Fluvial Processes pt5 by Ea18e2

VIEWS: 16 PAGES: 20

									     Part 5

CHARACTER OF
RIVER CHANNELS
• Stages in maturity of landscape evolution
  revealed by channel morphology and adjacent
  flood plains.
• The fluvial sediment package within any
  confined bedrock channel is a transient mass,
  which can be excavated during extreme flows,
  as shown here on the San Juan River in Utah.
                                Hoover Dam




A total of 6 million yards of
material was excavated at
the dam site.

A sawn 2 x 6 plank was found in the river bed
buried 50 feet beneath the low water surface.
Geologists suggested that it was deposited from
a debris flow in Callville Wash during the high
water of 1922.
Inner gorge at Hoover Dam




           About 2 million cubic yards of
           material was excavated out of the
           river channel beneath the dam,
           revealing an incised inner gorge with
           fluting and boulders up to 12 feet
           across.
  Pool and
   Riffle
• Natural channels
  tend to develop a
  pool and riffle
  system
• The equilibrium
  grade is the
  average slope of
  the energy grade
  line, as sketched
  at left
• Natural channels increase their hydraulic
  grade in proportion to curvature in order to
  compensate for increased energy expenditures
  to overcome friction around turns.
• Common channel cross sections. The thalweg is that
  point lying over the deepest portion of the channel.
• During high flows the channel bed is
  deepened by increased scour
• Point bars are deposited on the inside banks
  and bends along the channel, due to lower
  velocities and increased flow friction. These
  features typify the coarse fraction of the
  river’s bedload, commonly, gravel and sand.
 Sinuosity and
development of
   point bars
• Point bars develop
  on the inside turns
  of bends in the
  channel, due to
  increased friction
• The more sinuous
  the channel, the
  more point bar
  deposits can be
  expected
    BRAIDED
   CHANNELS
• Braided channels
  occur in rivers
  subject to large
  seasonal
  fluctuations in flow,
  common in arid or
  arctic regions. In
  these cases, the
  river has insufficient
  stream power to
  transport the
  sediment load
  imposed upon it.
Sediment carrying
capacity of rivers
• Sediment is
  transported in rivers
  and streams by two
  components: 1)
  suspended load; and,
  2) bed load
• Fast moving rivers on
  steep gradients, like
  that shown here, have
  gravel and cobble beds
                            Mechanics of saltation – which involves
                           kinetic energy transfer between bouncing
                                            particles


• The suspended sediment load is transported
  by the river’s current, aided by buoyancy
• The bed load is the coarse grained fraction
  that is transported via rolling, sliding, and
  saltation (shown at right) along the channel
  bed
  Bed Traction
• Upper - Distribution of
  velocity in unconfined
  channel, caused by
  frictional drag along
  the stream bed
• Lower- Distribution of
  boundary shear
  stress in a uniform
  density current,
  typical in deltas and
  large reservoirs
• As channel velocity increases, more particles can be
  transported by a given flow, everything else being
  equal. The terms competence and capacity are used
  to describe the efficiency of sediment transport in the
  channel.
    Channel
    gravels
• Channels aggrade
  locally where
  conditions are
  favorable to
  deposition
• These can be local
  bars (upper photo),
  or along broad
  stretches of
  channels, as
  shown below
    Flood
   Damage
• Before and after
  views of damage
  caused by the
  flood of January
  1916 along the
  Santa Margarita
  River in San Diego
  County, CA; at
  transition between
  confined bedrock
  canyon and
  alluvial valley
• Note railroad
  rolling stock and
  water tower for
  scale
 Impacts of
  channel
constrictions
• Before and after
  views of highway
  bridge over Verdugo
  Creek in Glendale,
  CA during March
  1938 flood.
• Approach fills in
  active channels are
  never a good idea,
  and usually removed
  by back-eddy scour
  from the downstream
  side of the crossing
• Extremely turbid flow of a ‘flash flood’ event in an
  semi-arid area, which has been developed into a golf
  course with adjoining homes. Short-lived, but highly
  turbid flows are common in arid and semi-arid regions.

								
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