Hydrologic Evaluation of Established Rain Gardens in Lincoln, Nebraska Using a Storm Runoff Simulator

Increased urbanization has resulted in water quality and flooding problems for many receiving waters in the United States. Bioretention, or rain gardens, is one of the most widely popular and effective best management practices in low impact development (LID), which strives to return a watershed to a predevelopment hydrologic regime. Many studies have evaluated large bioretention cells in research settings. There is little information on the effectiveness of homeowner-maintained rain gardens that rely on deep percolation as the method for water exfiltration. Additionally, few studies address rain garden performance in cold, arid, or semi-arid climates found in the Midwest U.S. The objective of this project was to evaluate the hydrologic properties of twelve established residential rain gardens using a stormwater runoff simulator. A volume-based design storm of 1.19 inches (90% Water Quality Volume) was applied as a synthetic SCS-Type II 30-minute runoff hydrograph in each garden based on their respective catchment characteristics. Data including ponding zone storage capacity, infiltration rate, drain time, soil characterization, and observations of berm, outflow, and grading performance were collected and analyzed to make performance conclusions for each site. Results indicate that rain gardens constructed on loamy to silty clay loam soils in a residential area of Lincoln, NE infiltrate at the rates recommended by state and national guidelines. The geometric mean infiltration rate for all sites for the first simulated runoff test was 4.13 cm h -1 and 2.75 cm h -1 for the design event and overflow event, respectively. Every rain garden tested drained in 30 h or less, with six gardens draining in less than one hour. Rain garden storage capacity was poor with only two gardens able to