Earthquakes by wuyunyi

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									   Floods

  GEOL 4093
Risk Assessment
                Resources
• http://lists.uakron.edu/geology/natscigeo/lec
  tures/streams/Miss_Flood.htm
• http://www.usgs.gov/themes/flood.html
• http://www.fema.gov/nfip
• Figures are from From Murck et al.,
  Dangerous Earth unless otherwise noted
           What is a flood?
• Occurs when the level of a body of water
  exceeds its natural or artificial confines
• Water then submerges land in surrounding
  areas
Rio de la Plata, Puerto Rico after Hurricane Hugo
Three Kings Day storm, 1992, Puerto Rico,
24-hour rainfall totals for selected areas.
Flash flood from 1992 Three Kings Day flood, PR
                  Some Historic Floods
River             Type of Flood    Date   Fatalities   Remarks
Huang Ho, China   River flooding   1887   900,000      130,000 sq km flooded

Johnstown, PA     Dam failure      1889   2,200        10-12 m high wave
                                                       rushed down valley
Yangtze, China    River flooding   1911   100,000      Formed lake 130 x 50
                                                       km
Yangtze, China    River flooding   1931   200,000      Extended over 800 km,
                                                       millions homeless
Vaiont, Italy     Dam failure      1963   2,000        Landslide into lake
                                                       caused wave
                                                       overtopping dam and
                                                       flooding village below
        Human Development
• Often near water, so highly susceptible to
  flooding
• Especially true in developing countries
• Bangladesh—half of land less than 8 meters
  above sea level
Humans tend to live near rivers—Paris and the Seine River
             Causes of Flooding
•   Unbalance in hydrologic cycle
•   Very high precipitation gives floods
•   Very low precipitation gives droughts
•   Often combined effects:
    –   Snow melt
    –   Inadequate drainage
    –   Water-saturated ground
    –   Dam failures
    –   High tides
             River Systems
• Channels—passageways
• Discharge—quantity of water passing a
  given point within some interval of time
  (for example cfs or cfm)
• Load—sediment and dissolved material
  carried by river
• Gradient—vertical drop over measured
  distance
These plots are called long profiles and clearly show river gradients.
Steep mountain streams (Sacramento) may reach 60 m/km or more;
gradients near mouths of very large rivers (Missouri) 0.1 m/km or less).
          Channel Patterns
• Straight—rare, occur for brief stretches
  only
• Meandering—series of bends and curves,
  very common, especially in low gradients
• Braided—channels divide and rejoin,
  usually indicate variable discharge and
  easily erodible banks
                   Channel Patterns




Straight Channel      Meandering Channel        Braided Channel


Pools are areas along a channel where water is deepest. Arrows
indicate direction of flow and trace path of deepest water.
           Drainage Basins
• Area that contributes water to a river
• Can be entire river system basin down to
  small tributary basin
• Basins separated by divides
Mississippi River drainage basin. Width of rivers reflect discharge.
          Drainage Patterns
• Arrangement of channels
• Controlled by geology
• We won’t get into this
           Streamflow Dynamics
•   Overland flow—not channelized
•   Streamflow—channelized
•   Runoff—streamflow plus overland flow
•   Main controls on flow:
    –   Gradient
    –   Cross-sectional area (width x average depth)
    –   Average velocity
    –   Solid load
               Discharge
• Q=VxA
• Discharge = average velocity times cross-
  sectional area
Change in cross-sectional area of the Colorado River at Lees Ferry,
Arizona, during a 6-month period in 1956. Note scour of the river
bottom during rising water and deposition during falling water.
             River Flooding
• Stage—height of a river
• Bankfull stage (or flood stage)—when a
  river’s discharge increases to fill channel
  completely
• Flood—water exceeds river’s banks
               Hydrograph
• Graph used to show discharge versus time
• Lag time—time between precipitation and
  peak flood stage
• Peak discharge—maximum discharge
  during time of study (could be single flood
  event or entire recorded history of river)
Hypothetical flood hydrograph:
(a) preflood stage, (b) bankfull
stage, (c) peak discharge.
Hydrograph
for 1993
Mississippi
River flood
                Floodplain
• Area surrounding river influenced by
  flooding
• Typically broad and flat, built of fine silt
  and mud from floodwaters
• Usually very good agricultural land
• Mississippi River floodplain covers 80,000
  square kilometers
                     Levees
• Ridge of sediment built up adjacent to river
  channel
• Boundary between channel and its floodplain
• When river floods, coarsest sediments deposit first
• Also, smaller floods are more frequent than larger
• Because of extensive river engineering we are
  now forced to use the term natural levee
Main features of a river valley.
           Infiltration Capacity
• Rate at which precipitation can be absorbed
  by ground
• Depends on ground composition,
  vegetation, slope, rate of precipitation
• Major control on runoff, thus flooding
• Runoff =
  precipitation – infiltration – interception – evaporation
Rainfall Distribution and Flooding
• Distribution of rainfall over an area is
  commonly shown on a contour map
Rainfall in the upper
Mississippi River
basin, January-July
1993, expressed in
terms of the percent
by which it exceeded
the 30-year average
rainfall for the same
period.

