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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