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Application of Time Domain Reflectometers in Urban Settings
OCONNOR, T., A. ROWE, E. STANDER, AND M. BORST. Application of Time Domain Reflectometers in Urban Settings. Cities and the Environment. Urban Ecology Institute, Chestnut Hill, MA, 3(1):19, (2010).
To inform the public.
Time domain reflectometers (TDRs) are sensors that measure the volumetric water content of soils and porous media. The sensors consist of stainless steel rods connected to a circuit board in an epoxy housing. An electromagnetic pulse is propagated along the rods. The time, or period, required for the signal to travel down the rods and back varies with the volumetric water content of the surrounding media and temperature. A calibration curve is needed for the specific media. TDRs were developed mostly for agricultural applications; however, the technology has also been applied to forestry and ecological research. This study demonstrates the use of TDRs for quantifying drainage properties in low impact development (LID) stormwater controls, specifically permeable pavement and rain garden systems. TDRs were successfully used to monitor the responses of urban fill, engineered bioretention media, and the aggregate storage layer under permeable pavement to multiple rain events of varying depth, intensity, and duration. The hydrologic performance of permeable pavement and rain garden systems has previously been quantified for underdrain systems, but there have been few studies of systems that drain to the underlying soils. We know of no published studies outlining the use of TDR technology to document drainage properties in media other than soil. In this study TDRs were installed at multiple locations and depths in underlying urban fill soils, engineered bioretention media, and recycled concrete aggregate (RCA) in a permeable pavement parking lot and associated series of rain gardens at EPA’s Edison Environmental Center. Bench- and pilot-scale tests were performed before permanent installation to test the sensors’ ability to detect the wetting front during both saturated and unsaturated flow conditions.
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