Hydrologic Impact of an Internal Water Storage (IWS) Layer on Bioretention Performance

Abstract

The bioretention drainage configuration of raising the outlet to force an internal water storage (IWS) layer in the media was originally intended to promote denitrifying conditions, so that nitrate and total nitrogen effluent concentrations could be reduced. Several field studies in regions of North Carolina with tighter underlying soils have shown that this design feature has also enhanced infiltration and reduced outflow from bioretention. Two bioretention cells in Rocky Mount, NC, were monitored for two, year-long periods, to measure the impact of varying IWS zone depths over sandier underlying soils. Nearly 99% of the runoff entering the bioretention cell with sand underlying soil (Mulch/Shrub cell) was not released as outflow. The other bioretention cell had a sandy-loam underlying soil (Grass cell), and it was able to completely capture (not release as outflow) 87% of runoff during the monitoring period with a 0.87 m IWS zone depth and 72% when the IWS zone depth was reduced to 0.56 m.