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Impact of Hydraulic Conductivity Variability in Bioretention Cells

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  2010 Pittsburgh, Pennsylvania, June 20 - June 23, 2010  1008678.(doi:10.13031/2013.29697)
Authors:   Glenn O Brown, Rebecca A Chavez, Daniel E Storm
Keywords:   Key words: bioretention cell, variability, modeling

Bioretention Cells are an established and growing Best Management Practice for stormwater quality treatment. Cells work by filtering and temporarily storing stormwater. The cell filter media provides physical filtration, chemical adsorption and a platform for plant biological uptake or microbial degradation of some contaminates. Treatment performance will be in large a function of the movement of water and the uniformity of contaminant transport through the cell. Ten demonstration cells have recently been constructed in Oklahoma that have a sand filter media amended with fly ash to improve phosphorus adsorption. Variability of the sand-fly ash mix produced small scale variations in the saturated hydraulic conductivity with a log-normal distribution that ranged over two orders of magnitude with 100 g samples. Preliminary hydraulic testing has suggested that flow variability through the cells may be significant. The impact of hydraulic conductivity variability is investigated with a 3-D, finite element model. A 84 m3 cell was modeled with more than 400,000 elements that had randomly assigned conductivity following the normal distribution of transformed data. Results indicate that flow through the cell is impacted by the variability, but is greatly reduced by the small scale of the variations. Short-circuiting of the filter media is only significant if larger scale variability is present.

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