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Tuesday, May 27, 2008 - 10:00 AM
139
Does longitudinal dispersion influence the measurement of uptake length? It depends on the Péclet number
J. Denis Newbold, Stroud Water Research Center, 970 Spencer Road, Avondale, PA 19311
Runkel (2007, L&O Methods) showed that longitudinal dispersion can affect the measured uptake length, SW, of solutes, and so may compromise the utility of SW in estimating uptake mass-transfer coefficients. I applied Runkel’s analysis to 57 measurements of SW in 17 streams in southeastern New York State encompassing flows of 0.02 to 7.7 m3/s. In 70% of the cases and for all flows >1 m3/s, the dispersion-induced error was <5%. In smaller streams the median error was 4%, but ranged up to 16%. Among all streams, the error was well described as a decreasing function of the dimensionless Péclet number (Pe), given by v SW /D, in which v is water velocity, and D is longitudinal dispersion. The error exceeded 4% for Pe <25 and was <4% for Pe>25. Combining predictive formulae for SW and D suggests that Pe varies as v d2/w, in which d is depth and w is width. Thus Pe generally increases with flow but, for a given flow, wider, shallower channels should be more error prone. These results suggest that dispersion is rarely a serious problem, particularly when compared to the much larger errors that arise from the saturating effects of experimental nutrient additions.
Web Page: spiraling, nutrient uptake, longitudinal dispersion
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See more of The Nabs 56th Annual Meeting (25-28 May, 2008)