Lecture 12 Running Water and Streams
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6.
Hydrologic cycle
Stream hydraulics
Stream erosion
Transportation of sediment by streams
Deposition of sediment by streams
Flooding
1. Hydrologic cycle:
•
Powered by solar energy, Earth's Water is
constantly moving among the hydrosphere,
the atmosphere, the solid Earth, and the
biosphere. This unending circulation of
Earth's water supply is called the
hydrologic cycle.
• Balance of water in the hydrologic cycle.
• What happens to the water after falling on
the land?
• Much of the water falling on the land returns to
the oceans by seepage into the ground (called
infiltration) and by runoff over the surface (when
rainfall is beyond the land's ability to absorb it).
Some of the water that infiltrates the ground is
absorbed by plants, which later release it into the
atmosphere (a process called transpiration).
2. Stream hydraulics
•
flow types
Water may flow in one of two ways, laminar flow or
turbulent flow. The stream's velocity is a primary
controlling factor.
•
Laminar flow: When velocity is low, water particles
flow steadily parallel to each other and to the channel,
without mixing.
•
Turbulent flow: When velocity is high, streamlines will
mix, cross each other, and form swirls and eddies.
• (Left) Most stream flows are turbulent. (Right) This stream is
closer to a laminar flow. Continuous records of discharge are
collected by the USGS at more than 7000 gauging stations
like this in the U.S. (Tarbuck and Lutgens)
• velocity
• When the channel is straight, the highest velocities occur
in the center of the channel just below the surface. It is
here the friction is least. Minimum velocities occur along
the sides and bottom (bed) of the channel where friction is
the greatest.
• When the channel is bent, the zone of highest velocities
shifts away from the center towards the outside of the bend.
Thus active erosion occurs on the outside of the bend and
deposition occurs on the inside.
• Stream velocity is controlled by
(1) the gradient,
(2) the shape, size, and roughness of the channel, and
(3) the discharge.
• The Manning equation:
v proportional to d2/3s1/2/n,
d: water depth
s: gradient
n: bed roughness
• Gradient:
Gradient is the vertical drop of a stream over a fixed
distance. For example, portions of the lower
Mississippi River have gradients of only 10 cm per
km or less.
• The shape of a channel:
The shape of the cross-section of a channel
determines the amount of water in contact with the
channel and hence affects the frictional drag and
stream velocity. The most efficient channel is one
with the least perimeter for its cross-sectional area.
•
Influence of channel shape on velocity. The cross-sectional areas of
(A) and (B) are the same, but water flows more rapidly in (B) because
it has less water in contact with the channel and hence less friction.
• discharge
• The discharge is the amount of water flowing past a certain
point in a given unit time, thus
discharge = (cross-sectional area) x (flow velocity)
• Discharge is measured in cubic feet per second (cfs) for
streams but for water supply and sewage treatment in millions
of gallons per day (MGD) (1 ft3/sec=0.646 MGD).
• The discharge of the World's largest river, Amazon, is 7.5
million cfs. Just one day's discharge of Amazon could supply
the water needs of New York City of for 9 years! The
Mississippi River ranks 7th in the world by discharge.
3. Stream erosion
•
Streams erode their channels by lifting loose particles
and by abrasion.
•
Erosive power is proportional to the square of the
velocity. Thus when discharge increases, the depth
increases and the velocity increases, resulting in
dramatic increase of erosive power.
•
The scour and removal of bed materials during flooding
may undermine foundations for engineering structures
located in stream channels (such as bridges, loading
facilities).
• Sediment-filled floodwaters. The greatest erosion and
sediment transport occur during these high-water periods.
(Davis/Stone Images)
• Sands and gravels are great tools of erosion. Transported by a
river, they act as powerful abrasives, cutting through the bedrock
as they are moved by the stream.
• Potholes in a river bed. The rotational motion of swirling
pebbles acts like a drill to create potholes. (T. Till)
4. Transportation of sediment by streams
•
Streams transport their loads in solution, in suspension, and
along the bottom of the channels (bedload).
•
Ions in solution may include calcium, magnesium, chloride,
nitrate, sulfate, and silica.
•
Usually only fine sand-, silt-, and clay- particles can be
carried in suspension, except during flood stage where larger
particles are carried as well.
• Sediment-filled floodwaters. The greatest erosion and
sediment transport occur during these high-water periods.
