Understanding Urban Rain Garden Performance through Observations and Modeling.

Many municipalities and agencies are exploring the use of green infrastructure to combat pollution and other hydrological impacts (e.g., groundwater recharge, flooding, superfluous treatment costs) related to excess stormwater. Rain gardens, where stormwater is collected and then allowed to infiltrate through soils and/or other porous media, are currently being utilized as a potentially low cost/low maintenance means to manage stormwater. However, the effects of rain garden installations on local groundwater levels and on stormwater return flows are not yet well-understood. In this study, we make use of high-resolution field measurements of soil physical data (e.g., hydraulic conductivity, drainage rates, and texture) and hydrological data (e.g., precipitation and potential evapotranspiration, inflow rates, groundwater levels, and soil water content) to quantify and evaluate the effect of two rain gardens installed in the city of Cleveland, Ohio. The rain gardens were installed in the fall of 2014 and were built with three substrate layers: 1) a biosoil, 2) a graded sand, and 3) an aggregate base, with a subsurface drain pipe installed at the base of each. Observational data collected at the site suggests that, due to soil degradation within the rain garden itself, preferential flowpaths may allow stormwater to bypass much of the rain garden. Therefore, to study the effects of 1) soil properties on rain garden performance and 2) this type of green infrastructure on urban hydrology (i.e., the movement of water through a variably-saturated subsurface zone), we constructed a set of calibrated models using HYDRUS-2D/3D. The modeling results show that the rain gardens may cause localized groundwater mounding; the extent of this mounding is moderated by the relatively low permeability of the native soil and by the presence of the subsurface drain pipes. When the model is run without the subdrains, rain garden infiltration rates become more sensitive to the saturated hydraulic conductivity and pore size distribution of the surrounding soils. Results of this study indicate the importance of green infrastructure design, soil properties, and water table dynamics on effectiveness of green infrastructure at managing urban stormwater flow.

The views expressed in this presentation are those of the author[s] and do not necessarily represent the views or policies of the U.S. Environmental Protection Agency or the U.S. Geological Survey.