HR: 09:45h
AN: H41G-08    [Abstracts]
TI: Mechanics of Horseshoe Waterfalls
AU: * Pasternack, G B
EM: gpast@ucdavis.edu
AF: University of California, Davis, 211 Veihmeyer Hall, LAWR 1 Shields Avenue, Davis, CA 95616 United States
AB: The interaction between flow, sediment transport, and channel morphology is very poorly known for step units in bedrock rivers. Unlike dams and weirs, natural steps have complex 3D morphologies. A detailed study of the fluid mechanics of horseshoe falls was performed using a scaled model with a 0.91-m vertical drop in a 2.75-m wide flume with flows up to 122 cfs. Five non-dimensional upstream energy levels with 3-5 non-dimensional tail depths were assessed for the resulting 3D water surface topography via digital elevation modeling, flow dynamics via digital videography, and overall energy dissipation via an energy and momentum conservation model. Regardless of tail depth, the horseshoe step was found to have 3 distinct zones beyond the brink: 1) a nappe whose degree of convergence depends on upstream energy input and brink configuration, 2) a convergence zone whose features vary strongly with upstream energy input, brink configuration, and tail depth, and 3) a downstream tailwater region whose dynamics primarily depend on tail depth. The centerline nappe profile and brink velocity were reasonably predicted using Rouse's jet trajectory equations when (H+P)/H$>$3. Peripheral profiles were not predictable using existing equations. For any arbitrary broad-crested step brink configuration, maximum energy dissipation was found to occur when no jump was present and downstream tail depth was exactly critical. DEM and process data including direct shear stress and sediment transport measurements from 4 natural horseshoe waterfalls in the Pacific Northwest will be presented for comparison against the flume study.
DE: 1803 Anthropogenic effects
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting