Techniques to Assess Rain Gardens as Stormwater Best Management Practices
Rain gardens are an aesthetically pleasing stormwater best management practice (BMP) that reduce runoff volume and remove stormwater pollutants through the processes of infiltration/filtration, adsorption, evapo-transpiration, 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 assessment program is aimed to assist municipalities in evaluating the effectiveness of BMPs for purposes of construction due diligence, NPDES permit requirements, and determining maintenance requirements.
The primary process through which runoff volume is reduced in rain gardens is infiltration of water through the soil. Thus, infiltration rate is a key assessment parameter for rain gardens. Two methods for determining the infiltration rates of rain gardens have been developed as part of a tiered four level assessment protocol:
1. visual inspection,
2. capacity testing,
3. synthetic runoff testing , and
Level 2: capacity testing and level 3: synthetic runoff testing will be used herein to determine infiltration rates in rain gardens which may be warranted after construction or when concerns arise from visual inspections.
Capacity testing for infiltration rates involves using an infiltrometer to make a number of point measurements throughout the rain garden. Synthetic runoff testing utilizes synthetic runoff (i.e. a flood test) to determine an overall infiltration rate of the rain garden rather than observation of natural runoff events for which the timing can be inconvenient. 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. 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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