A field evaluation of the formative conditions, wavelengths and heights of alternate bars in alluvial channels

This study investigates the fluvial dynamics of straight natural stream channels. In particular, this experimental field study quantitatively assesses: (1) six experimentally derived formulas that predict the conditions for alternate bar formation and equilibrium bar dimensions, and (2) a physically-based nonlinear mathematical theory of alternate bar formation under unsteady natural flow conditions within a straight alluvial stream (Tubino 1991). The study site is an artificially-straightened section of the Embarras River located approximately 16 km south of Champaign, Illinois. Data were collected on channel form, gradient, alternate bar dimensions, and flow conditions over a two-year study period.Experimental flume studies suggest that alternate bars form in wide, straight, shallow streams have wavelengths between 4 and 15 channel widths, and have heights that are roughly equal to the average depth of flow. Both linear and nonlinear steady flow hydrodynamic theories suggest that alternate bars are critical to the process of meander development. Tubino (1991) suggests that bar evolution for a flood hydrograph can be divided into three parts: (1) a period of limited bar growth during the rising stage of the flood, (2) a stage of modest bar decay near the peak of the flood, and (3) a stage of nonlinear bar growth during the prolonged falling stage of the flood.Quantitative evaluations of: (1) the experimentally-derived formulas of bar development under steady flow, and (2) Tubino's model of bar development under unsteady flow were performed using data on flow stage, bed topography, and bed sediment size data from a straight alluvial channel. Bar formation under unsteady flow conditions can be accurately predicted by the steady-flow formulas, but these formulas fail to successfully predict bar dimensions. Furthermore, although bars developed during the falling limb of a hydrograph, they exhibited sequential development rather than the uniform growth along the reach predicted by Tubino's model. Further work should focus on attempting to isolate the physical mechanisms responsible for alternate bar formation in straight natural streams with heterogeneous bed material and flashy hydrologic flow regimes through either additional analytical or empirical research.U of I OnlyETDs are only available to UIUC Users without author permissio