HR: 16:45h
AN: H54B-04    [Abstracts]
TI: Range Variability of Bed Shear Stress: a Postulate for Stream Evolution
AU: * PAIK, K
EM: kpaik@uiuc.edu
AF: Department of Civil and Environmental Engineering University of Illinois, 205 N. Mathews Ave., Urbana, IL 61801 United States
AU: Kumar, P
EM: kumar1@uiuc.edu
AF: Department of Civil and Environmental Engineering University of Illinois, 205 N. Mathews Ave., Urbana, IL 61801 United States
AB: Over the past decades, efforts to discover possible natural laws that drive the fluvial landscape evolution have led to countless hypotheses, such as minimum variance, minimum energy dissipation rate, minimum Froude number etc. However, these extremal theory based formulations have had limited degrees of success in explaining the various natural formations such as stream network, plan form, hydraulic geometry, grain size distribution along stream, and bed profile. We postulate, based on channel equilibrium theory, that the nature seeks a certain range of stability constrained by multiple thresholds of bed shear stress to avoid both systematic erosion and deposition. This argument especially questions the validity of the extremal hypotheses which pursue single objectives. To test the adequacy of this `range variability' theory, a 1-D stream evolution model, considering the bed shear stress as the control variable, is developed. For constant discharge above certain value, this model shows bed erosion which results in flatter and linear bed profile. The downstream variation of discharge seems a key for generating concave bed profiles. As discharge $Q$ varies as $Q\propto x^{\theta}$ where $x$ is the distance from the headwater, regardless of ${\theta}$ and width variation, the concave bed profile forms as the slope $S\propto Q^{z}$. The exponent $z$ widely varies depending on ${\theta}$ and width variation. However, $z$ approaches -0.3 for ${\theta}$ in the range 1.5 and 2, regardless of width variation. This signifies that ${\theta}$ between 1.5 and 2 is the necessary condition to induce concave bed profile evolution resulting in $z$ close to -0.3, under the `range variability' theory. It is noteworthy that these values of ${\theta}$ and $z$ correspond to the observed values for natural streams. When width varies downstream as $W\propto Q^{0.5}$, the stream evolution shows downstream hydraulic geometry very close to the observed relationships, i.e. $V\propto Q^{0.1}$ and $H\propto Q^{0.4}$. These agreements with observation support the validity of `range variability' theory. Implications of this theory for application to channel and network evolution under alter hydrologic variability are being explored.
DE: 1719 Hydrology
DE: 1800 HYDROLOGY
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1625 Geomorphology and weathering (1824, 1886)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting