yellow River go back to 2356
  B.C.E.
• In last 2,500 years, river has undergone ten
  major channel shifts moving location of mouth up
  to 1,100 km
• Sediment deposition on channel floor builds up
  channel in elevation ? eventually may be higher
  than surrounding floodplain
• During next flood river may adopt lower elevation
  course outside of old banks ? avulsion
• 1887 avulsion sent Huang River south to join
  Yangtze River, with floods that resulted in over
  1 million deaths
• 1938 dynamiting of levees resulted in 1 million
  deaths

11
Regional Floods

• China
• Huang River today is 20 m higher than adjacent
  floodplain kept in place by levees

Figure 14.28
Figure 14.29
12
Societal Responses to Flood Hazards

• Structural responses
• Dam construction
• Building levees
• Straightening, widening, deepening and clearing
  channel to increase water-carrying ability
• Sandbagging
• Nonstructural responses
• More accurate flood forecasting
• Zoning and land-use policies
• Insurance programs
• Evacuation planning
• Education

13
Societal Responses to Flood Hazards

• Dams
• Dam construction to create reservoirs gives sense
  of protection from floods, but dams do not
  control floods
• Life spans of dams are limited by construction
  materials, construction style, rate at which
  sediment fills reservoir
• Major floods occur downstream due to
• Overtopping
• Heavy rainfall below dam
• Dam failure
• 1981 study of dam safety by Army Corps of
  Engineers
• 2,884 of 8,639 dams unsafe

14
Societal Responses to Flood Hazards

• Levees
• Cost of building levees may be more than value of
  structures intended to protect
• Sense of security encourages further development
  of floodplain
• Research shows that peak floodwater heights
  increased 2 to 4 m (for same water volume) in
  last 150 years in upper Mississippi River
  sections with levees and engineered channels,
  while staying the same on unengineered upper
  Missouri River
• Floods in St. Louis crested at 11.6 m in 1903,
  15.1 m in 1993 for same water volume

15
Societal Responses to Flood Hazards

• Sandbagging
• Temporary levees of bags of sand and mud
• Estimated about 26.5 million sandbags used in
  1993 floods
• Lessened damage some places, but not others
  therapeutic value
• Forecasting
• Forecasts of height and timing of regional
  floodwaters have significantly reduced loss of
  life
• Do not offset ever-greater damages, losses
• Zoning and Land Use
• National Flood Insurance Program, FEMA ban
  building on floodplain covered by 100-year flood
• Discourages construction at frequently flooded
  sites but does not prevent all flooding of
  structures

16
Societal Responses to Flood Hazards

• Insurance
• Flood insurance available from National Flood
  Insurance Program since 1950s, rarely purchased
• Of 10,000 flooded households in Grand Forks,
  North Dakota in 1997, only 946 had flood
  insurance
• 300,000 media campaign by FEMA ? 73 households
  bought flood insurance
• U.S. Congress comes to rescue 1993 flood
  victims received 6.3 billion bill providing aid
• Presidential Disaster Declarations
• Such severity and magnitude that effective
  response is beyond the capabilities of the state
  and the affected local governments
• Disastrous floods caused 45 of PPDs in 51 years

17
Urbanization and Floods

• Hydrographs
• Plots volume of water (or stream depth) against
  time
• Time lag after rainfall for runoff to reach
  stream channel, then stream surface height rises
  quickly (steep rising limb of hydrograph)

• Stream level falls more slowly as underground
  flow of water continues to feed stream (gently
  sloped falling limb of hydrograph)

Figure 14.31
18
Urbanization and Floods

• Urbanization changes shape of hydrograph, making
  curve much steeper
• Good news urban flood might only last 20 as
  long
• Bad news urban flood could be four times higher

Figure 14.32
19
Urbanization and Floods

• Flood Frequencies
• Urbanization increases surface runoff of
  rainwater ? higher stream levels in shorter times
  (flash floods)

Figure 14.33
20
Urbanization and Floods

• Channelization
• Try to control floodwaters by making channels
  clear of debris, deeper, wider and straighter
• Push stream into too much discharge case
• Stream response to regain equilibrium erodes
  bottom and banks to pick up sediment and decrease
  gradient

21
Urbanization and Floods

• The Extreme Approach Los Angeles
• Cleared, straightened and deepened river channels
  also lined with concrete, to reduce friction
  and speed up flow
• While flood volumes are smaller than channel
  capacity ? no urban floods

• Dangerous if anyone falls into channel with
  racing floodwaters
• Obliterates habitat of riverine plants and
  animals, soul of community

Figure 14.34
22
Urbanization and Floods

• The Binational Approach Tijuana and San Diego
• Tijuana River passes through Tijuana, Mexico,
  then through San Diego, California on way to
  Pacific

• U.S. and Mexico agreed to construct concrete
  channel for river
• U.S. backed out after Mexico constructed channels
• High-velocity floods from Mexican channels
  inundate open farms and subdivisions of southern
  San Diego

Figure 14.35
23
Urbanization and Floods

• The Uncoordinated Approach San Diego
• Army Corps of Engineers constructed 245 m wide
  channel at mouth

Figure 14.36
24
Urbanization and Floods

• The Uncoordinated Approach San Diego
• Mission Valley was developed along natural
  channel, 7.5 m wide
• Part of Mission Valley has 110 m wide channel
  feeding into natural channel

Figure 14.37
25
Urbanization and Floods

• The Hit-and-Miss Approach Tucson
• Flooding of Santa Cruz River in 1983 was 1.76
  times bigger than FEMA estimates of 100-year
  flood
• Six of seven largest floods were between 1960 and
  1983, during years of peak city growth and
  urbanization
• Desert floods damage by bank erosion, not
  inundation
• Some Tucson stream banks moved laterally more
  than 300 m, so that 100-year floodplain moved
  also
• Protective walls concentrate erosion at end of
  wall
• Metropolitan Tucson Convention and Visitors
  Bureau The 100-year flood has come and gone,
  so, by all rights, Tucsonians should enjoy
  another century of great southwestern weather.

26
The Biggest Floods

• Ancient Tales of Deluge
• Tales of ancient floods are part of many cultures
• Are these floods larger than those today, or
  1,000-year floods?
• Flooding of fertile ground adjacent to rivers
  (floodplains) where entire ancient cultures
  thrived would have seemed like flooding of whole
  world

Figure 14.40
27
The Biggest Floods

• Ice-Dam Failure Floods
• Biggest floods during melting of continental ice
  sheets ? lakes behind ice dams that failed
  suddenly
• Evidence of flood from Lake Missoula after
  melting of ice dam

• Lake sediments
• Land stripped of soil, sediment cover
• High-elevation flood gravels
• Integrated system of braided channels
• Abandoned waterfalls
• High-level erosion
• Large-scale sediment deposits

Figure 14.41
28
The Biggest Floods

• Ice-Dam Failure Floods
• Huge volume of meltwater
• Changed paths of rivers
• Could change global circulation of deep ocean
  water ? change in global climate
• About 12,900 years ago, climate cooled about 5oC
  (Younger Dryas), possibly by gigantic meltwater
  flood through St. Lawrence River into North
  Atlantic Ocean
• Largest known floods in Earth history, raising
  sea level by about 130 m

Figure 14.42
Figure 14.43
29
End of Chapter 14