Rain gardens have become a widespread stormwater practice in the United States, and their use is poised to continue expanding as they are an aesthetically pleasing way to improve the quality of stormwater runoff. The terms rain garden and bioretention, are now often used interchangeably to denote a landscape area that treats stormwater runoff. Rain gardens are an effective, attractive, and sustainable stormwater management solution for residential areas and urban green spaces. They can restore the hydrologic function of urban landscapes and capture stormwater runoff pollutants, such as phosphorus (P), a main pollutant in urban cities and residential neighborhoods. Although design considerations such as size, substrate depth, substrate type, and stormwater holding time have been rigorously tested, little research has been conducted on the living portion of rain gardens. This paper reviews two studies—one that evaluated the effects of flooding and drought tolerance on the physiological responses of native plant species recommended for use in rain gardens, and another that evaluated P removal in monoculture and polyculture rain garden plantings. In the second study, plants and substrate were evaluated for their ability to retain P, a typical water pollutant. Although plant growth across species was sometimes lower when exposed to repeated flooding, plant visual quality was generally not compromised. Although plant selection was limited to species native to the southeastern U.S., some findings may be translated regardless of region. Plant tissue P was higher than either leachate or substrate, indicating the critical role plants play in P accumulation and removal. Additionally, polyculture plantings had the lowest leachate P, suggesting a polyculture planting may be more effective in preventing excess P from entering waterways from bioretention gardens. The findings included that, although monoculture plantings are common in bioretention gardens, polyculture plantings can improve biodiversity, ecosystem resilience, and rain garden functionality.

Urbanization causes alteration of the thermal regime (surface, air, and water) of the environment. Heated stormwater runoff flows into lakes, streams, bays, and estuaries, which potentially increases the base temperature of the surface water. The amount of heat transferred, and the degree of thermal pollution is of great importance to the ecological integrity of receiving waters. This research reports on a controlled laboratory scale test to assess low impact development (LID) stormwater control measure impacts on the thermal characteristics of stormwater runoff. We hypothesize that LID stormwater control measures (SCMs) such as pervious surfaces and rain gardens/bioretention can be used to mitigate the ground level thermal loads from stormwater runoff. Laboratory methods in this study captured and infiltrated simulated stormwater runoff from four infrared heated substrate microcosms (pervious concrete, impervious concrete, permeable concrete pavers, and turf grass), and routed the stormwater through rain garden microcosms. A data logging system with thermistors located on, within, and at exits of the microcosms, recorded resulting stormwater temperature flux. Researchers compared steady state temperatures of the laboratory to previously collected field data and achieved between 30% to 60% higher steady state surface temperatures with indoor than outdoor test sites. This research helps establish baseline data to study heat removal effectiveness of pervious materials when used alone or in combination as a treatment train with other stormwater control measures such as rain gardens/bioretention.

The initial abstraction ratio (λ) used in the SCS-CN method plays an important role in calculation of runoff volume and consequently hydrograph peak. The recommended value for λ in the SCS handbook is 0.2. However, recent studies suggest that λ varies between 0.05 and 0.2, closer to the lower bound. Yet, the SCS recommended value of 0.2 is still being widely used in many hydrologic models. In this study, we explored the effects of variation in λ on simulating urbanization impacts in a coastal Alabama watershed with HEC-HMS. The potential impacts of future development scenarios on peak flows were scrutinized for λ values of 0.2 and 0.05. Results showed that the use of λ = 0.05 provides superior results when model generated hydrographs and peak flows were compared to observed counterparts.