Performance and Water Table Responses of Retrofit Rain Gardens

The alteration of natural flow regimes associated with land use change increases storm water runoff volume, increases peak flows, and reduces the time to peak flow. This can cause flooding, erosion, and higher pollutant loading in streams, rivers, lakes, and estuaries. Rain gardens represent a sustainable and economic method to decrease the volume of water that flows into rivers and streams from impervious areas during storm events. For developments that were built without permanent storm water controls, rain gardens may be a viable retrofit instead of centralized and more costly alternatives. However, there is a lack of knowledge about the performance of rain gardens in urban retrofit applications. In this rain garden study, the hydrologic performance of terraced, street-side rain gardens was examined by monitoring inflow and outflow volumes and water tables during simulated runoff events. Areas of uncertainty that were addressed include general relationships between inflow and hydrologic performance, and analyzing the behavior of the internal saturation zone. The performance variables quantified were runoff volume reduction, reduction in peak flow, and peak delay. For eight simulated runoff events of equivalent rainfall depths ranging from 0.1–1.7 cm, the street-side rain gardens reduced storm water volume by an overall total of 37% with mean individual simulation values for volume reduction, peak flow reduction, and peak delay of 52%, 62%, and 16 min, respectively. Regression relationships between equivalent rainfall depth and volume reduction were developed from the simulated runoff events for rain gardens of different surface area to catchment area ratios. From these relationships, it was conservatively predicted that the rain gardens retained the entire runoff volume for 26% of the 38 natural storm events monitored during the study. The results of this study suggest that rain gardens can benefit existing developments by reducing runoff volume and peak flow, and provide a dynamic internal saturation zone with the potential for water quality benefits. The findings also show the importance of understanding interactions with the in situ soil, the existing drainage system, and the entire catchment area when sizing rain gardens in urban retrofits to meet desired reduction objectives.