Hydrologic Performance of Retrofit Rain Gardens in a Residential Neighborhood (Cleveland Ohio USA) with a Focus on Monitoring Methods
Citation:
Shuster, W. AND R. Darner. Hydrologic Performance of Retrofit Rain Gardens in a Residential Neighborhood (Cleveland Ohio USA) with a Focus on Monitoring Methods. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-18/191, 2018.
Impact/Purpose:
Green infrastructure refers to a range of urban stormwater management tools that can be flexibly implemented. These practices can aid in mitigating the negative impacts of runoff by increasing catchment detention capacity. We studied two engineered rain gardens (Cleveland OH) that were designed to infiltrate and detain direct runoff volume generated from an adjacent roadway, and sheet flow from pervious areas of each catchment area. We also accounted for hydrologic interactions between the engineered and upslope basic (non-engineered) rain gardens. A whole water-cycle monitoring approach was employed to fully assess the role of green infrastructure interventions on performance as inflows captured, duration of outflow drainage (i.e., excess moisture), hydrologic losses (e.g., evapotranspiration), and groundwater table dynamics. We found that these tandem rain gardens had good capacity for runoff inflow volumes over the course of over 100 storm events.The integration of green infrastructure in urban landscapes and long-term monitoring for effectiveness and its key functions produces novel data that can be used by researchers and other interested parties to conduct assessments of urban ecosystem functions and leverage these unique datasets by integrating with other datasets as per good scientific practice. We role model good monitoring practice, discuss unique ways to interpret challenging hydraulic circumstances, and conclude with a discussion of monitoring techniques that scale between the simple, passive and elegant; to full-blown research-grade monitoring infrastructure such as that employed in this study.
Description:
Green infrastructure refers to a range of urban stormwater management tools that can be flexibly implemented. These practices can aid in mitigating the negative impacts of runoff by increasing catchment detention capacity. We studied two engineered rain gardens (Cleveland OH) that were designed to infiltrate and detain direct runoff volume generated from an adjacent roadway, and sheet flow from pervious areas of each catchment area. We also accounted for hydrologic interactions between the engineered and upslope basic (non-engineered) rain gardens. A whole water-cycle monitoring approach was employed to fully assess the role of green infrastructure interventions on performance as inflows captured, duration of outflow drainage (i.e., excess moisture), hydrologic losses (e.g., evapotranspiration), and groundwater table dynamics. The 75th Street South rain garden, captured nearly 180,000 gallons of stormwater with a total duration of inflows of 308 hours. The duration of outflows in this 17-month period was 54 hours, indicating that drainage of excess moisture from the garden was a relatively small proportion (1/6) of the total inflow period. Additional evidence indicates that these outflows were shallow and never approach surcharging the outflow pipe. Overall, the 75th Street South rain garden effectively contributed sufficient detention capacity, as the garden design ponding depth of 0.75 ft was not exceeded for any of the monitored storm events. Post-event shallow ponding (max. 0.4 ft) – an indicator of rooting zone saturation – persisted for less than a day, and for only 13 out of 138 possible events. Analysis of groundwater level data showed that the upslope basic rain gardens interacted with the downslope 75th Street South rain garden, and that the nature of this interaction shifted with growing versus senescent seasons. The 75th Street North rain garden, which is downstream of the 75th Street South rain garden, came on line about 5 months after the 75th Street South rain garden. The 75th Street North rain garden collected a cumulative (12-month) total of more than 100,000 gallons of stormwater, over a period of 500 hours. There was ouflow from the 75th Street North rain garden for more than double (640 hours) the duration recorded for the 75th Street South rain garden. The 75th Street North rain garden outflow events typically had long recession times at very low flows. The 75th Street North rain garden was at the lowest point in a larger, vegetated catchment area, and experienced backflow (through the outflow pipe) from the combined sewer (CS) conveyance. These situational features enhanced subsurface runoff volume into the garden, and backflows from the CS contributed to an increased total duration of outflows. The comprehensive, full water-cycle monitoring approach ensured qualification of important hydrologic processes that contribute to the overall effectiveness of these rain garden technologies. Due to the small difference in the invert elevations of the garden overflow pipe and its connection to the CS conveyance, we found that sewer flow can and does backup into the monitored outfall. This malfunction highlights the importance of properly siting and plumbing the engineered rain garden system. We present an overview of rain garden monitoring practices and discuss the suitability and appropriateness of both passive low-cost, and research-grade monitoring strategies with regard to monitoring objectives; and provide example equipment-parts lists to aid in scaling level-of-effort and associated costs.