Lecture Outlines Natural Disasters, 7th edition

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Lecture OutlinesNatural Disasters, 7th edition

• Patrick L. Abbott

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FloodsNatural Disasters, 7th edition, Chapter 14
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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

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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
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How Rivers and Streams Work

• Longitudinal Cross Section of a Stream
• Cross-sectional plot of streams 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 into which
  stream flows
• Profiles are similar for all streams because of
  equilibrium processes

Figure 14.3
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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)

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How Rivers and Streams Work

• 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

Figure 14.4
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How Rivers and Streams Work

• Case 1 Too Much Discharge
• Too much water ? 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

Figure 14.6
Figure 14.7
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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
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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
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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
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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

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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
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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

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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

Figure 14.12
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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

Figure 14.13

• For each years 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

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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

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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

Figure 14.14
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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
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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
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Flash Floods

• Big Thompson Canyon, Colorado, 1976
• 139 people killed, damage totaling 36 million

Figure 14.18
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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.

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

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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 into still
  frozen sections, causing floods

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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

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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

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Regional Floods
Mississippi River System
Figure 14.20