IX: Shaping Topography
1.
Dynamic Rivers
Where do rivers flow?
All rivers are surrounded by a certain amount
of land that is higher in altitude (upgradient)
than the actual river. Precipitation that falls
in this area eventually flows downhill towards
the river. At any particular point on a river,
the land upgradient of the point is the river's
drainage basin (often known as the watershed).
This example of a drainage basin gives a rough
idea of how precipitation flows downhill into
rivers (and lakes).
[
EPA Watershed Interactive Website]
Which direction do rivers flow?
What separates two drainage basins from each
other are ridges of higher land. The Continental
Divide, runs along the highest ridges of the
Rocky Mountains. Precipitation falling on the
western side of the Divide will flow towards
the Pacific Ocean and precipitation falling
on the eastern slopes will flow towards the
Atlantic Ocean, via the Gulf of Mexico. The
United States has many drainage basins of many
sizes, and many, many ridges that define these
drainage basins. In Atlanta, Ga, the main street
through the city is Peachtree Street. It is
actually built on top of a ridge that is a drainage-basin
divide. Rain falling on the eastern side of
Peachtree Street will flow towards the Atlantic
Ocean, while runoff on the western side heads
to the Gulf of Mexico.
[
Map of Continental Divide ]
Rivers
cause erosion:
The Grand Canyon was formed primarily by erosion
due to flowing water and wind.
[
Location of the Colorado River ]
[
Angel's Window, Cape Royal, north rim ]
Photo
courtesy of Grand Canyon Explorer web site,
www.kaibab.org
[
Inner Gorge, Colorado River, looking west from
Tonto Trail just east of Salt Creek ]
Photo courtesy of Grand Canyon Explorer
web site, www.kaibab.org
[
Layers of the Grand Canyon ]
Rivers
Can Change Direction
As
the Misssissippi river enters the Gulf of Mexico,
it loses energy and dumps its load of sediment
that it has carried on its journey through the
mid-continent. This pile of sediment, or mud,
accumulates over the years, bulding up the delta
front. As one part of the delta becomes clogged
with sediment, the delta front will migrate
in search of new areas to grow. The area shown
on this image is the currently active delta
front of the Mississippi. The migratory nature
of the delta forms natural traps for oil and
the numerous bright spots along the outside
of the delta are drilling platforms. Most of
the land in the image consists of mud flats
and marshlands. There
is little human settlement in this area due
to the instability of the sediments. The main
shipping channel of the Mississippi River is
the broad red stripe running northwest to southeast
down the left side of the image. The bright
spots within the channel are ships.
[
Mississippi Delta from Space ]
2. Wave Action - Ocean erosion
Landslides (Mass Wasting)
[
Landslides caused by Wave Erosion]
Along bluff-backed and slide-backed shorelines,
processes of mass wasting are the primary controls
on shoreline stability. Mass wasting refers
generally to a broad range of gravity-driven
rock, soil, or sediment mass movements. This
includes weathering processes that result in
gradual bluff recession, such as direct wind
and rain impact. Here, the term mass wasting
refers to episodic slope movements also known
as landslides. The distinction between bluff-backed
and slide-backed shorelines represents differences
in the scale of slope movement. Simple surface
sloughing is the dominant process along bluff
backed shorelines. Complex deep-seated landsliding
and slumping is the dominant process along slide-backed
shorelines. A number of factors affecting slope
stability increase driving forces and/or reduce
resisting forces. Material composition is a
primary control on slope stability. Headlands,
for example, while not immune to mass wasting,
do not readily give way. In contrast, soft bluff-forming
sandstones and mudstone are highly susceptible
to slope movement. Prolonged winter rains saturate
these porous bluff materials, both loading the
slope and lowering cohesive strength, to further
decrease slope stability. The geometry and structure
of bluff materials also affects slope stability.
They define lines of weakness and control surface
as well as subsurface drainage. By removing
sediment from the base
of bluffs and by cutting into the bluffs themselves,
processes of wave attack may also affect slope
stability. The extent to which the beach fronting
the bluff acts as a buffer is important in this
regard.
[ Landslides in England ]
The
direction from which waves approach the shore
affects where sand or other material is transported.
