Effects of bioretention structural layer parameters on detention and retention for road runoff

Based on theoretical unsaturated soil water movement, the effects of different bioretention structural layer parameters on ponding, outflow, and runoff regulation were studied under four types of rainfall using a numerical simulation method. Based on the results, ponding was significantly affected by the depth of the surface aquifer. With increasing depth from 20 cm to 30 cm, the overflow control water was increased by about 0.196 m³, while the ponding duration increased by up to 85 min. Bioretention structural layer parameters all had some influence on the outflow of the perforated pipe. As the soil layer to sand layer thickness ratio or the internal water storage zone height increased, the outflow of the perforated pipe was delayed and the peak of the flow decreased, while the increase in aquifer depth led to the advancement of the outflow time and increase in the flow peak. Under the four types of rainfall studied, the average runoff reduction rate of the five types of bioretention ranged from 16.71% to 37.31%, the average reduction rate of the runoff peak ranged from 41.53% to 63.90%, and the average runoff delay time ranged from 97.75 min to 166.50 min. In the case of overflow, the performance of bioretention methods on runoff regulation was significantly reduced, and the effects of structural layer parameters on runoff regulation were weakened.