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Lecture Outlines Natural Disasters, 5 th edition

Lecture Outlines Natural Disasters, 5 th edition. Patrick L. Abbott. Floods Natural Disasters, 5 th edition, Chapter 13. Christiane Stidham, Stonybrook University. Floods. Rainfall varies in intensity and duration In small drainage basin, short-lasting maximum floods

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Lecture Outlines Natural Disasters, 5 th edition

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  1. Lecture OutlinesNatural Disasters, 5th edition Patrick L. Abbott

  2. FloodsNatural Disasters, 5th edition, Chapter 13 Christiane Stidham, Stonybrook University

  3. Floods • Rainfall varies in intensity and duration • In small drainage basin, short-lasting maximum floods • In large drainage basin, maximum floods for weeks • Events of past  forecast of future events • Largest past event likely to be exceeded at some point • Floods of River Arno through Florence

  4. A Different Kind of Killer Flood • Unusually hot January in Boston caused molasses in heated tank to expand and burst container • 2.3 million gallons of molasses flooded out as 9 m high wave • Killed 21 people and injured 150 people • Many trapped in molasses after it cooled and congealed

  5. How Rivers and Streams Work Longitudinal Cross Section of a Stream • Cross-sectional plot of stream’s bottom elevation vs. distance from source • Profile for almost any stream is smooth, concave upward, with steeper slope near source and flatter slope near mouth • Base level – level below which stream can not erode • Ocean, lake, pond or other stream that stream flows into • Profiles are similar for all streams because of equilibrium processes

  6. The Equilibrium Stream • Streams act to seek equilibrium, state of balance • Change causes compensating actions to offset • Factors: • Discharge: rate of water flow, volume per unit of time • Independent variable (stream can not control) • Available sediment to be moved • Independent variable (stream can not control) • Gradient: slope of stream bottom • Dependent variable (stream can control) • Channel pattern: sinuosity of path • Dependent variable (stream can control)

  7. Case 1 – Too Much Discharge Too much water stream will flow more rapidly and energetically Response: • Excess energy used to erode stream bottom and into banks • Sediment produced by erosion – energy is used up carrying sediment away • Erosion of stream bottom results in less vertical drop  flatter gradient  slower, less energetic water flow • Erosioninto stream banks creates meandering pattern  longer stream path, lower gradient  slower, less energetic water flow

  8. Case 2 – Too Much Load Too much sediment stream becomes choked Response: • Excess sediment builds up on stream bottom • Buildup results in increased gradient water flows faster and more energetically  can carry away more sediment • Channel pattern becomes straighter  minimum energy needed to flow distance • Islands of sediment form within channel, creating braided stream pattern • Similar to stream overflowing and eroding away landslide dam

  9. Graded Stream Theory • Delicate equilibrium maintained by changing gradient of stream bottom  graded stream • Typical stream: • Too much load, too little discharge in upstream portion  braided pattern • Too much discharge, less load in downstream portion  meandering pattern • Change in response to seasonal changes, changes in global sea level, tectonic events

  10. The Floodplain • Floors of streams during floods • Built by erosion and deposition • Occupied during previous floods, and will be occupied again in future floods • About 2.5% of U.S. land is floodplain – home to about 6.5% of population

  11. Feedback Mechanisms • Negative feedback: system acts to compensate for change, restoring equilibrium • Positive feedback: change provokes additional change, sending system in “vicious cycle” dramatically in one direction • Desirable in investments accumulating interest • Undesirable in debts accumulating interest charges

  12. Flood Frequency • Larger floods  longer recurrence times between each • Analyze by plotting flood-discharge volumes vs. recurrence interval, construct flood-frequency curve • Can be used to estimate return time of given size flood • 100-year flood used for regulatory requirements • Difference between yearly probability, cumulative probability • Flood-frequency curves different for different streams

  13. Constructing Flood Frequency Curves Impossible to know exactly when floods will occur, but can predict statistical likelihood over period of time Steps in construction: • Record peak discharge for each year, rank years accordingly • For each year’s maximum flood, calculate recurrence interval = (N + 1) / M, where N = number of years of records, M = rank • Plot recurrence interval vs. discharge for each year, connect points as best-fit line Longer records of floods  better flood frequency curves

