Mississippi River System

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, andabandons 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 behigher 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 is20 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 about26.5 million sandbagsused 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 946had flood insurance • $300,000 media campaign by FEMA 73 householdsbought flood insurance • U.S. Congresscomes to rescue – 1993 flood victims received$6.3 billionbill 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