Impact of Variable Hydraulic Conductivity on Bioretention Cell Performance and Implications for Construction Standards

Bioretention cells have become an accepted technology for storm-water management due to their potential benefits in runoff volume reduction and water quality improvement. Over the years, a considerable amount of research has been done to improve performance, whether by improved functionality or by introducing engineered filter media to target specific contaminants. The design for a full-scale demonstration project carried out in Oklahoma incorporated a sand and fly ash filter for enhanced phosphorus adsorption and surface sand plugs for improved infiltration. Samples taken from the filter media during construction exhibited heterogeneity in fly ash content, and therefore, hydraulic conductivity due to the pozzolanic property of fly ash. Maximizing contaminant attenuation will require achieving uniform flow through the cell to ensure use of the entire filter volume. Thus, it is important to understand how the small-scale filter heterogeneity and the large-scale sand plugs impact performance of this design. To do this, a three-dimensional finite element model was developed in a software tool to simulate saturated flow through a bioretention cell. Three general configurations were modeled for three different scenarios. A filter-only configuration was evaluated to assess the effect of filter media hydraulic conductivity heterogeneity on flow through the cell. The second configuration added a top soil and sand plug layer with six sand plugs measuring 1.5 m by 1.5 m, which was similar to the constructed cells. The final configuration evaluated a top soil and sand plug layer with 14 smaller sand plugs only 1 m by 1 m. Three different scenarios were evaluated for each configuration that varied by size and distribution of the filter media heterogeneity. The first scenario used the measured scale and range variability, while the second used the same scale with double the variation, and the third used the same variability, but increased the scale volume by a factor of 27. Model results indicate that variability in fly ash content creates complex flow through the filter medium, but does not result in significant preferential flow. Sand plugs create some flow concentration but do not dominate flow within the cell, and the number of sand plugs is not significant provided that their total area is appropriate to maintain the desired drainage rate.