Note effect a retention pond has on water flow

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• Note effect a retention pond has on water flow
• Temporary storage lessens magnitude of discharge
  and spreads out impact

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Channelization

• Pros
• Flood and erosion control
• Better navigation
• Cons
• Disturbs ecosystems
• Plants, animals, feeding/breeding areas changed
• Aesthetics upset

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Flood frequency

• Recurrence interval
• Length of time in years separating floods of a
  similar size measured as volume of flow of
  stream
• Larger floods have a longer time interval
• We speak of the 100-yr flood, one that has a 1
  probability of occurring once in a given year
• 20-yr flood would have a 5 likelihood of
  occurring in a given year
• Oak Creek to the south of Flagstaff experienced
  three 100-yr floods in a decade due to
  combinations of rare conditions

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Types of floods

• Flash
• Localized areas, usually thunderstorms common in
  deserts
• Examples
• Antelope Canyon, AZ August 1997 11 died
• Big Thompson Canyon, CO July 1976 145 died
• Need to consider where storms areseldom directly
  overhead but up drainage area
• Regional, downstream
• Prolonged rain which saturates ground or delayed
  snowmelt
• Floodplain covered

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Flash flood vs downstream flood
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Large rivers in conterminous U.S.
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Mississippi River basin facts

• Greatest inundation floods in US occur here 3rd
  largest river basin in world
• 11 of 28 largest rivers in US are part of the
  Mississippi River system
• Avg flow 645,000 cu ft/sec
• 1 cu ft 7.5 gal, so this is 4.83 million
  gals/sec or 100 swimming pools PER SEC

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Major flooding between 1993 and 1997
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Missouri River in 1993
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Our response to flooding

• Dams
• Only provide some flood protectionnot control
  if full, lakes must be drained failure of dam is
  major problem
• Levees
• Create false sense of security if water breeches
  levee, it doesnt drain well
• Sandbagging
• Very short term solution
• Zoning and insurance
• Cant build in 100-yr floodplain most people
  dont buy floodplain insurance knowing fed govt
  will bail them out
• Channelization
• Clear, deeper, wider, straighter paths for water
  everything goes against idea of graded stream
  one seeking equilibrium

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Levee erosion and results
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Contributing factors and results for Tucson
floodingOct 1983

• Tropical storm Octave dropped 60 annual rainfall
  in a few days
• Increased development increases pavement and roof
  tops
• Desert flooding generates bank erosion rather
  than inundation

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Flooding in Tucson Oct 1983
Pantano Wash
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Bank erosion is major problem in deserts Tucson
Oct 1983
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Glacial control Channeled Scablands of WA state
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D Best