Techniques to assess rain gardens as stormwater best management practices

Rebecca Nestingen, Brooke C. Asleson, John S. Gulliver, Raymond M. Hozalski and John Nieber


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. 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. Monitoring programs are often used to evaluate the performance of stormwater BMPs such as rain gardens. Monitoring a large number of rain gardens, however, is impractical due to the time and cost requirements. It is of interest, therefore, to develop other techniques to determine the effectiveness of rain gardens. The primary process through which runoff volume is reduced in rain gardens is infiltration. Therefore, infiltration rate is a key assessment parameter for rain gardens. Two methods for determning 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: (1) a visual inspection, (2) capacity testing, (3) simulated runoff testing, and (4) monitoring. The first method for determining infiltration rates (level 2 assessment) involves using a permeameter or infiltrometer to make a number of point measurements throughout the rain garden and the second method (level 3 assessment) utilizes simulated runoff (i.e. a flood test) to determine an overall infiltration rate of the rain garden. Each of the infiltration rate measurement techniques was refined through numerous field tests performed over the course of a growing season for which the collected data has been analyzed and results are given within this paper. The results of our research and the widespread application of our testing methods for the periodic assessment of rain gardens will provide useful insight to proper design and maintenance schedules to achieve stormwater treatment goals.

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