HR: 0800h
AN: H51E-0801    [Abstracts]
TI: Stepped Hydraulic Geometry in Stepped Channels
AU: Comiti, F
EM: francesco.comiti@unipd.it
AF: Dept. Land and Agroforest Environments - University of Padova, viale Universita' 16, Legnaro, 35020, Italy
AU: * Cadol, D D
EM: cadol@warnercnr.colostate.edu
AF: Colorado State University, Department of Geosciences, Fort Collins, CO 80523-1482, United States
AU: Wohl, E
EM: ellenw@warnercnr.colostate.edu
AF: Colorado State University, Department of Geosciences, Fort Collins, CO 80523-1482, United States
AB: Steep mountain streams typically present a stepped longitudinal profile. Such stepped channels feature tumbling flow, where hydraulic jumps represent an important source of channel roughness (spill resistance). However, the extent to which spill resistance persists up to high flows has not been ascertained yet, such that a faster, skimming flow has been envisaged to begin at those conditions. In order to analyze the relationship between flow resistance and bed morphology, a mobile bed physical model was developed at Colorado State University (Fort Collins, USA). An 8 m-long, 0.6 m-wide flume tilted at a constant 14% slope was used, testing 2 grain-size mixtures differing only for the largest fraction. Experiments were conducted under clear water conditions. Reach-averaged flow velocity was measured using salt tracers, bed morphology and flow depth by a point gage, and surface grain size using commercial image-analysis software. Starting from an initial plane bed, progressively higher flow rates were used to create different bed structures. After each bed morphology was stable with its forming discharge, lower-than-forming flows were run to build a hydraulic geometry curve. Results show that even though equilibrium slopes ranged from 8.5% to 14%, the reach-averaged flow was always sub-critical. Steps formed through a variety of mechanisms, with immobile clasts playing a dominant role by causing local scouring and/or trapping moving smaller particles. Overall, step height, step pool steepness, relative pool area and volume increased with discharge up to the threshold when the bed approached fully- mobilized conditions. For bed morphologies surpassing a minimum profile roughness, a stepped velocity- discharge relationship is evident, with sharp rises in velocity correlated with the disappearance of rollers in pools at flows approaching the formative discharge for each morphology. Flow resistance exhibits an opposite pattern, with drops in resistance being a function of the height of the drowned steps. Step formation seems to occur under a hydraulic regime different from the lower flows, because spill resistance begins below step-forming flows.
DE: 1625 Geomorphology and weathering (0790, 1824, 1825, 1826, 1886)
SC: Hydrology [H]
MN: 2007 Fall Meeting