Lecture Outlines Natural Disasters, 7 th edition

1. Lecture OutlinesNatural Disasters, 7th edition Patrick L. Abbott

2. FloodsNatural Disasters, 7th edition, Chapter 14

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 eventlikely to beexceededat some point • Floods of River Arno through Florence

4. Side Note: 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 Figure 14.2

5. How Rivers and Streams Work Longitudinal Cross Section of a Stream • Cross-sectional plot of stream’s bottom elevation vs. distancefrom 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 into which stream flows • Profiles are similar for all streams because ofequilibrium processes Figure 14.3

6. How Rivers and Streams Work 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) • Availablesediment(load)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. How Rivers and Streams Work Case 1 – Too Much Discharge Too much water stream will flow more rapidly and energetically Response: • Excess energy used toerode stream bottom and into banks • Sedimentproduced 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 Figure 14.4

8. How Rivers and Streams Work Case 1 – Too Much Discharge Too much water stream will flow more rapidly and energetically Response: • Erosioninto stream banks createsmeandering patternlonger stream path, lower gradient  slower, less energetic water flow Figure 14.6 Figure 14.7

9. How Rivers and Streams Work 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 Figure 14.8

10. How Rivers and Streams Work Case 2 – Too much load Too much sediment stream becomes choked Response: • Channel pattern becomes straighter  minimum energy needed to flow distance • Islands of sediment form within channel, creating braided stream pattern Figure 14.9

11. How Rivers and Streams Work Case 2 – Too much load Too much sediment stream becomes choked Response: • Similar to stream overflowing and eroding away landslide dam Figure 14.10

12. How Rivers and Streams Work 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

13. The Floodplain • Floors of streams during floods • Built by erosion and deposition • Occupied during previous floods, and will be occupied again in future floods Figure 14.11

14. Side Note: 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

15. Flood Frequency • Larger floods  longer recurrence times between each • Analyze by plotting flood-discharge volumes vs. recurrence interval, construct flood-frequency curve • Flood-frequency curvesdifferent for different streams • Can be used to estimate return time of given size flood • 100-year floodused for regulatory requirements, has 1% chance of occurrence in any given year • Difference between yearly probability, cumulative probability Figure 14.12

16. In Greater Depth: 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 dischargefor each year,rankyears accordingly Figure 14.13 • For each year’s maximum flood, calculaterecurrence interval = (N + 1) / M, where N = number of years of records, M = rank • Plotrecurrence interval vs. dischargefor each year, connect points asbest-fit line Longer records of floods  better flood frequency curves

17. Flood Styles Several causes: • Localthunderstormflash (upstream) floodlasting few hours, building and ending quickly • Rainfallover days regional (downstream) floodslasting weeks, building and dissipating slowly • Storm surge of hurricaneflooding coastal areas • Broken ice on rivers can dam up, block water flow  fail inice-jam flood • Short-lived natural dams(landslide, log jam, lahar) fail in flood • Human-built levees or damsfail in flood

18. 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 arevehicle-related • Only two feet of moving waterrequired to lift and carry away average car Figure 14.14

19. 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 Figure 14.15

20. Flash Floods 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 Figure 14.17

21. Flash Floods Big Thompson Canyon, Colorado, 1976 • 139 people killed, damage totaling $36 million Figure 14.18

22. 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 Lakedam broke,flooding Rapid City • 238 people killed, $664 million in damages • Floodplain remainsundeveloped  greenbelt • “No one should sleep on the floodway.”

23. Flash Floods Rapid Creek, Black Hills, South Dakota, 1972 Figure 14.19

24. Regional Floods Inundation of area under high water for weeks • Few deaths, extensive damage • Large river valleys with low topography • Widespread cyclonic systems  prolonged, heavy rains • In U.S., about 2.5% of land is floodplain, home to about 6.5% of population

25. Regional Floods 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 intostill frozensections, causing floods

26. Regional Floods Red River of the North: • 1997 record floods: • Fall 1996 rainfall four times greater than average • Winter 1996 freezing began early, causing more ice in soil • Winter 1996-7 snowfalls more than 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

27. Regional Floods 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

28. Regional Floods Mississippi River System Figure 14.20