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Mississippi River System • Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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Page 1: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

Mississippi River System

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

Regional Floods

Figure 14.21

Page 2: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

Page 3: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

Page 4: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

Page 5: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

Mississippi River System – The Great Midwestern Flood of 1993

Regional Floods

Figure 14.22

Page 6: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

Regional Floods

Figure 14.24

Page 7: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

Page 8: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

Mississippi River System – Role of Levees• Saturated levees can be compromised by wave attack, erosion by

overtopping, failing by slumping, undermining by piping

Regional Floods

Figure 14.25

Page 9: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

Page 10: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

Page 11: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

China• Huang River

today is 20 m higher than adjacent floodplain – kept in place by levees

Regional Floods

Figure 14.28 Figure 14.29

Page 12: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

Page 13: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

Page 14: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

Page 15: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

Page 16: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

Societal Responses to Flood Hazards

Page 17: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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)

Urbanization and Floods

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

Figure 14.31

Page 18: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

• 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

Urbanization and Floods

Figure 14.32

Page 19: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

Flood Frequencies• Urbanization increases

surface runoff of rainwater higher stream levels in shorter times (flash floods)

Urbanization and Floods

Figure 14.33

Page 20: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

Page 21: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

Urbanization and Floods

• Dangerous if anyone falls into channel with racing floodwaters

• Obliterates habitat of riverine plants and animals, “soul” of community

Figure 14.34

Page 22: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

The Binational Approach: Tijuana and San Diego • Tijuana River passes through Tijuana, Mexico, then through San

Diego, California on way to Pacific

Urbanization and Floods

Figure 14.35

• 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

Page 23: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

mouth

Urbanization and Floods

Figure 14.36

Page 24: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

Urbanization and Floods

Figure 14.37

Page 25: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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.”

Urbanization and Floods

Page 26: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

Page 27: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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:

The Biggest Floods

– 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

Page 28: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

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

The Biggest Floods

Figure 14.42

Figure 14.43

Page 29: Mississippi River System Carries sediment to sea and deposits in Birdfoot delta lobe in Gulf of Mexico Regional Floods Figure 14.21

End of Chapter 14