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PP 1470. Simon, Andrew, 1994, Gradation processes and channel evolution in modified West Tennessee streams; process, response, and form: U.S. Geological Survey Professional Paper 1470, 84 p.
Channelization of alluvial channels in West Tennessee has increased energy conditions along main stems and tributaries and initiated systematic trends in channel adjustment. Gradation processes and adjustment trends are a function of the magnitude and extent of an imposed disturbance on a stream channel and the location of the adjusting reach in the fluvial network.
Degradation at a site is described by power-decay equations. Exponents denoting the nonlinear rate of downcutting with time decrease with distance upstream from the area of maximum disturbance, and generally range from -0.002 to -0.04.
Aggradation occurs in reaches immediately downstream from the area of maximum disturbance and in upstream reaches following overadjustment by degradation, and also can be described by power equations. Aggradation rates increase linearly with distance downstream and reach a maximum of 0.12 meters per year.
Adjustment of channel width by mass-wasting processes follows degradation and continues through the aggradational phases. Bank instabilities are induced after downcutting creates bank heights and angles that exceed the critical conditions of the material. Piping in the loess-derived bank materials enhances bank-failure rates.
Development of the bank profile is defined in terms of three dynamic and observable surfaces: vertical face (70° to 90°), upper bank (25° to 50°), and slough line (20° to 25°). Both the vertical face and upper bank sections represent major failure planes, and masses of failed bank material often come to rest on the upper bank. The slough line develops from additional flattening and downslope movement by low-angle slides and fluvial reworking, and is the initial site of reestablishing riparian vegetation and stable bank conditions.
A six-stage, semiquantitative model of channel evolution in disturbed channels is developed by quantifying gradation trends, by interpreting process-response relations during stages of bank-slope development, and by interpreting changes over space as changes over time.
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U.S. Department of the Interior, U.S. Geological Survey
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