Incorrect Sizing Calculation Methods for Bioretention Cells

Water quality control bioretention cells are usually sized so that they are large enough to provide storage of runoff generated from its catchment resulting from the water quality control design storm of the location of interest. A bioretention cell can have three parts where runoff may be temporarily stored: (1) the depressed surface ponding area, (2) the growing media layer, and (3) the storage layer. In this study, the detailed sizing calculation methods adopted by 21 jurisdictions in the US and Canada were reviewed and compared. It was found that for satisfying the required storage volume to store runoff generated from the water quality control design storm, some jurisdictions only allow the storage provided by the surface ponding area to be counted, whereas other jurisdictions allow the sum of the storages provided by two of the three or all of the three parts to be counted. These differences can result in significant differences in the sizes and performances of bioretention cells. By examining and analyzing the detailed hydrologic processes involved in the operation of bioretention cells, it was demonstrated in this study that many of the adopted sizing calculation methods are flawed or inappropriate. Presented here are a summary of the findings and the recommended sizing calculation method. The possible degrees of over- or underdesigns resulting from some of the incorrect sizing calculation methods are also estimated.