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Part 5. CHARACTER OF RIVER CHANNELS. Stages in maturity of landscape evolution revealed by channel morphology and adjacent flood plains. .

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Part 5
Part 5

CHARACTER OF RIVER CHANNELS




Hoover Dam channel is a transient mass, which can be excavated during extreme flows, as shown here on the San Juan River in Utah.

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

Asawn 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
Inner gorge at Hoover Dam channel is a transient mass, which can be excavated during extreme flows, as shown here on the San Juan River in Utah.

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
Pool and Riffle channel is a transient mass, which can be excavated during extreme flows, as shown here on the San Juan River in Utah.

  • 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



  • Common channel cross sections. The proportion to curvature in order to compensate for increased energy expenditures to overcome friction around turns. thalweg is that point lying over the deepest portion of the channel.



  • Point bars scour 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
Sinuosity and development of point bars scour

  • 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 scour

  • 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 carrying capacity of rivers scour

  • 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


  • The scoursuspended 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

Mechanics of saltation – which involves kinetic energy transfer between bouncing particles


Bed traction
Bed Traction scour

  • 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



Channel gravels
Channel gravels transported by a given flow, everything else being equal. The terms

  • 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
Flood Damage transported by a given flow, everything else being equal. The terms

  • 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
Impacts of channel constrictions transported by a given flow, everything else being equal. The terms

  • 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



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