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: |
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Relevant References: |
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.
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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.
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