Investigating the influences of concave depths on stormwater runoff and pollution retention of urban grasslands.

In this study, scale-based runoff plots of concave grasslands were designed and simulated rainfall experiments were conducted to investigate their retention effectiveness for runoff volume and pollutant loads, and to analyze the influences of concave depths on runoff and pollution retention of grasslands. Results showed that mean time to runoff of concave grasslands was 88.5 minutes, which was 5.3 times than that of flat grassland. Average peak flow rate of concave grasslands was reduced by 36.2% compared with flat grassland. Concaved grasslands averagely retained 58.2% of stormwater runoff. Deeper concave depths significantly increased runoff detention and retention performance of grasslands. Total suspended solids (TSS) load reduction rates of concave grasslands were ranged from 50.8% to 97.3%. Total nitrogen (TN) load reduction rate was 49.8% for concave depth of 10 cm. Total phosphorus (TP) load reduction rates were 45.0% and 93.9% for grasslands with 5 cm and 10 cm concave depths, respectively. Pollution load reduction rates of TSS, TN and TP enhanced along with the increase in concave depths. The estimated minimum area ratios of upslope impervious surface to grasslands of 5 cm and 10 cm concave depths were approximately 1:1 under 20 mm rainfall events, and 38:1 under 5 mm rainfalls, respectively.