(Davis/Stone Images)
• Particles too large to be carried in suspension may be
moved by streams along the stream bottom as bedload. The
maximum-size particle a stream can move is determined
by its velocity. (Tarbuck and Lutgens)
5. Deposition of sediment by streams
•
When stream velocity decreases due to
reduced depth or gradient, the particles of
sediment are deposited. The coarsest
particles of bedload are deposited first,
followed by finer and finer particles in
suspension.
• base level:
• defined as the lowest elevation to which a stream can cut
its channel. The ocean is the ultimate base level of all
streams. Local base levels include lakes, a dam, and
resistant layers of rock.
• The ability of a stream to do work is closely related to its
base level. When a base level is raised, e.g. by a dam, the
reduced gradient lowers its velocity, causing sediment to
deposit until the stream again has a gradient sufficient to
carry its load.
•
The base level of a stream is raised when a dam is built and a reservoir
forms. This reduces the gradient and leads to the reduction of velocity
and deposition of sediment upstream from the reservoir.
• Landform features of streams
• Both through erosion and deposition,
streams alter the appearance of the land
surface.
Drainage patterns. (a) Dendritic, forming on gentle slope and uniform substrate. (b) Radial,
forming on a cone-shaped mountain flow. (c) Rectangular, forming on a rectangular grid of
vertical joints. (d) trellis, forming on parallel valleys and ridges. (W.W. Norton)
The continental divide separate drainage basins that flow into different oceans. The
Mississippi drainage basin is one of the several in North America. (W.W. Norton)
Niagara Falls. The resistant Lockport dolostone serves as
a local base level for Lake Erie. (W.W. Norton)
Niagara Falls. The undercutting of the soft shale layers causes the
resistant dolostone to break off. (W.W. Norton)
• More common than straight channels, meandering streams
tend to form in gently sloping areas of unconsolidated
sediments. Meanders in the Sevier River west of Yuba
Reservoir, Utah.
Erosion occurs faster on the outer bank while deposition takes
place on the inner curve (to form point bars). An oxbow lake is
formed when the stream eventually cuts through the meander
neck. (W.W. Norton)
Evolution of a Meandering Stream
People building communities along a riverbank
mistakenly assume that the shape of a meandering
stream will remain fixed for a long time. In fact, in a
natural meandering river system, the river channel
migrates back and forth across the floodplain. View 1
illustrates the processes of erosion and deposition, and
View 2 shows the evolution, in map view, of a
meandering stream. [by Stephen Marshak]
Play Animation Windows version >>
Play Animation Macintosh version >>
• The outside of a meander is a zone of active erosion (often
referred to as the cut bank). The Neaukum River,
Washington in January 1965 (A) and March, 1965 (B).
(P.A. Glancy, USGS)
• Alluvial fans develop on land where the gradient of a stream
changes abruptly from steep to flat (e.g. from mountain
terrain to flat valley floor). Death valley has many large
alluvial fans as shown (Hamblin and Christiansen).
• Deltas form where a stream flows into a standing body of water (e.g.,
oceans, lakes). The transported sediment is deposited because of decreased
velocity. Today New Orleans located in the Mississipi Delta is built where
there was ocean less than 5000 years ago. The river flows over the delta to
form the tributaries (Modified from Hamblin and Christiansen, 1988
Tarbuck and Lutgens)
6. Flooding
•
When the discharge of a stream becomes so
great that it exceeds the capacity of its channel,
it overflows its banks as a flood.
•
The 1931 great flood of Yellow River in China
killed 4 million people. The 1993 Midwest flood
in the upper Mississippi River Basin caused
direct property damage exceeding $10 billion.
• Satellite views of the Missouri River flowing into the
Mississippi River before (top) and during (bottom) the
1993 flood.
• Water rushes through a break in an artificial levee in
Monroe County, Illinois during the record-breaking 1993
Midwest floods.
• The cause of flood is weather. But human
interference can make it worse. One example is
urbanization, which shortens the lag time
between rainfall and flood peak and increases the
flood peak because of less infiltration and more
rapid runoff.
• When an area changes from rural to urban, the lag time between
rainfall and flood peak is shorted and the flood peak is higher because
of less infiltration and more rapid runoff. (Hamblin and Christiansen)
A channel was constructed to take additional run-off from
the new parking lot.