TREE BOX PERFORMANCE IN EXFILTRATING STORMWATER RUNOFF

This study determines the exfiltration rates in six tree boxes and analyzes their performance over time. The study site is in Louisville, KY, where we monitored the performance of six tree boxes and other stormwater control measures (SCMs). Each tree box is 1.5 m wide, 1.5 m long, and 1.8 m deep. Street and parking lot runoff enters tree boxes though a curb cut. A 46-cm diameter shaft was drilled at the bottom of each tree box to reach the underlying permeable soil layer. A pressure transducer installed at the bottom of the shaft measured the water level depth and water temperature. From the water level data, the exfiltration rate of six tree boxes was calculated for 121 rain events. For each rain event the exfiltration rate was calculated at different water levels inside the shaft. The effects of water level inside the shaft, temperature, and age on the exfiltration rate were analyzed. Exfiltration rate increased with water level and exfiltration rate in first year is significantly larger than second year. Overall in second year the decrease in geometric mean exfiltration rate was largest for moderate depth. The exfiltration rate of the tree boxes is significantly larger for warmer rain events and significantly smaller for cooler rain events. This paper highlights the use of continuous two years’ water level monitoring data to quantify the potential for local recharge via exfiltration, and the effect of different parameter on the exfiltration rate of six tree boxes.