Vertical and Lateral Extent of the Influence of a Rain Garden on the Water Table

Abstract

Rain gardens use natural processes, such as infiltration and evapotranspiration, to control runoff from rain events. Although prior research has shown that rain gardens are effective at controlling the volume of runoff and pollutants associated with runoff, the extent of groundwater mounding under rain gardens remains a concern for landowners, practitioners, and regulators. A rain garden on Villanova University’s campus was instrumented with monitoring wells to observe the influence of the rain garden on the water table. This rain garden receives the runoff from approximately 0.52 ha that consists of grassy areas as well as parking lots. This paper investigates vertical and lateral behavior of localized mounding caused by infiltration in a rain garden subsequent to a storm event. The data used for the analysis were selected from 1.5 years of continuous monitoring at the site. A statistical model was developed, describing event-based peak groundwater rise as a function of radial distance from the rain garden’s center and infiltrated water depth. Statistical analysis was performed to demonstrate significance of the model parameters. Analyses of time to peak and time to return to prestorm levels (dissipate) were performed. The system exhibited a very pronounced dissipation behavior in peak rise, normalized to infiltrated depth of water, with an increase in radial distance. Temporal lag in mounding peak and dissipation was investigated with respect to radial distance. In light of the observed behavior, a methodology for estimating site-specific vertical and lateral impacts was proposed. The methodology was applied to the rain garden, and the results reviewed in the context of geospecific design storms. The paper concludes with a discussion on the expected effects of localized mounding from the rain garden on foundations of adjacent buildings.