Shaded area is
general area of
flooding during the
summer of 1993.
          Upstream Flooding
• Intense, infrequent storms of short duration
• Cause flooding that is severe but local in
  extent
• Called ―upstream flooding‖ because effects
  of the storm runoff usually do not extend to
  the larger streams further downstream
Upstream floods are
generally local, with
short lag times
              Flash Floods
• Floods with exceptionally short lag time
• Peak discharge reached only hours or
  minutes after storm has passed
• Deadly
       Downstream Flooding
• Usually from storms that last a long time
  and extend over large area
• Total discharge increases downstream as
  tributaries collect floodwaters
Downstream floods are regional in extent
with longer lag times and higher peak
discharges.
Examples of Flood Hazards from
the 1993 Mississippi River Flood
• Primary Effects
  – Water damage to household items
  – Structural damage to buildings
  – Destruction of roads, rail lines, bridges, levees,
    boats, barges
  – Historical sites destroyed
  – Crop loss
  – Cemeteries flooded, graves disrupted
  – Loss of life
   1993 Miss. River Flood, cont.
• Secondary and Teritiary Impacts
  – Destruction of farmlands
  – Destructions of parklands and wildlife habitat
  – Health impacts
     •   Disease related to pollution
     •   Injuries (back, electric shock, etc.)
     •   Fatigue
     •   Stress, depression
  – Disruption of transportation/electrical services
  – Gas leaks
  – Lack of clean water
Secondary, Tertiary, continued
– Impacts on crop prices; food shortages
– Job loss and worker displacement
– Economic impacts on industries
   •   Construction (beneficial impact)
   •   Insurance (negative impact)
   •   Legal (beneficial impact)
   •   Farming (negative impact)
– Misuse of government relief funds
– Changes in river channels
– Collapse of whole community structures
     Predicting River Flooding
• Statistical techniques to predict frequency
  of floods of a given magnitude
• Models and mapping to determine areal
  extent of flood hazards (paleohydrology)
• Monitoring a storm, forecasting warnings
Observing modern floods is a good way to learn what areas may
be flooded in future events. This is the 1993 Mississippi River
Flood near St. Louis, before and after.
          Recurrence Interval
• Average interval between occurrences of
  two floods of equal magnitude
• Not actual recorded interval but statistical
  average
• 100-year floods don’t always occur 100
  years apart
            Flood Frequency Curve

• Plot of flood
  discharges versus
  recurrence interval for
  a flood of that
  magnitude
• Flood frequency curve
  for Skykomish River
  at Gold Bar, WA
           Coastal Flooding
• Storm surge
• Recall causes, controls from previous
  lecture
       Flood Hazard Mapping
• To depict the impact and areal extent of a
  flood
• Actual floods
• Modeling
• Paleohydrology
• FIRMs—Flood Insurance Rate Maps
              Flood Zones
• A-zone = 100-year flood
• V-zone = 100-year flood with waves
• Old terms
  – B-zone = 100-500-year flood
  – C-zone = above 500-year flood
• New term
  – X-zone = outside 100-year flood zone
Coastal Flood Zones
FIRM for a coastal NC community
FIRM for a river system in
Puerto Rico
USGS
topographic
map, Boca de
Cangrejos
area, Puerto
Rico
Same area
showing
historical
river
flooding
Same area, this time showing
storm surge flooding
             Q3 Flood Layer
• FEMA Digital flood maps
• ArcInfo Coverages
  –   ―X‖ zone
  –   ―A‖ zone
  –   ―AE‖ zone
  –   ―X500‖ zone
  –   ―VE‖ and ―UNDES‖ (undesignated) zone
             SLOSH Maps
• Sea, Lake, and Overaland Surges from
  Hurricanes
• FEMA, Army Corps, National Hurricane
  Center, State Emergency Management
  Agencies
• Colors, shading show areas predicted to be
  inundated by various category hurricanes
           Channel Modification

• Channelization (engineering)
• For ―safety‖ but has unintended consequences
   –   Interferes with natural habitats and ecosystems
   –   Aesthetics
   –   Groundwater problems
   –   Pollution
   –   May cause more severe flooding downstream
• See previous lectures for graphics of levee
  problems on the Mississippi River
Natural river
system,
undisturbed
Channelized
river
River control structures built to maintain course of Mississippi River
     Development and Floods
• Subsidence from groundwater removal
  (remember Baytown, TX)
• May decrease cross-sectional area of river,
  so flooding occurs with less discharge
• Runoff increases with more paving and with
  storm sewer drainage (less infiltration),
  called urban runoff
Development on a floodplain can alter cross-sectional area of channel
Development impacts shown on flood hydrographs—note
decrease in lag time, increase in peak discharge.
Structural Responses to Flood Hazards

 • Channelization
 • Dams, retention ponds, and reservoirs
   – Controlled release of flood water
 • Levees, dikes, flood walls
   – May worsen flooding upstream and
     downstream
      Nonstructural Approaches
• Reduction of Vulnerability
• National Flood Insurance Act, late 1960s
  started ball rolling
  –   Floodplain zoning
  –   Specialized building codes
  –   Open-space planning in flood zones
  –   Floodplain buyout programs
• FEMA plays big role

								
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