Hydrology: Discharge, Hydrographs, Floods, and Sediment Transport

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http//news.bbc.co.uk/2/shared/spl/hi/world/03/hur
ricane_guide/html/default.stm
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http//weather.about.com/gi/dynamic/offsite.htm?zi
1/XJsdnweathercdneducationtm294gps215_118
2_1020_580f11tt3bt0bts0zuhttp3A//www.cb
snews.com/htdocs/natural_disasters/hurricanes/fram
esource_flash.html - Excellent hurricane
animation
Tropical Cyclone Structure
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• Since 1953, Atlantic tropical storms have been
  named from lists originated by the National
  Hurricane Center. They are now maintained and
  updated by an international committee of the
  World Meteorological Organization. The original
  name lists featured only women's names. In 1979,
  men's names were introduced and they alternate
  with the women's names. Six lists are used in
  rotation. Thus, the 2006 list will be used again
  in 2012.
• The only time that there is a change in the list
  is if a storm is so deadly or costly that the
  future use of its name on a different storm would
  be inappropriate for reasons of sensitivity. If
  that occurs, then at an annual meeting by the WMO
  committee (called primarily to discuss many other
  issues) the offending name is stricken from the
  list and another name is selected to replace it.
• Several names have been changed since the lists
  were created. For example, on the 2004 list
  (which will be used again in 2010), Gaston has
  replaced Georges and Matthew has replaced Mitch.

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Hazards storm surge
several factors combine to determine the severity
of the surge a) wind speed higher winds "push"
more water onto the land b) low pressure lower
pressure causes more water to "dome" up under the
hurricane center c) tide stage at the time of
hurricane landfall d) coastal geomorphology
coasts with shallow bays and concave inlets have
more severe flooding Calculated by computer
model SLOSH Sea Lake and Overland Surges from
Hurricanes http//www.nhc.noaa.gov/HAW2/english/su
rge/slosh.shtml
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Winds Hurricane wind velocity
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Angle of Hurricane Approach to Coast
The amount of damage that occurs when a hurricane
approaches a coast depends on the angle of
approach. Two extreme examples illustrate this
point. Coast-parallel vs. Coast-normal
tracks Damage is less inland with parallel
track Extensive damage inland if normal track
Coast parallel
Most intense winds blow offshore
Coast-normal
Most intense winds blow onshore
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• Wind damage
• responsible for the loss of power and utilities
• wind damage affects large areas
• interaction with structures
• winds increase with height (problem for tall
  buildings) due to less frictional drag
• can channelize between buildings increasing the
  velocity (wind tunnel effect)

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Mitigation/Hurricanes

• evacuation/warnings
• Warning and Evacuation - With modern techniques
  of forecasting and tracking hurricane paths, it
  is always possible to issue warnings about the
  probable locations that will be affected by any
  given hurricane.
• Ships at sea transmit weather reports that help
  meteorologists locate centers of low pressure
  that may develop into tropical disturbances.
• Images from weather satellites, which are
  collected every 30 minutes, are then scanned to
  look for any development or growth of the
  disturbance. In particular, the images are
  examined to detect any rotational development of
  the storm, an indication that it may be
  approaching tropical storm strength.

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• Planes collect data on wind speed, air pressure,
  and moisture content by dropping devices called
  dropsondes into the storm.
• These dropsondes transmit the meteorological data
  continuously has they fall to the ocean surface,
  and thus provide information on the vertical
  structure of the storm.
• In addition, radar devices carried on the plane
  collect data about the intensity of the rainfall
  and wind velocities. The planes fly completely
  through the storm, passing though the eye,
  sometimes making several passes. The data
  collected give meteorologists a 3 dimensional
  picture of the structure of the storm.

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• Radar from land based stations allow scientists
  to track the position of the storm and report it
  to all agencies that may be affected if the storm
  makes landfall.

