HESS Hydrology and Earth System Sciences HESS Hydrol. Earth Syst. Sci. 1607-7938 Copernicus Publications Göttingen, Germany 10.5194/hess-14-1321-2010 A flume experiment on the effect of constriction shape on the formation of forced pools Thompson D. M. 1 McCarrick C. R. 2 Department of Physics, Astronomy and Geophysics, Connecticut College, Campus Box 5585, 270 Mohegan Avenue, New London, Connecticut, 06320, USA Excel Environmental Resources, Inc., 111 North Center Drive, North Brunswick, NJ 08902, USA 20 07 2010 14 7 1321 1330 Copyright: © 2010 D. M. Thompson 2010 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://hess.copernicus.org/articles/14/1321/2010/hess-14-1321-2010.html The full text article is available as a PDF file from https://hess.copernicus.org/articles/14/1321/2010/hess-14-1321-2010.pdf

A series of 18 flume runs were conducted in a 6-m long, 0.5-m wide recirculating flume with a bed gradient of 0.8% to determine the influence of obstruction shape on the formation and characteristics of forced pools. Six different-shaped obstructions were added to the flume with the maximum width of the obstruction held constant at 20 cm, which equaled a 40% constriction of flow. The obstruction shapes used included a square, a rectangle, a right triangle with the hypotenuse-facing upstream, a right triangle with the hypotenuse-facing downstream, a combination of a square and triangle with the hypotenuse-facing upstream, and a rectangle and semi-circle shape. Three flume runs were conducted with each obstruction shape. A profile of the flume bed was taken after each experiment and a grid measurement of bed elevations for the last run were conducted to create topographic maps of the flume bed to compare pool-riffle morphologies. ANOVA results indicate pool depth, pool location, and the distance between the pool center and the riffle crest all vary with the obstruction shape. Obstructions with a more blunt upstream face created deeper pools, more total scour and longer pool-riffle sequence lengths than pools formed by obstructions with a more gradual narrowing of flow. The increased volume of scour associated with obstructions that rapidly narrow flow also creates larger volume riffles that cover a greater extent of the channel bed.

 References Buffington, J. M., Lisle, T. E., Woodsmith, R. D., and Hilton, S.: Controls on the size and occurrence of pools in coarse-grained forest rivers, River Res. App., 18, 507–531, 2002. Carling, P. A.: Hydrodynamic models of boulder berm deposition, Geomorphology, 2, 319–340, 1989. Clifford, N. J.: Differential bed sedimentology and the maintenance of a riffle-pool sequence, Catena, 20, 447–468, 1993. De Serres, B., Roy, A. G., Biron, P. M., and Best, J. L.: Three-dimensional structure of flow at a confluence of river channels with discordant beds, Geomorphology, 26, 313–335, 1999. Dolan, R., Howard, A., and Trimble D.: Structural control of the rapids and pools of the Colorado River in the Grand Canyon, Science, 202, 629–631, 1978. Harrison, L. R. and Keller, E. A.: Modeling forced pool-riffle hydraulics in a boulder-bed stream, southern California, Geomorphology, 83, 232–248, 2007. Hassan, M. A. and Woodsmith, R. D.: Bedload transport in an obstructed-formed pool in a forested gravel-bed stream, Geomorphology, 58, 203–221, 2004. Jia, Y., Kitamura, T., and Wang, S. S.: Simulation of scour process in plunging pool of loose bed-material, J. Hydraul. Eng., 127, 219–229, 2001. Kieffer, S. W.: The 1983 hydraulic jump in Crystal Rapid: Implications for river-running and geomorphic evolution in the Grand Canyon, J. Geol., 93, 385–406, 1985. Lisle, T. E.: Stabilization of a gravel channel by large streamside obstructions and bedrock bends, Jacoby Creek, northwest California, Geol. Soc. Am. Bull., 97, 999–1011, 1986. Lisle, T. E. and Hilton, S.: