Effect of rain gardens on storm runoff reduction

Rain gardens can be used effectively for retaining stormwater runoff and mitigating the negative impact of urban development on the aquatic environment. By monitoring the inflow and outflow processes of a rain garden over four years in Xi'an, China, this paper presents a study on the effect of a rain garden on storm runoff reduction; the sustainability of the rain garden operation was examined on the basis of soil sampling for particle size analysis and onsite measurements of infiltration capacity. The results showed that, for the experimental rain garden constructed with ponding depth of 15 cm and flow catchment area ratio of 20:1, only 4 storm events produced overflow from the rain garden out of the 28 monitored events, and the overflow volumes were generally small; there were no overflow occurred in two years, all inflow infiltrated in the rain garden; there was maximum 3 overflow events in one year, but the runoff reduction rate was as high as 96.8％; the storms that caused the rain garden overflow were all high intensity and short duration events. The onsite infiltration measurements showed no significant change in the infiltration capacity over the four years, the rain garden infiltration capacity was maintained at 2.400 m/d. The soil particle size analysis showed that, sand content at the surface layer of the rain garden increased from 7.36％ to 20.55％, while the silt and clay contents decreased accordingly. These results indicate that rain gardens can reduce storm runoff significantly, and the stable infiltration capacity entitles them a promising future in the study area. Findings from this research may provide theoretical basis and technical support for the sponge city construction in China.