Rain garden design and function: A field monitoring and computer modeling approach

Date of Completion

January 2005


Environmental Sciences




Rain gardens have been recommended as a best management practice to treat stormwater runoff. Replicate rain gardens were constructed in Haddam, CT, to capture roof runoff. The gardens were sized to store the first 2.54 cm (1 inch) of runoff. Flow was measured using tipping buckets. Weekly composite water samples were analyzed for total phosphorus (TP), total Kjeldahl nitrogen (TKN), ammonia-nitrogen (NH3-N), and nitrite+nitrate-nitrogen (NO 3-N). Monthly composite samples were analyzed for copper (Cu), lead (Pb) and zinc (Zn). Redox potential and water temperature were also measured. The first year of results indicated high retention of NH3-N, but low retention of other nutrients and metals measured. Export of TP and Pb from the system was noted. Over 98% of inflow volume was retained by the system. The second phase of the study was then initiated; the paired watershed study design and analysis of covariance (ANCOVA) were then used to assess whether the creation of a saturated zone in a rain garden impacted retention of pollutants by the garden. Results show high retention of flow; only 0.9% overflowed. Overall, according to ANOVA analysis, the rain garden significantly reduced concentrations of NO3-N, NH3-N, and TN in influent roof runoff. TP concentrations significantly increased, likely due to release from the soil. Poor retentions of TKN, organic-N, Cu, Pb, and Zn were observed. As a result of saturation, ANCOVA results showed significant reductions in TN (18%) concentrations. Redox potential also decreased due to treatment. Water temperature was significantly (p = 0.01) increased by the rain gardens during the fall and winter. Overall, these rain gardens provided runoff control, but water quality renovation was not good. It may be necessary to install a rain garden with an underdrain in some situations. However, given the high overall retention of flow found for the 2.54 cm design method used in this study, a rain garden could be an effective BMP in reducing flow and pollutant loads if an underdrain were not connected to the stormwater system. An evaluation of several rain garden sizing methods was also performed, and two Stella ® models were constructed using data obtained from the study. ^