Pool Effects on Longitudinal Dispersion in Streams and Rivers

Wei Zhang, Michel C. Boufadel

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DOI: 10.4236/jwarp.2010.211114                

Abstract

Surface storage (pools, pockets, and stagnant areas caused by woody debris, bars etc) is very important to solute transport in streams as it attenuates the peak of a spill but releases the solute back to the stream over a long time. The latter results in long exposure time of biota. Pools as fundamental stream morphology unit are commonly found in streams with mixed bed materials in pool-riffle or pool-step sequences. Fitting the transient storage model (TSM) to stream tracer test data may be problematic when pools present. A fully hydrodynamic 2-D, depth averaged advection-dispersion solute transport numerical simulation study on hypothetical stream with pool reveals that a pool can sharply enhance longitudinal spreading, cause a lag in the plume travel-time and radically increase solute residence time in the stream. These effects fade like a “wake” as the solute plume moves downstream of the pool. Further, these effects are strongly influenced by a dimensionless number derived from the 2-D transport equation ? or , which outlines the relative transverse mixing intensity of a stream or river, where, of the stream reach concerned, W is the flow width, Q0 is the volumetric flow rate, q is the longitudinal flux density, and Dt is the transverse turbulent diffusion coefficient. The breakthrough curves (BTCs) downstream of a pool may be “heavy tailed” which cannot be modeled accurately by the TSM. The internal transport and mixing condition (including the secondary circulations) in a pool together with the pool’s dimension determine the pool’s storage effects especially when >> 1. Results also suggest that the falling limb of a BTC more accurately characterizes the pool's storage because the corresponding solute has more chance to sample the entire storage area.

Keywords

Pool Effects, Solute Transport, Longitudinal Dispersion, Transient Storage, Open Channel

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	S. H. Keefe, L. B. Barber, R. L. Runkel, J. N. Ryan, D. M. McKnight, and R. D. Wass, “Conservative and reactive solute transport in constructed wetlands,” Water Resources Research, Vol. 40, W01201, 2004, pp. 12.
[2]	J. G. M. Derksen, G. B. J. Rijs and R. H. Jongbloed, “Diffuse Pollution of Surface Water by Pharmaceutical Products,” Water Science and Technology, Vol. 49, No. 3, pp. 213-221, 2004.
[3]	M. Velicu and R. Suri, “Presence of steroid hormones and antibiotics in surface water of agricultural, suburban and mixed-use areas”, Environ Monit Assess, Vol. 154, 2009, pp. 349-359.
[4]	H. B. Fisher, E J. List, R. C.Y. Koh, J. Imberger and N.H. Brooks, “Mixing in inland and coastal waters,” Academic Press, 1979.
[5]	J. R. Webster and T. P. Ehrman, “Solute Dynamics,” In: F. R. Hauer and G. A. Lamberti, ed., Methods in Stream Ecology, Academic Press, Inc., San Diego, Calif., 1996, pp. 145 -160.
[6]	R. Runkel, D. M. McKnight and H. Rajaram, “Modeling hyporheic zone processes”, Advance Water Resources, Vol. 26, No. 9, 2003, pp. 901-905.
[7]	G.I. Taylor, “The dispersion of matter in turbulent flow through a pipe,” In: Proceedings of the Royal Society London Series A, Vol. 123, 1954, pp. 446-468.
[8]	H.B. Fischer, “The Mechanics of Dispersion in Natural Streams,” Journal Hydraulics Division Proceedings, ASCE, Vol. 93, No. 6, 1967, pp.187-216.
[9]	H.B. Fischer, “Dispersion Predictions in Natural Streams,” Journal Sanitary Engineering Division, ASCE, Vol. 94, No. SA5, 1968, pp. 927-943.
[10]	E. L Thackston, and K. B Schnelle, “Predicting effects of dead zones on stream mixing”. Journal of the Sanitary Engineering Division. Proceeding of The American Society of Civil Engineering, Vol. 96, No. 2, 1970, pp. 319- 331.
[11]	K. E. Bencala and R. A. Walters, “Simulation of solute transport in a mountain pool and riffle stream: a transient storage model,” Water Resources Research, Vol. 19, No. 3, 1983, pp. 718-724.
[12]	C. F. Nordin and B. M. Troutman, 1980. “Longitudinal dispersion in rivers: the persistence of skewness in observed data,” Water Resources Research, Vol. 16, No. 1, 1980, pp.123-128.
[13]	E. M. Valentine, and I. R. Wood, “Longitudinal dispersion with dead zones”. Journal of the Hydraulics Division, Vol.103, No. HY9, 1977, pp. 975-991.
[14]	R. L. Runkel, “One-dimensional transport with inflow and storage (OTIS): a solute transport model for streams and rivers (WRIR 98-4018),” US Geological Survey, Denver, CO 73 pp. 1998.
[15]	Y.Ge and M. Boufadel, “Solute transport in multiple-reach experiments: Evaluation of parameters and reliability of prediction,” Journal of Hydrology, Vol. 323, No. 1-4, May 2006, pp. 106-119.
[16]	M. Gabriel, “The role of geomorphology in the transport of conservative solutes in streams. M.S. thesis, Temple University, Philadelphia, 2001.
[17]	P. A. Carling and H.G. Orr, “Morphology of riffle-pool sequences in the river severn,” England. Earth Surface Processes and Landforms, Vol. 25, No. 4, 2000, pp. 369- 384.
[18]	A.Chin, “The periodic nature of step-pool mountain streams,” American Journal of Science, Vol. 302, February 2002. pp. 144-167.
[19]	DHI Water and Environment, 2003. MIKE 21- Coastal Hydraulic and Oceanography(Hydrodynamic Module: Scientific Documentation; Advection-Dispersion Module: Scientific Documentation).
[20]	J. C. Rutherford, “River mixing,” John Wiley and Sons, New York, 1994.