Analysis and Classification of Semi-Arid Bioswales

Abstract

Green infrastructure (GI) is a tool used by cities to mitigate runoff and improve water quality. However there are a wide variety of design choices, found within dozens of guides, both official manuals and informal neighborhood workshops, leading to a lack of uniformity. This diversity is evident in Tucson with the recent adoption and installation of “bioswales”: vegetated basins intended to capture and clean street runoff during precipitation events. Due to a lack of comprehensive studies on basin performance, it is unclear if the hundreds of bioswales, representing thousands of dollars and many of hours of maintenance each year, are performing as their original designers envisioned. In order to comprehensively test bioswale performance, this study evaluated bioswales both qualitatively and quantitatively following a series of traditional soil tests. Based on current data there is significant variation in bioswale infiltration capacity despite relatively similar soil properties. The presence of vegetation and rocky additives may generally improve infiltration rates, but differing degrees of mulch decomposition can have mixed effects on overall hydraulic conductivity. The capture rate of runoff during storms also needs to be carefully monitored for long-term evaluation. When attempting to balance runoff mitigation with street aesthetics in the form of mature vegetation, there is no single “best” bioswale, though design standardization can lead to more efficient use of city and neighborhood resources.