Inverted versus Raised: The Impact of Bioretention Underdrain Height on Internal Water-Storage Hydraulics

Abstract

Bioretention design can include a saturated internal water storage (IWS) layer to promote denitrification. Typical practice connects an underdrain, at the IWS base, to an upturned elbow; alternatively, the underdrain can be located at the top of the IWS. We coupled laboratory column studies and transport modeling to evaluate hydraulic efficiency, ev (tracer residence time/theoretical residence time), as a function of underdrain height and IWS width to depth (w/d) ratio. Tracer tests revealed ev decreased from 1.0 to 0.76 as underdrain height increased from 0 to 30 cm due to the presence of immobile zones below raised underdrains. For IWS w/d ratios greater than 1, ev was less sensitive to underdrain location and ranged 0.89 to 0.96. For raised underdrains, higher hydraulic loading rate (HLR) reduced IWS immobile zone size (22%) and enhanced exchange between old and new water. Site-specific features (i.e., IWS geometry and HLR) influence optimal IWS underdrain design and transport modeling provides more accurate predictions of mean residence time compared to theoretical calculations.