When waves approach parallel to the shore, sand
is moved in and out from a beach in cycles of
fine and stormy weather, and there is generally
no significant loss of sand from the area. Waves
approaching at an angle to the shore generate
currents which move sand in one direction along
the shore. This process of longshore drifting
can cause significant beach erosion, especially
on long beaches uninterrupted by headlands,
estuaries and inlets.
[
Sand Motion ]
from
http://www.ecy.wa.gov/programs/sea/coast/erosion/jetty.html
[
Wave Action ]
Jetties
and shoreline change
In the early 1900s, jetties were built at the
entrances to the Columbia River and Grays Harbor.
These jetties were designed to scour out sandbars
and keep navigation channels open. Beaches,
inlet entrances, and the nearby sea floor are
still changing as a result of jetty construction
over a century ago. River deltas scoured. The
jetties narrowed inlets and increased tidal
currents. These currents flushed sand out of
tidal deltas. Beach growth and erosion Over
decades, sand from the scoured deltas accumulated,
causing beaches near the jetties to grow. Campgrounds
and condominiums were built on this newly accreted
land. The delta sand is now gone and beaches
near the jetties are experiencing erosion.
[
Washington State shoreline change - demonstrates
the effect of a jetty ]
from
http://www.ecy.wa.gov/programs/sea/coast/erosion/oc_shores.html
[
New method for determining detrimental waves
]
Photos courtesy of Dr.
Thomas C. Lippmann Civil & Environmental
Engineering and Geodetic Science Ohio State
University
3. Natural Disasters
Floods & flood
plains
On
one level, floods are pretty easy to understand.
Most of us have studied the "water cycle",
or "hydrological system", in school.
Water circulates from clouds, to the soil, to
streams, to rivers, to the oceans and then returns
to the clouds. When that system backs-up, there
is a flood. A number of factors can contribute
to that imbalance, including: heavy, intense
rainfall, run-off from a deep snow cover, over-saturated
soil, when the ground can't hold anymore water.
Frozen soil, high river, stream or reservoir
levels caused by unusually large amounts of
rain, ice jams in rivers, urbanization, or lots
of buildings and parking lots.
There are two basic
types of floods. In a regular river flood, water
slowly climbs over the edges of a river. The
more dangerous type, a flash flood, occurs when
a wall of water quickly sweeps over an area.
Almost three-quarters of the approximately 92
deaths from floods each year are due to flash
floods.
Human activity
that changes the surface of the Earth also effects
the water cycle, and can cause floods. Buildings,
parking lots and roads, replace grass and dirt
with concrete. Under normal circumstances, soil
acts like a sponge and soaks up a fair portion
of rainwater. But in crowded towns and cities,
rainwater flows into storm sewers and drainage
ditches, and, at times, overloads them. An urban
area can be flooded by an amount of rainfall
that would have had no impact in a rural area.
The
destruction of the nation's wetlands may also
contribute to moderate floods. The wetlands
are the swampy land along the edges of some
rivers. When it rains, the wet soil and mud
of a wetland acts like asponge and stores the
extra water. But much of America's wetlands
have been drained for farmland or to build houses.
The only place flood water can go is up and
over its normal riverbanks and into areas where
it can cause major damage.
[
Footage of 1927 flood on the Mississippi River
]
[
Comparison of 1927 and 1993 floods on the Mississippi
River - Maps and Pictures
via PBS http://www.pbs.org/wgbh/amex/flood/maps/index.html
]
Glaciers
Presently,
10% of land area is covered with glaciers. Glaciers
store about 75% of the world's freshwater. Glacierized
areas cover over 15,000,000 square kilometers.
Antarctic ice is over 4,200 meters thick in
some areas. In the United States, glaciers cover
over 75,000 square kilometers, with most of
the glaciers located in Alaska. During the last
Ice Age, glaciers covered 32% of the total land
area. If all land ice melted, sea level would
rise approximately 70 meters worldwide. Glacier
ice crystals can grow to be as large as baseballs.
The land underneath parts of the West Antarctic
Ice Sheet may be up to 2.5 kilometers below
sea level, due to the weight of the ice. North
America's longest glacier is the Bering Glacier
in Alaska, measuring 204 kilometers long. The
Malaspina Glacier in Alaska is the world's largest
piedmont glacier, covering over 8,000 square
kilometers and measuring over 193 kilometers
across at its widest point. Glacial ice often
appears blue when it has become very dense.
