The development and application of a four-level rain garden assessment methodology

Rain gardens are a commonly used stormwater best management practice (BMP) in urban areas that reduce stormwater runoff volume via infiltration and evapotranspiration and remove pollutants via filtration, sorption, microbial degradation and other processes. In response to the implementation of phase II of the National Pollutant Discharge Elimination System (NPDES) program, small municipal separate storm sewer systems (MS4s) are required to evaluate the effectiveness of their stormwater BMPs. Currently monitoring is the most widely used approach for evaluating the performance of stormwater BMPs, but this approach has many limitations including excessive time, effort, and cost. For the research reported in this manuscript a tiered four level assessment approach was developed consisting of visual inspection, capacity testing, synthetic runoff testing, and monitoring. Several rain gardens were evaluated using this approach. Of the rain gardens assessed eight were functioning properly based on the visual inspection (level 1) and the saturated hydraulic conductivity (Ksat) was then measured at several locations throughout the basin. The median values of Ksat for each rain garden ranged from 0.0196 to 0.0008 cm/s. This resulted in drain times well below the required 48 hours. The drain time determined by the synthetic runoff tests (level 3) corresponded well with the estimates based on the point measurements, with the exception of one rain garden which contained a restrictive soil layer beneath the topsoil. The results demonstrate that this assessment approach provides effective tools for determining if a rain garden is functioning properly and optimally, for developing maintenance tasks and schedules, and for ensuring due diligence during construction.
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Minnesota Water Research Digital Library
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