  14. Flood Styles Several causes: • Local thunderstorm  flash (upstream) flood lasting few hours, building and ending quickly • Rainfall over days  regional (downstream) floods lasting weeks, building and dissipating slowly • Storm surge of hurricane flooding coastal areas • Broken ice on rivers can dam up, block water flow  fail in ice-jam flood • Short-lived natural dams (landslide, log jam, lahar) fail in flood • Human-built levees or dams fail in flood

  15. Flash Floods • Thunderstorms can release heavy rainfall, creating flash floods in steep topography • Flash floods cause most flood-related deaths • 50% of flood-related deaths are vehicle-related • Only two feet of moving water required to lift and carry away average car

  16. Flash Floods Antelope Canyon, Arizona, 1997 • Narrow slot canyons of tributaries to Colorado River • Thunderstorm releasing rain to form flash flood may occur too far away to hear or see • 12 hikers killed by flash flood in 1997 Big Thompson Canyon, Colorado, 1976 • Centennial celebrations brought thousands to canyon • Stationary thunderstorm over area dumped 19 cm of rain in four hours • Runoff created flash flood up to 6 m high, 25 km/hr • 139 people killed, damage totaling $36 million

  17. Flash Floods Rapid Creek, Black Hills, South Dakota, 1972 • Pactola Dam built in 1952 to give flood protection and water supply to Rapid City, on Rapid Creek  increased development of floodplain • Stationary thunderstorm poured up to 38 cm in six hours • Canyon Lake overflowed as Canyon Lake dam broke, flooding Rapid City • 238 people killed, $664 million in damages • Floodplain remains undeveloped  greenbelt • “No one should sleep on the floodway.”

  18. Regional Floods Inundation of area under high water for weeks • Few deaths, extensive damage Red River of the North: unusual northward flow (spring floods) • Geologically young – shallow valley • Very low gradient – slow flowing water tends to pool • As winter snow melts, flows northward into still frozen sections, causing floods • 1997 record floods: • 1996 rainfall four times greater than average • Winter 1996 freezing began early, causing more ice in soil • Winter 1996-7 snowfalls three times greater than average • Rapid rise in temperature melted snow and ice in soil • Floodwaters flowed slowly northward, flooding huge areas of North and South Dakota, Minnesota and Manitoba

  19. Mississippi River System • Greatest inundation floods in U.S. • Third largest river basin in world • Drains all or part of 31 states, two Canadian provinces • System includes almost half of major rivers in U.S. • Average water flow in lower reaches is 18,250 m3/sec • Water flow can increase fourfold during flood

  20. 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

  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 • High water levels from April to August • Levees created higher water levels, by confining rivers • Levees damaged by waves, overtopping erosion, slumping, piping • 1,083 out of 1,576 levees in upper Mississippi River system overtopped or damaged • Submerged 75 towns, caused $12 billion of damage • Mississippi River wants to change course to join shorter, steeper Atchafalaya River but held in place by Army Corps of Engineers

  22. 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 to join Yangtze River, with floods that resulted in over 1 million deaths • 1938 dynamiting of levees resulted in 1 million deaths • Huang River today is 20 m higher than adjacent floodplain – kept in place by levees

  23. 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

  24. 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

  25. 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

  26. Sandbagging • Temporary levees of bags of sand and mud • Estimated about 26.5 million sandbags used in 1993 floods • Lessened damage in some places, but not in 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

  27. 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” • Severity of flood damages, increase in population and wealth over time, political concerns: differences in number of declarations for different presidents

  28. Hydrograph • Hydrograph: 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)

  29. 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

  30. Flood Frequencies • Urbanization increases surface runoff of rainwater  higher stream levels in shorter times (flash 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 • Extreme approach: Los Angeles – concrete lined channels

  31. 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 The Uncoordinated Approach: San Diego • Army Corps of Engineers constructed 245 m wide channel at mouth • Mission Valley was developed along natural channel, 7.5 m wide • Part of Mission Valley has 110 m wide channel feeding into natural channel

  32. 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 have damage due to 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

  33. 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”

  34. 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

  35. End of Chapter 13

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