Submerged and Unsubmerged Natural Hydraulic Jumps in a Bedrock Step-Pool Mountain Channel

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Geomorphology 82 (2006) 146 – 159 www.elsevier.com/locate/geomorph

Submerged and unsubmerged natural hydraulic jumps in a bedrock step-pool mountain channel Brett L. Vallé ⁎, Gregory B. Pasternack
Department of Land, Air, and Water Resources, University of California, Davis, 95616, USA Accepted 2 September 2005 Available online 19 June 2006

Abstract High-resolution digital elevation models (DEMs) for natural submerged and unsubmerged jump regions were tested against the classical hydraulic jump (CHJ) and engineering analogues that have dominated previous geomorphic and engineering research. DEMs were compared for two discharge conditions. The data showed bed and water surface features of the natural jump regions differ significantly from CHJ conditions and engineering analogues with respect to boundary conditions. Roughness elements were highly irregular in frequency, spacing, size, and orientation, and were comprised of form obliquity and bed scour in boulder-bed and pure-bedrock conditions. The data also showed bed and water surface features of natural jump regions have similarities and deviations from CHJ conditions and engineering analogues with respect to hydraulic characteristics. Several variations were observed in the hydraulic characteristics between the ballistic and sloping jets. Revised conceptual models based on idealized corollaries were developed for the natural jump regions based on upstream energy head relative to the downstream tailwater depth and step geometry. The DEMs highlight the importance of recognizing the interrelationship between spatial transcritical flow structures and localized topographic heterogeneities in bedrock channels. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydraulic jump; Mountain channel; Supercritical flow; High-resolution data; Rapidly varied flow

1. Introduction Tumbling flow features such as supercritical jets and hydraulic jumps have increasingly been recognized as important morphologic components of mountain river channels (Kieffer, 1985; Grant et al., 1990; Carling, 1995; Grant, 1997; Montgomery and Buffington, 1997; Parker and Izumi, 2000; Vallé and Pasternack, 2002a). Supercritical jet scour has been documented in a wide ⁎ Corresponding author. Philip Williams & Associates, Ltd., 720 California St., Suite 600, San Francisco, CA 94108, USA. Tel.: +1 415 262 2300; fax: +1 415 262 2303. E-mail address: b.valle@pwa-ltd.com (B.L. Vallé). 0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2005.09.024

variety of settings, including agricultural furrows, bed sills, and stilling basins (Peterka, 1983; Alexandrowicz, 1994; Alonso et al., 2002; Comiti, 2003; Bollaert and Schleiss, 2003; Lenzi et al., 2003). Structural damage and morphologic changes from hydraulic jumps have also been documented at hydraulic structures (Peterka, 1983; ASCE, 1986; Hager, 1992; Fiorotto and Rinaldo, 1992; Vischer and Hager, 1998). While extensive research has characterized the kinematics of different jet and jump types in controlled settings such as laboratory flumes (Rajaratnam, 1967; Peterka, 1983; Hager, 1992; Vischer and Hager, 1998; Bombardelli et al., 2002), no previous geomorphic investigations have attempted to classify natural jump regions beyond the

B.L. Vallé, G.B. Pasternack / Geomorphology 82 (2006) 146–159

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Fig. 1. Classical hydraulic jump with partially developed inflow conditions. F1 = 13.6, V1 = 4.7 m/s, B = 0.25 m, h = 0.020 mm, d1 = 0.012 mm, Q = 14 L/s. Photo courtesy of Dr. Hubert Chanson.

classical hydraulic jump (Kieffer, 1985, 1987; Carling, 1995; Parker and Izumi, 2000). The most well understood hydraulic jump is the classical hydraulic jump (CHJ). The CHJ is a steady unsubmerged jump formed in a smooth, horizontal, and rectangular laboratory flume (Fig. 1). The CHJ is comprised of four principal regions: a horizontal supercritical jet, a rapidly varied aerated roller, a gradually varied outflow,...
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