Natural Disasters. Tenth Edition. Chapter 13 Floods. Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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1 Natural Disasters Tenth Edition Chapter 13 Floods 13-1

2 Table 13.1 U.S. Flood Deaths and Damages by Decade, Decade Deaths Damages (in billions of 2011$) , , , * 1, Totals 50 Years 6,752 Deaths $ Billion Average Deaths per Year = 135 Average Damages per Year = $6.76 billion * Includes Katrina adjustment. Data from National Weather Service, NOAA. 13-2

3 Floods Rainfall varies in intensity and duration In small drainage basins, short-lasting maximum floods In large drainage basins, maximum floods for weeks Events of past forecast of future events Largest past event likely to be exceeded at some point 13-3

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 13-4

5 Figure 13.2 How Rivers and Streams Work 13-5

6 Figure 13.3 Side Note: A Different Kind of Killer Flood 13-6

7 How Rivers and Streams Work 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 an ocean, lake, pond or other stream into which stream flows Profiles are similar for all streams because of equilibrium processes 13-7

8 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) Available sediment (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) 13-8

9 How Rivers and Streams Work: Case 1 Too Much Discharge; Too much water (1 of 2) Stream will flow more rapidly and energetically Response: Excess energy used to erode stream bottom and into banks Sediment liberated by erosion energy is used carrying sediment away Erosion of stream bottom results in less vertical drop flatter gradient slower, less energetic water flow 13-9

10 How Rivers and Streams Work: Case 1 Too Much Discharge; Too much water (2 of 2) Stream will flow more rapidly and energetically Response: Erosion into stream banks creates meandering pattern longer stream path, lower gradient slower, less energetic water flow 13-10

11 Figure 13.4 How Rivers and Streams Work 13-11

12 Figure 13.6 How Rivers and Streams Work 13-12

13 Figure 13.7 How Rivers and Streams Work Drawing suggested by Gabi Laske 13-13

14 How Rivers and Streams Work: Case 2 Too much load; Too much sediment (1 of 3) 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 13-14

15 How Rivers and Streams Work: Case 2 Too much load; Too much sediment (2 of 3) Stream becomes choked Response: Channel pattern becomes straighter minimum energy needed to flow distance Islands of sediment form within channel, creating braided stream pattern 13-15

16 How Rivers and Streams Work: Case 2 Too much load; Too much sediment (3 of 3) Stream becomes choked Response: Similar to stream overflowing and eroding away landslide dam 13-16

17 Figure 13.9 How Rivers and Streams Work 13-17

18 Figure How Rivers and Streams Work 13-18

19 Figure How Rivers and Streams Work 13-19

20 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 13-20

21 The Floodplain Floors of streams during floods Built by erosion and deposition Occupied during previous floods, and will be occupied again in future floods 13-21

22 Figure The Floodplain 13-22

23 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 13-23

24 Flood Frequency Larger floods longer recurrence times between each Analyze by plotting flood-discharge volumes vs. recurrence interval, construct flood-frequency curve Flood-frequency curves different for different streams Can be used to estimate return time of given size flood 100-year flood used for regulatory requirements, has 1% chance of occurrence in any given year Difference between yearly probability, cumulative probability 13-24

25 Figure Flood Frequency 13-25

26 Table 13.2 Cumulative Probabilities of Floods Percentage Chance of This Size Flood Occurring Return Period (years) Any 1 Year 10 Years 25 Years 50 Years 100 Years , , , , Source: B. M. Reich, Water Resources Bulletin, 9 (1973),

27 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 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 13-27

28 Figure In Greater Depth: Constructing Flood- Frequency Curves 13-28

29 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 Hot weather causing rapid melting of snow Short-lived natural dams (landslide, log jam, lahar) fail in flood Human-built levees or dams fail in flood 13-29

30 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 vehiclerelated Only two feet depth of moving water required to lift and carry away average car 13-30

31 Figure Flash Floods 13-31

32 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

33 Figure Flash Floods 13-33

34 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 139 people killed, damage totaling $36 million 13-34

35 Figure Flash Floods 13-35

36 Figure Flash Floods 13-36

37 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

38 Figure Flash Floods 13-38

39 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 13-39

40 Regional Floods: Red River of the North (1 of 2) 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 13-40

41 Regional Floods: Red River of the North (2 of 2) 1997 record floods: Fall 1996 rainfall four times greater than average Winter 1996 freezing began early, causing more ice in soil Winter 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 13-41

42 Figure Flow volumes in Red River of the North,

43 Regional Floods: Mississippi River System (1 of 2) 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 m 3 /sec Water flow can increase fourfold during flood 13-43

44 Regional Floods: Mississippi River System (2 of 2) 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 13-44

45 Figure Regional Floods 13-45

46 Figure Regional Floods 13-46

47 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 km

48 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 13-48

49 Figure Regional Floods Mississippi River System The Great Midwestern Flood of

50 Figure The Missouri River flood of 1993 surrounds the State capitol in Jefferson, Missouri 13-50

51 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 13-51

52 Figure Regional Floods 13-52

53 Regional Floods: Mississippi River System 2011 Flood Management April and May heavy rains brought record or near-record river levels in multiple states Agonizing decisions needed to be made regarding diverting flows from upstream areas onto historic floodplains to reduce downstream flooding in highly populated and industrial areas Damages were limited to $4 billion instead of the much greater losses that would have occurred 13-53

54 Regional Floods: China (1 of 2) 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 13-54

55 Regional Floods: China (2 of 2) 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 Huang River today is 20 m higher than adjacent floodplain kept in place by levees 13-55

56 Figure Regional Floods 13-56

57 Figure Regional Floods 13-57

58 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 13-58

59 Societal Responses to Flood Hazards: Dams, Reservoirs and Natural Storage Areas Flood water can be stored behind dams in reservoirs or diverted to areas with low topography 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 13-59

60 Societal Responses to Flood Hazards: Levees (1 of 2) There is much debate about the value of levees Cost of building levees may be more than value of structures intended to protect Proponents claim that the reduced frequency of flooding saves billions of dollars Saturated levees can be compromised by wave attack, erosion by overtopping, failing by slumping, undermining by piping 13-60

61 Societal Responses to Flood Hazards: Levees (2 of 2) 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 13-61

62 Figure Societal Responses to Flood Hazards 13-62

63 Figure Breach in Kaskaskia Island levee, Illinois, August

64 Societal Responses to Flood Hazards: Sandbagging and Forecasting 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 13-64

65 Societal Responses to Flood Hazards: 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 13-65

66 Figure (a) After Hurricane Sandy flooding, house is raised above ground in New York, (b) Sand-bagged structure built to keep Missouri River away in South Dakota

67 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 13-67

68 Societal Responses to Flood Hazards: 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 64% of PDDs in 50 years 13-68

69 Urbanization and Floods: Hydrographs (1 of 2) 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) 13-69

70 Urbanization and Floods: Hydrographs (2 of 2) 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 13-70

71 Figure Urbanization and Floods 13-71

72 Figure Urbanization and Floods 13-72

73 Figure Urbanization and Floods Flood Frequencies Urbanization increases surface runoff of rainwater higher stream levels in shorter times (flash floods) 13-73

74 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 13-74

75 Urbanization and Floods: The Concrete 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 13-75

76 Figure Urbanization and Floods 13-76

77 Urbanization and Floods: The Uncoordinated Approach: San Diego Army Corps of Engineers constructed 245 m wide channel at mouth in 1940s Mission Valley was developed along natural channel, 7.5 m wide in 1950s Part of Mission Valley has 110 m wide channel feeding into natural channel in 1980s 13-77

78 Figure Urbanization and Floods 13-78

79 Urbanization and Floods: The Hit-and- Miss Approach: Tucson (1 of 2) 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 13-79

80 Urbanization and Floods: The Hit-and- Miss Approach: Tucson (2 of 2) 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

81 Figure Bank Erosion in Pantano Wash, Tucson, Arizona removed 30 mobile homes and their spaces 13-81

82 The Biggest Floods 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 13-82

83 Figure The Biggest Floods 13-83

84 The Biggest Floods: Ice-Dam Failure Floods (1 of 2) 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 13-84

85 The Biggest Floods: Ice-Dam Failure Floods (2 of 2) Huge volume of meltwater: Changed paths of rivers Could change global circulation of deep ocean water change in global climate Largest known floods in Earth history, in total, raising sea level by about 130 m 13-85

86 Figure The Biggest Floods 13-86

87 Figure The Biggest Floods 13-87

88 Figure The Biggest Floods 13-88

Floods Lecture #21 20

Floods Lecture #21 20 Floods 20 Lecture #21 What Is a Flood? Def: high discharge event along a river! Due to heavy rain or snow-melt During a flood, a river:! Erodes channel o Deeper & wider! Overflows channel o Deposits sediment