Hurricane Watch Meaning a hurricane is possible
in the watch area within the next 36 hours.
Hurricane Warning A hurricane is expected in
the warning area with twenty four hours. If
advised to evacuated, do so immediately!
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• The problem, however, is that it may not always
  be possible to issue such a warning in time for
  adequate evacuation of these areas.
• Because the storm surge and even high winds can
  reach an area many hours before the center of the
  storm, warnings must be issued long enough before
  the storm strikes that the surge and winds do not
  hinder the evacuation process.
• beach width/dune height
• acts as a natural buffer zone for the storm surge
• If maintained or increased by artificial
  processes, could prevent more damage for
  structures inland
• Wetlands and coastal marshes also act as natural
  buffer zones
• Less development along coastlines would reduce
  damage as well

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Damages caused by weather hazards (in 2006 US
million)
Weather hazards are most damaging and costly
hazards on Earth
Average cost of weather related disasters,
including hurricanes, tornadoes, severe storms,
flooding, droughts and extreme temperatures Appr.
60 billion US
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Damages caused by geological hazards (in 2006 US
million)
Average cost of geologic disasters, including
earthquakes, volcanoes and tsunamis Appr. 18
billion US
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Occurrence of natural disasters in the period of
1987-2006
Floods
Geological hazards
Droughts
Wind Storms
www.em-dat.net
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Discharge and hydrographs
modified from www.usda.gov/stream_restoration/chap
1.html
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• Usually rivers in Canada have a substantial
  seasonal variation in runoff. There are different
  patterns for flow variations.
• Most high flows in Canada are caused by snowmelt,
  which usually peaks in the spring. Soils are
  saturated and have low storage capacities.
• Consequently, this is the season when floods are
  most likely to occur in Canada.

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Lag Time - The time difference between when
heavy precipitation occurs and when peak
discharge occurs in the streams draining an area
is called lag time.
Lag time depends on such factors as the amount of
time over which the rain falls and the amount of
infiltration and interception that takes
place along the path to a stream.
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Effects of urbanization on a storm hydrograph
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Overland flow and development
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Upstream flooding and flash floods

• In areas where large amounts of rain fall over a
  short period of time within a small area, streams
  in the local area may flood, with little or no
  effect on areas downstream.
• Such floods are referred to as upstream floods.
  In such floods, water rises quickly and flows
  away quickly after the storm has passed..

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• If these floods are sudden with of relatively
  great volume they are called Flash floods occur
  when the rate of infiltration is low and heavy
  rains occur over a short period of time.
• Flash floods are most common in arid and semiarid
  environments, and in areas with steep topography
  and little vegetation.
• Because they come with little warning, flash
  floods are the most dangerous to human lives.

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Downstream flooding

• If large amounts of rain fall over an extended
  period of time over a large region, downstream
  floods may occur.
• Tributary streams continually increase the
  discharge into larger streams due to prolonged
  rainfall on saturated soils.

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Peterborough's Flood of Two Centuries
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Channelization
The Kissimmee River in Florida has been heavily
channelized. The103 mile long river was
originally only a few feet deep. Channelization
involved building a 56 mile long drainage canal
30 feet deep and 300 feet wide. This has resulted
in drainage of areas of the Everglades that were
previously wetlands.
Kissimmee River prior to channelizationPre-1961
Choked Remnant River Channelafter channelization
Structure and canal withdrained floodplain
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Retention Ponds Retain water and hold it
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• What is the difference between Detention Ponds
  and Retention Ponds?
• Detention ponds capture water and release it at
  a slower rate. Decreases the peak discharge to
  river, but not necessarily the total amount of
  runoff that reaches the river.
• Also removes some pollutants and sediment.
• Retention ponds capture water and hold it until
  it infiltrates or is removed by
  evaporation/transpiration.
• Retention areas reduce stormwater volume, peak
  discharge rate, off-site pollutant loading. They
  may also increase the recharge to shallow
  groundwater aquifers.

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Flooding

• We tend to have a biased view of floods as
  unpredictable, disastrous events, while in
  reality they are predictable and necessary
  occurrences.
• One important tool to predict floods is to
  calculate the recurrence interval of a flood,
  which we will discuss next Monday!

www.aquatic.uoguelph.ca/rivers/chphys.htm