Surface Water Sediment Transport by Streams

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Hydrologic cycle
Surface water, ground water, and water elsewhere on
earth is connected together as part of a large cycle
Surface Water
Hydrologic (water) Cycle
Sediment transport in rivers and streams
Why do river channels bend and change location over time?
Longitudinal profile – downstream change in gradient (slope)
Terraces
Drainage basins and discharge
Flooding
Human interventions to prevent flooding
Fig. 12.2 Understanding Earth
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Sediment Transport by Streams
• Dissolved material - from chemical weathering
• Dissolved load
• Solid material - clastic material from physical weathering
• Suspended load - smaller particles
• Bed load - larger particles
• Capacity: how much sediment a stream can transport (related to how
much water or discharge)
Streams may carry 4
clastic sediment either
as particles suspended
in the water or
moving along the
bottom. Particle size
and water velocity
determine which
particles will be
suspended and which
will be transported
along the bed.
Streams also carry
dissolved material.
• Competence: size of the largest particles which a stream can move
(related to water velocity)
Fig 13.2 Understanding Earth
Changes in a river system from the mountains to the sea
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1. Steep, narrow river
valley in the mountains
2. Broader, shallower
valley further downstream
3 A delta and nearly
3.
flat land where the
river flows into a
lake or the sea
from Chapter 13 Understanding Earth
Nile River delta,
Egypt - NASA
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Formation of
meanders
Faster-flowing water
at the outside of a
river bend erodes the
bank forming a cut
bank. With continued
erosion, the river
channel moves
toward the side
where erosion
occurs. Deposition at
the point bar on the
inside of the bend
fills in behind the
shifting channel.
Fig 13.10 Understanding Earth
Next: Stream Dynamics.mov
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How river channels change position (meander) over time.
Fig 13.10 Understanding Earth
Formation of an oxbow lake
When a river has welldeveloped meanders, the river
may cut a new, shorter channel.
Often this occurs when the river
floods. The abandoned part of
the original channel becomes
an oxbow lake.
Inside of
bend:
Point bar
Fig 13.10 Understanding Earth
Outside
of bend:
Cut bank
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Ridges (meander scrolls)
on an old point bar
These ridges record the
change in river position
over time.
Meandering river near
Anchorage, Alaska. Note the
accumulation of sand (white
areas) at point bars.
Fig 13.11 Understanding Earth
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Oxbow lakes and other
remnants of old river
channels
Drainage basins The entire area that contributes water to a river is
Next: rivers animation
called its drainage basin or watershed. Basins are separated by areas of
higher elevation called drainage divides. On one side of the divide, surface
water flows into one stream. On the other side of the divide, surface water
flows into another stream. Fig 13.21 Understanding Earth
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Flooding
Drainage basin of
the Colorado River
Flooding is a natural river process. Almost all floods result
from heavy rainfall, rapid snow melt, or a combination of
these two factors. During a flood the amount of water
(di h
(discharge)
) iis more th
than will
ill fit in
i the
th river
i
channel.
h
l
Fig 13.22 Understanding Earth
Flooding becomes a problem when people get in the way.
1996 flood in Liuzhou, China
Flooding is a normal river and stream
process. This city is built on a river floodplain as are many other cities.
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Estimated Flood Losses in the U.S.
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Chapter 13 Understanding Earth
http://www.weather.gov/hic/flood_stats/flood_trends.JPG
A river floodplain: A floodplain is the relatively flat area at the bottom
of a river valley. This area may be covered in water during a flood. In
larger floods, more of the floodplain is covered in water. During floods,
fine sediment is deposited on the floodplain.
Fig 13.8 Understanding Earth
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River terraces A river terrace is
an old area of floodplain that was
left behind when the river eroded
downward and formed a new
floodplain at a lower level.
Fig 13.20 Understanding Earth
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Formation of natural
levees by flooding.
The thickest and
coarsest deposits of
sediment accumulate
near the edges of the
river channel.
Discharge, the amount of water
flowing in a river channel, is
calculated by multiplying the
velocity of water flow by the
width and depth of the river.
Fig 13.14 Understanding Earth
Fig 13.13 Understanding Earth
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Flood frequency
curve
The recurrence
interval for the largest
flood often is
estimated based on
data for smaller
floods. Forecasting
based on past flood
history only works if
enough
g data has been
collected and the river
system hasn’t been
changed significantly.
Many river systems
have been changed by
human activity, and
this can have a big
effect on the accuracy
of flood probabilities.
Natural levees
during a flood
Fig. 14-29 Geology 2nd edition Chernicoff
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Flood-frequency curve for the
Skykomish River at Gold Bar, WA
A different river would have a different curve
Variation in discharge pattern with land use type
Relationship between flood
size (discharge) and frequency
or probability of occurrence. A
“ten-year” flood has a 10%
chance of happening in any
particular year.
Fig. 14-28 Geology 2nd edition - Chernicoff
Fig 13.15 Understanding Earth
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Older, 1982 map compared
to a newer, 1998 map.
The newer map uses both
more accurate land elevation
data and a more accurate
(larger) estimate of flood
discharge.
USGS Open File Report 98-200
Improving
Flood Hazard
Mapping
USGS Open File Report 98-200
http://wa.water.usgs.g
ov/pubs/ofr/floodgis/
Map of flood depth using the improved data.
USGS Open File Report 98-200
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What can people do about flooding?
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Current River Observations (map)
http://www.weather.gov/ahps/hic/index.php/index/
•
•
•
•
Research studies to better understand the problems
Make maps showing what areas are most hazardous
Educate people
Land use planning and zoning regulations for future development
(e.g. avoid building in the most hazardous areas)
preparation
p
• Disaster p
• Supplies
• Emergency plans & practice
• Control flood waters
• Fix and improve levees
• Dams
• Build or improve flood channels and bypasses
• Raising existing houses
Explore Online
Explore Online
Flood Warnings
http://www.weather.gov/ahps/hic/WFOs/FLOOD/
Hourly Precipitation records
htt //
http://www.srh.noaa.gov/rfcshare/precip_analysis_hourly.php
h
/f h /
i
l i h l h
James River Hydrographs
http://newweb.erh.noaa.gov/ahps2/hydrograph.php?wfo=rnk&gage=HRKV2
Area Hydrographs (Note Maury, James, and South rivers)
http://newweb.erh.noaa.gov/ahps2/area.php?wfo=rnk&view=1,1,1,1,1,1,1,1&t
oggles=10,7,8,2,9,15,6
NWS Blacksburg VA and region http://www.erh.noaa.gov/rnk/
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U.S. Geological Survey (USGS) Waterwatch http://water.usgs.gov/waterwatch
real time stream flow, daily/yearly/weekly stream flow, drought, flooding
National Weather Service - flooding, high/low flow
http://www.nws.noaa.gov/oh/hic/current/river_flooding/
USGS Surface Water News http://water.usgs.gov/osw/
Principles of Flood Management
http://www.friendsoftheriver.org/fotr/BeyondFloodControl/no1.html
Flood Statistics – damages and loss of life
http://www.weather.gov/hic/flood_stats/index.shtml
Rethinking Development on Floodplains article
http://www.crcwater.org/issues4/calfloodplain.html
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