Treatment Performance Of Rain Gardens For Urban Road Runoff In South China

This research study investigated urban road runoff quality in a city of South China, Shenzhen and the efficiency of runoff quality and quantity control of rain garden systems in a labortarty-scale, which is one of the most typical Low Impact Development (LID) systems. Additionally, a design of rain garden system was undertaken for an urban road in order to treat stormwater runoff and an evaluation for the rain garden performance was caught out in this study.It is found that urban road runoff was polluted. The event mean concentrations (EMC) of SS, COD, TP, TN and NH4+-N were44.6-915.0mg/L,25.4-333.0mg/L,0.1-0.6mg/L,1.0-9.9mg/L and0.1-5.3mg/L individually. This was worse than the quality specifyied in the IV class of surface water quality standard (GB3838-2002). Furtheremore, the research outcomes show a medium magnitude of first flush on urban road runoff and a3-5mm of initial rainfall depth removal was suggested for fisrt flush capturing device degisn. It is also noted that nitrogen concentrations in the runoff were higher in low-intensity rainfall than high-intensity rainfall while the opposite held true for SS. COD and TP did not show close relationships with rainfall characteristics.In the labortary-scale rain garden investigation, SS and TP removal percentages were relatively stable in the case of low-intensity rainfall events (<10mm/h), more than90%and80%respectively while the corresponding values were39.47%,78.2%and62.0%for COD, TN and NH4+-N. In the case of high-intensity rainfall events (≥10mm/h)(no overflow), the removal percentages of COD, TP, TN, NH4+-N and N03--N were29.7%,49.7%,39.5%,41.5%and29.1%individually while for the extreme rainfall events(overflow), the removal percentages of COD, TP, TN, NH4+-N and N03--N were76.1%,80.0%,71.6%,81.4%and81.3%separately after24hours. The research outcomes also indicated that pollutant species and media types are the two key influential factors of rain garden system removing pollutants from stormwater runoff. The rain garden performed well for quantity control under a low-intensity rainfall event condition. It is proposed that the greening rate should be more than10%along the urban roads.In the evaluation of a rain garden designed for an urban road which the catchment area is542.5m2with a greening rate of20%, it is observated that the average load reduction rate of SS, COD, TP, TN and NH4+-N which through the overflow to the receiving water were75.0%,83.6%,72.0%,71.8%and78.4%reppectively while the mean removal capacity of the designed rain garden system were3059.0mg/m2,2930.7mg/m2,3.3mg/m2,41.0mg/m2and34.9mg/m2for SS, COD, TP, TN and NH4+-N individually. Moreover, this rain garden system reduced about95%of annual runoff quantity, increasd the time to peak and decreased the peak flow by30.4%. This has led to an incease of drainage system capacity from2-year ARI (average recurrence interval) to4-year ARI. Therefore, the designed rain garden is able to control runoff pollution and hence assist in the urban water environment improvement.