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Pool-Riffle Maintenance Mechanisms

The persistence of pool-riffle sequences as stable morphologies in alluvial rivers is counter intuitive when one considers that the highest velocities (and presumably most erosive energy) are usually over riffles and the slowest velocities are often found in nearby pools (Clifford and Richards, 1992). Yet pools appear to be preserved by scour and riffles by deposition.

Geomorphic investigators have proposed a range of working hypotheses to explain the maintenance of pool-riffle sequences (Sear 1996). Keller’s (1965; 1971) velocity reversal hypothesis and its offshoots (e.g. entrainment reversal, shear stress reversal) remain the most debated of these.

Others have proposed more detailed mechanisms such as width constrictions at pool heads giving rise to secondary flow structures (Thompson 1999 & 2001), sediment routing around pools (Booker et al. 2003), turbulent bursts evacuating fines from riffles at low flows (Sear 1996) and many variations. A number of one-dimensional hydraulic simulations have been undertaken in pool-riffle sequences to test for velocity-reversals (e.g. Carling and Wood 1994; Keller and Florshiem 1993). More recently, three-dimensional CFD simulations have been used to compare the evidence for velocity-reversals in comparison to some of the more detailed mechanisms mentioned above (e.g. Cao et al. 2003; Booker et al. 2003; MacWilliams 2004).

In collaboration with Micahel MacWilliams and Greg Pasternack, we undertook a side-by-side comparison of a 2D and 3D CFD model in Keller’s (1965) Dry Creek. The study site was the orgin of the velocity-reversal hypothesis and we thought it would be interesting to explore the ability of both CFD codes to a) reproduce Keller’s observed velocity reversal and b) make mechanistic inferences about processes responsible for maintaining pool-riffle sequences. The findings and our suggested flow convergence-routing hypothesis is reported in an article in Water Resources Research (MacWilliams, et al. 2006).

Case Studies:

• Dry Creek, Winters, California, USA
• Mokelumne River, California USA
• Highland Water, New Forest, Hampshire, England.

Outputs from this research:

MacWilliams, M. L. Jar., Wheaton, J.M., Pasternack, G.B., Street, R. L. and Kitanidis, P. K. , 2006. Flow convergence and routing hypothesis for pool-riffle maintenance in alluvial rivers, Water Resources Research, 42, W10427.

Cited References:

• Booker, D.J., Sear, D.A. and Payne, A.J., 2001. Modeling three-dimensional flow structures and patterns of boundary shear stress in a natural pool-riffle sequence. Earth Surface Processes and Landforms, 26(5): 553-576.
• Cao, Z., Carling, P. and Oakey, R., 2003. Flow Reversal Over a Natural Pool-Riffle Sequence: A Computational Study. Earth Surface Processes and Landforms, 28: 689-705.
• Carling, P.A. and Wood, N., 1994. Simulation of Flow over Pool-Riffle Topography - a Consideration of the Velocity Reversal Hypothesis. Earth Surface Processes and Landforms, 19(4): 319-332.
• Clifford, N.J. and Richards, K., 1992. 2: The Reversal Hypothesis and the Maintenance of Riffle-Pool Sequences: A Review and Field Appraisal. In: P. Carling and G. Petts (Editors), Lowland Floodplain Rivers: Geomorphological Perspectives. John Wiley and Sons Ltd, Chichester, U.K., pp. 43-70.
• Keller, E.A. and Florsheim, J.L., 1993. Velocity-Reversal Hypothesis: A Model Approach. Earth Surface Processes and Landforms, 18: 733-740.
• Keller, E.A., 1971. Areal sorting of bed-load material: the hypothesis of velocity reversal. Geological Society of America Bulletin, 82: 753-756.
• Keller, E.A., 1965. Form and Fluvial Processes of Dry Creek, Near Winters, California. Masters Thesis Thesis, University of California at Davis, Davis, CA, 73 pp.
• MacWilliams, M., 2004. (Chapter Four): Numerical Evaluation of the Velocity-Reversal Hypothesis. PhD Dissertation Thesis, Stanford University, Palo Alto, CA.
• Sear, D.A., 1996. Sediment transport processes in pool-riffle sequences. Earth Surface Processes and Landforms, 21(3): 241-262.
• Feeling Lazy? If you don’t feel like pulling the references cited here but need a quick citation you can can just cite this page (see example). Although, non-peer reviewed website citations aren’t nearly as credible or informative as the original source material.