The volume of fine sediment in pools: an index of sediment supply in gravel-bed streams, Water Resour. Bull., 28, 371–383, 1992. Liu, P. Q., Dong, J. R., and Yu, C.: Experimental investigation of fluctuation uplift on rock blocks at the bottom of the scour pool downstream of Three-Gorges spillway, J. Hydraul. Res., 3, 55–68, 1998. MacVicar, B. J. and Roy, A. G.: Hydrodynamics of a forced riffle pool in a gravel bed river: 1. Mean velocity and turbulence intensity, Water Resour. Res., 43, W12401, https://doi.org/10.1029/2006WR005272, 2007a. MacVicar, B. J. and Roy, A. G.: Hydrodynamics of a forced riffle pool in a gravel bed river: 2. Scale and structure of coherent turbulent events, Water Resour. Res., 43, W12402, https://doi.org/10.1029/2006WR005274, 2007b. McLean, S. R., Nelson, J. M., and Wolfe, S. R.: Turbulence structure over two-dimensional bed forms: Implications for sediment transport, J. Geophys. Res., 99, 12729–12747, 1994. MacWilliams Jr., M. L.,, Wheaton, J. M., Pasternack, G. B., Street, R. L., and Kitanidis, P. K.: Flow convergence routing hypothesis for pool-riffle maintenance in alluvial rivers, Water Resour. Res., 42, W10427, https://doi.org/10.1029/2005WR004391, 2006. Milan, D. J., Heritage, G. L., Large, A. R. G., and Charlton, M. E.: Stage dependent variability in tractive force distribution through a riffle pool sequence, Catena, 44, 85–109, 2001. Miller, A. J.: Debris-fan constrictions and flood hydraulics in river canyons: Some implications from two-dimensional flow modeling, Earth Surf. Process. Landforms, 19, 681–697, 1994. Montgomery, D. R., Buffington, J. M., Smith, R. D., Schmidt, K. M., and Pess, G.: Pool spacing in forest channels, Water Resour. Res., 31, 1097–1105, 1995. Nelson, J. M., McLean, S. R., and Wolfe, S. R.: Mean flow and turbulence fields over two-dimensional bed forms, Water Resour. Res., 29, 3935–3953, 1993. Petit, F.: The relationship between shear stress and the shaping of the bed of a pebble-loaded river, La Rulles-Ardenne, Catena, 14, 453–468, 1987. Roy, A. G., Biron, P. M., Buffin-Bélanger, T., and Levasseur, M.: Combined visual and quantitative techniques in the study of natural turbulent flows, Water Resour. Res., 35, 871–877, 1999. Schmidt, J. C., Rubin, D. M., and Ikeda, H.: Flume simulation of recirculating flow and sedimentation, Water Resour. Res., 29, 2925–2939, 1993. Thompson, D. M.: Random controls on semi-rhythmic spacing of pools and riffles in constriction-dominated rivers, Earth Surf. Process. Landforms, 26, 1195–1212, 2001. Thompson, D. M.: Channel-bed scour with high versus low deflectors. American Society of Civil Engineers, J. Hydraul. Eng., 128, 640–643, 2002. Thompson, D. M.: The influence of pool length on local turbulence production and energy slope: A flume experiment, Earth Surf. Process. Landforms, 29, 1341–1358, 2004. Thompson, D. M.: The role of vortex shedding in the scour of pools, Adv. Water Resourc., 29, 121–129, 2006. Thompson, D. M.: Turbulence characteristics in a shear zone downstream of a channel constriction in a coarse-grained pool, Geomorphology, 83, 199–214, 2007. Thompson, D. M. and Hoffman, K. S.: Equilibrium pool dimensions and sediment-sorting patterns in coarse-grained, New England channels, Geomorphology, 38, 301–316, 2001. Thompson, D. M., Nelson, J. M., and Wohl, E. E.: Interactions between pool geometry and hydraulics, Water Resour. Res., 34, 3673–3681, 1998. Thompson, D. M., Wohl, E. E., and Jarrett, R. D.: Pool sediment sorting processes and the velocity-reversal hypothesis, Geomorphology, 17, 142–156, 1999. Wohl, E. E. and Thompson, D. M.: Velocity fluctuations along a small step-pool channel, Earth Surf. Process. Landforms, 25, 353–367, 2000. Wohl, E. E. and Legleiter, C.: Controls on pool characteristics along a resistant-boundary channel, J. Geol., 111, 103–114, 2003.