Citation:

Alikhani, J., C. Nietch, S. Jacobs, W. Shuster, AND A. Massoudieh. Modeling and Design Scenario Analysis of Long-Term Monitored Bioretention System for Rainfall-Runoff Reduction to Combined Sewer in Cincinnati, OH. Journal of Sustainable Water in the Built Environment. American Society of Civil Engineers (ASCE), New York, NY, 6(2):04019016-1, (2020). https://doi.org/10.1061/JSWBAY.0000903

Impact/Purpose:

In this study, The Green Infrastructure Flexible Model (GIFMod) was used to determine the long-term performance of a dual rain gardens system (i.e., two separate bioretention facilities constructed in-series) that has been extensively monitored over a period of more than three years. The environmental data, including solar radiation, wind speed, relative humidity, ambient temperature, and precipitation collected at high-frequency was used for the model input time series. There was comprehensive hydrologic as well as water quality (namely, nutrient species) data. The model consists of the contributing catchment area and the rain gardens. Different layers within the rain gardens, as well as the native soil and recharge into the groundwater were modeled explicitly. Flow and water quality in the influent and effluent of the rain garden system were used to calibrate and validate the model. The main objective of the study was to use the model to evaluate the relative importance of design factors dictating the post-construction effectiveness of the rain gardens system for mitigating stormwater runoff to a combined sewer.

Description:

Green infrastructure is a contemporary approach to management of stormwater and limit its intrusion into wastewater collection and conveyance systems. Green infrastructure (GI) practices, such as rain gardens, effect the local water cycle by encouraging detention, infiltration and evapotranspiration of intercepted runoff. Therefore, for a complete understanding of GI effects a whole water-cycle monitoring and simulation approach is needed. The Green Infrastructure Flexible Modeling (GIFMod) package allows users to consider GI physio-chemical processes of subsurface flow, infiltration, evapotranspiration in the catchment’s hillslope and recharge to the groundwater table, evapotranspiration, and dominant biogeochemical transformations. In this study GIFMod was used to simulate hydraulic and water quality processes in a GI practice consisting of two rain gardens, built in series, and that drain a small catchment (3.41 ac) consisting of a parking lot (0.41 ac) and wooded hillslope (~3 ac). The goal was to evaluate the long-term effectiveness of the system at reducing hydrologic and nutrient loads to a combined sewer. Observed flow and water quality data was used along with the built-in GIFMod algorithms hybrid genetic and Markov Chain Monte-Carlo to calibrate the model. Then the model was used to determine cumulative load reductions afforded by the GI system over a 3 yr simulation. Next, the model was used to evaluate the degree to which different design scenarios change GI effectiveness, These included considering effluent riser height, depth of the aggregate layer, and expansion of the contributing catchment area. The results show that hydraulic retention time regulates the effectiveness of the rain garden system for flow reduction and retention of nutrients, which are both relevant to combined sewer operational capacity. Based on the current design and drainage configuration, the rain gardens system reduced about 50% of the runoff volume from the site. It is more effective in the retention of loads from small to medium rain events than large ones. The engineered soils of the rain gardens, which are maintained for landscaping aesthetics, prove to be a net source of nitrate release from the site. Given the ornamental garden-like features favored for these systems this challenges future designs intent on nutrient reduction. The results of the multiple design scenarios indicated that adding a riser-type outlet and increasing the storage volume of the aggregate layer would result in moderate improvement to reduction efficiency, while increasing the catchment area would significantly reduce its effectiveness. Of these, adding a riser feature constitutes a cost-effective design feature that in this case was easily implemented post construction, and, therefore, would be recommended for similar rain garden installations.

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