Years of compression gradually make the ice
denser over time, forcing out the tiny air pockets
between crystals. When glacier ice becomes extremely
dense, the ice absorbs all other colors in the
spectrum and reflects primarily blue, which
is what we see. When glacier ice is white, that
usually means that there are many tiny air bubbles
still in the ice. The Kutiah Glacier in Pakistan
holds the record for the fastest glacial surge.
In 1953, it raced more than 12 kilometers in
three months, averaging about 112 meters per
day. In Washington state alone, glaciers provide
470 billion gallons of water each summer. Antarctic
ice shelves may calve icebergs that are over
80 kilometers long. Almost 90% of an iceberg
is below water--only about 10% shows above water.
The Antarctic ice sheet has been in existence
for at least 40 million years. From the 17th
century to the late 19th century, the world
experienced a "Little Ice Age," when
temperatures were consistently cool enough for
significant glacier advances.
How does a glacier
cause erosion?
As
a glacier moves, particularly a warm glacier,
it causes erosion of the underlying surface.
Material from underlying bedrock or sediment
is picked up by the glacier and 'held' in the
ice as it moves. Material falling onto the surface
(often the result of freeze thaw activity, or
frost shattering, on the surrounding rock walls)
is also transported, and often finds its way
down through crevasses to the base of the glacier.
Material held within the glacier is called 'englacial
moraine'. It is this material trapped in the
ice, that allows the glacier to erode its surroundings.
With it's load of abrasive rock fragments, the
base of the glacier acts like a belt sander,
scraping across the rock, eroding it, producing
characteristic erosional features, and creating
a supply of material that leads eventually to
the formation of depositional features as well.
This scraping process is called Abrasion.
[
Where are glaciers today? http://www.glacier.rice.edu/land/5_warmice.html
]
Hurricanes
A
hurricane is a severe tropical storm, that forms
in the southern Atlantic Ocean, Caribbean Sea,
Gulf of Mexico or in the eastern Pacific Ocean.
Hurricanes need warm tropical oceans, moisture
and light winds above them. If the right conditions
last long enough, a hurricane can produce violent
winds, incredible waves, torrential rains and
floods. Hurricanes rotate in a counterclockwise
direction around an "eye." Hurricanes
have winds at least 74 miles per hour. There
are on average six Atlantic hurricanes each
year; over a 3-year period, approximately five
hurricanes strike the United States coastline
from Texas to Maine. When hurricanes move onto
land, the heavy rain, strong winds and heavy
waves can damage buildings, trees and cars.
The heavy waves are called a storm surge. Storm
surge is very dangerous and a major reason why
you MUST stay away from the ocean during a hurricane
warning or hurricane.
[
Interactive hurricane tracker via
http://html.nbc5i.com/sh/idi/weather/hurricanes/hurricanetracker.html
]
[
Current data on Hurricane Formation http://www.cpc.ncep.noaa.gov/products/Epac_hurr/irtempanim.html
]
[
Formation and Location of Hurricanes via nbc
]
Storm
surge is simply water that is pushed
toward the shore by the force of the winds swirling
around the storm. This advancing surge combines
with the normal tides to create the hurricane
storm tide, which can increase the mean water
level 15 feet or more. In addition, wind driven
waves are superimposed on the storm tide. This
rise in water level can cause severe flooding
in coastal areas, particularly when the storm
tide coincides with the normal high tides. Because
much of the United States' densely populated
Atlantic and Gulf Coast coastlines lie less
than 10 feet above mean sea level, the danger
from storm tides is tremendous
[
Storm Surge ]
Tornadoes
Tornadoes are one of nature's most violent storms.
In an average year, about 1,000 tornadoes are
reported across the United States, resulting
in 80 deaths and over 1,500 injuries. A tornado
is a violently rotating column of air extending
from a thunderstorm to the ground. The most
violent tornadoes are capable of tremendous
destruction with wind speeds of 250 mph or more.
Damage paths can be in excess of one mile wide
and 50 miles long. Tornadoes come in all shapes
and sizes and can occur anywhere in the U.S.
at any time of the year. In the southern states,
peak tornado season is March through May, while
peak months in the northern states are during
the summer.
[
http://www.noaa.gov/tornadoes.html ]
[
Formation of a tornado ]
[
Images of tornadoes ]
4.
Sensor Data from NOAA
[
Resource for all kinds of earth data http://coastwatch.noaa.gov/interface/interface.html
]
