Novel Use of Time Domain Reflectometry in Infiltration-Based Low Impact Development Practices

Abstract

Low impact development (LID) practices intercept storm-water runoff and infiltrate it through a range of media types and underlying soils. Hydrologic performance is typically evaluated by comparing inlet and underdrain outlet flows, but there is no standard practice for defining and measuring performance in LID structures designed without underdrains that infiltrate into the ground. Water content reflectometer (WCR) sensors were installed in the aggregate storage layer under permeable pavement and rain garden media to test their ability to measure the size and timing of the wetting front in infiltrating LID practices. WCR data were also used to monitor infiltration rates in the underlying soil beneath both practices. Bench-scale testing was performed to quantify the response of WCRs to saturated and unsaturated conditions and calibrate sensors to a range of water content values. Bench-scale testing revealed that WCRs installed in the aggregate were calibrated to volumetric water content (VWC) as the aggregate approached saturation. At partial saturation levels, however, mixing models did not accurately predict VWC; apparent permittivity is therefore the appropriate response metric to use under unsaturated flow conditions in the aggregate. WCR responses to a wide range of storm characteristics during the first 6 months of parking-lot and rain-garden monitoring are presented. These monitoring data demonstrated the effects of parking surface and rain-garden cell size on the maximum magnitude of WCR response, time lag from the onset of rain to WCR response, and degree of return to antecedent water content conditions following the end of the storm event. Spatial and temporal variability outweighed any effects of parking surface or cell size on wetting front rates in underlying soil. Methods to protect WCRs placed in the aggregate layer from construction activities of the permeable parking lot are also discussed.