Understanding of Road-Bioretention Strip Design and Improvement from Hydraulic Perspective

Abstract

This study is to understand and quantify the hydraulic and hydrological performance of road bioretention facilities as well as the basic problems in urban drainage and road bioretention design. An open-source two-dimensional flow simulation program, FullSWOF_2D, which fully (Full) solves shallow water (SW) equations for overland flow (OF) and river flow, was updated and applied to this study. The particle tracking method (PTM) module was first added into FullSWOF_2D program to estimate the time of concentration (Tc) for impervious and pervious surfaces. The updated program FullSWOF-PTM was applied to 446 impervious modeling cases to simulate and calculate Tci of overland flow on impervious surfaces. Tci equation derived using PTM correlates well with Tci from other five published equations, which proves PTM can also be used to estimate Tcp of overland flow on pervious surfaces. Seven hundred fifty (750) pervious modeling cases were developed and simulated to explore the Tcp equation. A regression equation for Tcp was developed and has higher accuracy compared to Akan’s equation wide ranges of input parameters. The FullSWOF_2D program was also revised to include 2D plane zones with different rainfall and infiltration parameters and a 2D-1D grate inlet flow interception module. The updated program called FullSWOF-ZG was tested with 20 locally depressed Texas type D curb inlet cases to simulate the inlet efficiency (Eci). It was also validated with 80 laboratory tests to simulate the curb inlet length of 100% interception (LT). These validation runs indicated that the FullSWOF-ZG program can be used to determine LT and Eci. One thousand (1000) undepressed curb inlet modeling cases of the road with 10 longitudinal slopes S0, 10 cross slopes Sx, and 10 upstream inflows Qin were established and modeled to determine LT. The second 1000 road modeling cases of undepressed curb inlet with 10 S0, 10 Sx, and 10 curb inlet opening lengths Lci and constant Qin (10 L/s) were established and modeled to determine Eci. Two new regression equations of LT and Eci were developed and compared with three previous methods, and the newly developed equations give more accurate estimations of LT and Eci over a wide range of input parameters and can be applied to design urban drainage and road bioretention facilities. Twenty road-bioretention strip (RBS) modeling cases were designed based on the commonly used parameters and evaluated using FullSWOF-ZG. The simulation results were analyzed and demonstrated that the RBS’s hydrological performance was jointly influenced by several parameters such as road gemoetry, inlet design and efficiency, bioretention infiltration capacity, and overflow discharge capacity. When the road, curb inlet, and bioretention strip were modeled together as an integral system, it was found that the RBS’s curb inlet could be the bottleneck of RBS’s hydrologic performance and should be designed based on hydraulic calculation. The curb inlet and road grate inlet combination is necessary for continuous RBS because the road surface runoff could not be 100% intercepted by the curb inlet alone. The FullSWOF-ZG program was applied to explore whether the deep cut over the curb and the road-curb cut inlets can improve the curb-inlet’s efficiency Eci. The deep-cut curb inlets were used in some retrofitting projects for improving Eci. However, the simulation results show that the Eci improvement of the deep cut over the curb inlet only is very small compared to corresponding undepressed curb inlet. The curb inlets with the road-curb deep cuts were also simulated, and it proved that the curb inlets with the road-cut width = 0.10 m could improve the efficiency compared to the undepressed curb inlets.