Estimation of Rain Garden Field Hydraulic Conductivity Based on Spot Infiltration Tests

Rain gardens are increasingly being used to control stormwater. Infiltration is a key component of volume control. Thus, determining the infiltration rate or field saturated hydraulic conductivity (Ksat) of rain gardens is critical to their continued successful operation. Designers and inspectors of rain gardens need to rapidly and efficiently determine the field Ksat. Prior research has found that single-ring infiltrometers (Princeton Hydro, Trenton, NJ, USA) can reliably be used to determine the infiltration rates of soils. The question often posed by designers and inspectors is "how many spot-infiltration tests are needed to sufficiently characterize the infiltration capacity of a rain garden?" Five rain gardens, varying in size from 62 to 429 m2, were analyzed for this study. Three different spot infiltration methods were used: single-ring (Princeton Hydro, Trenton, NJ, USA) (least sophisticated and expensive), modified Philip-Dunne (Villanova University, Villanova, PA, USA), and SATURO (METER Group, Pullman, WA, USA) (most sophisticated and expensive). These rain gardens also had been instrumented to capture the recession rates during either natural or artificial ponding events. The linear portion of the recession curve obtained during ponding events was used to provide the rain-garden-wide Ksat. It was found that the geometric mean of six spot infiltration tests provided a reliable Ksat value similar to that found by the recession rate, which best represents the value of Ksat for the entire rain garden. This indicates that an inspector can reliably determine the infiltration capacity of a rain garden in less than a day.