Abstract:Urbanization caused hydrological change and increased stormwater runoff volumes, which led to flooding, erosion and the degradation of instream ecosystem health. Low impact development (LID) options had been proposed as an alternative approach to mimic the natural flow regime by using decentralized designs to control stormwater runoff at the source, rather than at a centralized location in the watershed. Hydrological regulation performances of these LID practices can be influenced by rainfall characteristics, such as rainfall intensity and duration. To evaluate the impacts of different rainfall reappearance periods, rainfall analysis was conducted to determine the rainfall characteristics and SCS II type was selected for the analysis. A modeling approach based on SWMM was described to incorporate these LID practices into an existing hydrological model to estimate the impacts of LID practices on the surface runoff. Results demonstrated that the LID practices led to significant stormwater control for different rainfall reappearance periods. Hydrological regulation performances of the LID practices were varied with rainfall reappearance periods. For LID practices with the same surface area, the detention pond performed the best in reducing peak flow rate, which was followed by infiltration trench, bioretention cell and porous pavement. Detention pond was capable to reduce the peak flow rate of 100-year storm to the value of 10year storm, indicating significant performances. Differences in peak flow reduction were due to structure differences in the LID practices. For the infiltration regulation performances, infiltration trench had the highest recharge ratio for all the rainfall reappearance periods, followed by bioretention cell and porous pavement. Porous pavement, though made of 100% pervious material, infiltrated small runoff which was limited by the native soil infiltration rate when the rainfall volume exceeded the storage capacity. Deep analysis was conducted to determine the reasons that the LID practices performed differently when they had the same surface area. Results showed that the “effective storage”, which was the water volume that a facility can contain, was the crucial factor. When rainfall intensity was larger than native soil infiltration rate, the excessive water was stored in the facility, and then it was released or infiltrated to the groundwater, depending on the facility structure. Consequently, the water exceeded the “effective storage” was flowed over the LID practices and made contribution to the surface runoff directly. Calculation results showed that the “effective storage” for the detention pond was 1861.20m3, which was the largest among the four LID practices, and it explained the reason that detention pond worked the best in peak flow reduction. The “effective storage” for the infiltration trench, porous pavement and bioretention cell were 744.48m3, 80.37m3 and 565.14m3, respectively.
