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Lecture 12 Running Water and Streams

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Lecture 12 Running Water and Streams Hydrologic cycle Stream hydraulics Stream erosion Transportation of sediment by streams Deposition of sediment by streams – PowerPoint PPT presentation

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Title: Lecture 12 Running Water and Streams


1
Lecture 12 Running Water and Streams
  1. Hydrologic cycle
  2. Stream hydraulics
  3. Stream erosion
  4. Transportation of sediment by streams
  5. Deposition of sediment by streams
  6. Flooding

2
  • 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.

3
  • Balance of water in the hydrologic cycle.

4
  • 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).

5
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6
  • 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.

7
  • (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)

8
  • 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.

9
  • 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

10
  • 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.

11
  • 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.

12
  • 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/sec0.646 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.

13
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14
  • 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).

15
  • Sediment-filled floodwaters. The greatest erosion
    and sediment transport occur during these
    high-water periods. (Davis/Stone Images)

16
  • 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.

17
  • Potholes in a river bed. The rotational motion of
    swirling pebbles acts like a drill to create
    potholes. (T. Till)

18
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19
  • 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.

20
  • Sediment-filled floodwaters. The greatest erosion
    and sediment transport occur during these
    high-water periods. (Davis/Stone Images)

21
  • 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)

22
  • 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.

23
  • 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.

24
  • 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.

25
  • Landform features of streams
  • Both through erosion and deposition, streams
    alter the appearance of the land surface.

26
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)
27
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)
28
Niagara Falls. The resistant Lockport dolostone
serves as a local base level for Lake Erie. (W.W.
Norton)
29
Niagara Falls. The undercutting of the soft shale
layers causes the resistant dolostone to break
off. (W.W. Norton)
30
  • 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.

31
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)
32
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 gtgt Play Animation Macintosh version gtgt
33
  • 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)

34
  • 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).

35
  • 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)

36
  • 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.

37
  • Satellite views of the Missouri River flowing
    into the Mississippi River before (top) and
    during (bottom) the 1993 flood.

38
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39
  • Water rushes through a break in an artificial
    levee in Monroe County, Illinois during the
    record-breaking 1993 Midwest floods.

40
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41
  • 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.

42
  • 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)

43
A channel was constructed to take additional
run-off from the new parking lot.
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