Performance Assessment of Low Impact Stormwater Practices

In a time of continuing urban sprawl, using stormwater best management practices (BMPs) to effectively control and treat stormwater remains an important topic. Rain gardens are one such BMP that qualifies as a low impact development. Rain gardens are an aesthetically pleasing option that reduce runoff volume and remove stormwater pollutants through the processes of infiltration/filtration, adsorption, evapotranspiration, and plant uptake. However, before choices can be made, performance specifications and maintenance requirements need to be developed. Monitoring programs are often used to evaluate the performance of stormwater BMPs such as rain gardens. Monitoring a large number of low impact development (LID) practices, however, is impractical due to the time and cost requirements and the large uncertainty in the performance results. It is of interest, therefore, to develop other techniques to determine the effectiveness of these infiltration practices. Infiltration rate is a key assessment parameter for rain gardens. Three additional methods for determining the infiltration rates of rain gardens have been developed as part of a tiered four level assessment protocol. The four levels of assessment in the protocol include: a visual inspection, infiltration capacity testing, stimulated runoff testing, and monitoring. The results can provide useful insight to proper design, operation and maintenance schedules to achieve stormwater treatment goals.