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Novel Use of Time Domain Reflectometry in Infiltration-based Low Impact Development Practices
STANDER, E. K., A. A. ROWE, M. BORST, AND T. P. OCONNOR. Novel Use of Time Domain Reflectometry in Infiltration-based Low Impact Development Practices . Journal of Irrigation and Drainage Engineering. American Society of Civil Engineers (ASCE), Reston, VA, 139(8):625-634, (2013).
To inform the public.
Low impact development (LID) practices are structures that intercept stormwater runoff and infiltrate it through a range of media types, including aggregate, rain garden media, 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 to native soil. Water content reflectometer (WCR) sensors were installed in the aggregate storage layer under permeable pavement, rain garden media, and underlying soil to test their ability to measure the size and timing of the wetting front in infiltrating LID practices. WCR data was also used to calculate infiltration rates in the underlying soil beneath both practices. Bench-scale testing was performed to quantify the response of WCRs to saturated and unsaturated flows, calibrate sensors to a range of water content values, and test the use of PVC as a protective housing for WCRs installed in large diameter aggregate. WCR responses to a wide range of storm characteristics during the first six months of parking lot and rain garden use 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 infiltration rates in underlying soil. Bench-scale testing revealed that WCRs installed in aggregate were calibrated to volumetric water content as the aggregate approached saturation. At partial saturation levels, however, mixing models did not accurately predict volumetric water content; apparent permittivity is therefore the appropriate response metric to use under unsaturated flow conditions. WCRs installed in protective PVC housing quantified drainage patterns under saturated flow conditions but failed to detect the passage of the wetting front during unsaturated flows. Installation of the WCRs in a protective bedding of smaller aggregate within the larger aggregate storage layer ensured the continued functionality of the sensors during parking lot use.