Regional Floods
Mississippi River
System
• Carries
sediment to sea
and deposits in
Birdfoot delta
lobe in Gulf of
Mexico
Figure 14.21
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
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
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
Regional Floods
Mississippi River System – The Great Midwestern Flood
of 1993
Figure 14.22
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
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
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
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
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
Regional Floods
China
• Huang River
today is 20 m
higher than
adjacent
floodplain –
kept in place
by levees
Figure 14.28
Figure 14.29
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
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
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
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
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 PDDs in 51 years
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
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
Urbanization and Floods
Flood Frequencies
• Urbanization increases
surface runoff of
rainwater  higher
stream levels in shorter
times (flash floods)
Figure 14.33
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
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
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
Urbanization and Floods
The Uncoordinated Approach: San Diego
• Army Corps of Engineers constructed 245 m wide channel at
mouth
Figure 14.36
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
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.”
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
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
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
End of Chapter 14