Environmental Science ENSC 2800

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Environmental Science ENSC 2800

• Spring 2003
• Class 7
• The Bay Delta Issue 2
• Floods, Global Warming and Other
• Environmental Effects
• Part 1

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Flooding Risk in the Bay Delta

• The Central Valley and Delta would naturally
  flood with high periodicity if not for dams and
  levees.
• Hydrologists talk of flood levels using
  probabalistic language the 150, 1100, 1200
  year floods for example.
• These represent, statistically speaking, the peak
  volumetric flows expected, given historical
  records, at least once in the given number of
  years.
• Every stream system, based on its cross-sectional
  area, longitudinal slope and bed and bank
  friction, has a conveyance capacity.

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Flooding

• If the peak volumetric flows in a given year
  exceed the conveyance capacity, the stream will
  overflow onto the surrounding floodplain.
• Depending on the proportion of the peak flow not
  contained in the channel itself, the topography
  of the adjacent floodplain, and the average speed
  of the water, a given area of land will be
  flooded to varying depths.
• Topography is critical as to how far the water
  will flood, how deep the water will be at a given
  floodplain location, and how long the water will
  take to recede.

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Recurrence Intervals (probability), Floodplains
and Flood Stages
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Risky Business

• Much of the Sacramento metropolitan area is
  threatened by flooding from the American and
  Sacramento Rivers.
• The estimated 100-year floodplain covers over
  100,000 acres and contains over 400,000
  residents, 160,000 homes and structures, and over
  37 billion in developed property.
• As our hydrological knowledge improves, as record
  events change calculations of flood
  periodicities, and if climatic change brings
  wetter weather and more spring runoff, the
  100-year floodplain will prove to be even larger
  and the potential impacts greater than this.
• What was thought to be the 1100 flood limit was
  recalculated to be only about 177 after the 1997
  storms.

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Are you at risk?

• In response to public right-to-know policies and
  to help manage federal flood insurance programs,
  FEMA provide interactive maps to identify
  locations within 100-year floodplains.
• See http//mapserver2.esri.com/cgi-bin/hazard.ado
  l?s0c-121.493333,38.581667p3cdzd0
• Real time flood data can be gotten from a variety
  of web pages and other information sources
  including http//ceres.ca.gov/topic/flood/fmap/nc
  al.html

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The power of water!
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Flooding can be devastating and leave lasting
environmental impacts