Abstract
This paper presents a three-dimensional (3D) model developed using COMSOL Multiphysics to understand the 3D ex-filtration process of a soak-away rain garden. With a design hyetograph of 3-month average rainfall intensities of Singapore, it is found that the average vertical ex-filtration rate that is obtained by dividing the average vertical ex-filtration (drained through bottom of the soak-away rain garden, averaged over the simulation period = 720 min, and expressed in m3) by the surface area of the soak-away rain garden and the simulation time step is almost constant regardless of increase in saturated hydraulic conductivity (K) of the in-situ soil and the surface area of the soak-away rain garden as a percentage of catchment area. However, as depth to groundwater table which is measured from bottom of the filter media increases, in between 0.5 m and 1 m of depth to groundwater table, the average vertical ex-filtration rate decreases significantly (by around 15 - 20 mm/hr) and the decrease is almost twice, compared with that between 1 m and 1.5 m of depth to groundwater table. Furthermore, this study shows that for a given K of in-situ, K of filter media, and depth to groundwater table, as the surface area of the soak-away rain garden increases, the horizontal flow coefficient which is defined as the ratio between total horizontal ex-filtration (drained through sides of the soak-away rain garden, summed over the simulation period, and expressed in m3) and total vertical ex-filtration (drained through bottom of the soak-away rain garden, summed over the simulation period, and expressed in m3) decreases. Moreover, for a given surface area of the soak-away rain garden, K of in-situ, and depth to groundwater table, the horizontal flow coefficient decreases as K of the filter media increases. However, it is found that for a given surface area of the soak-away rain garden, K of in-situ, and K of filter media, the horizontal flow coefficient increases as depth to groundwater table increases.
Highlights
To mitigate the adverse impact of urbanization around the world, several best management practices, in other words, green infrastructures, have been used in a way that protect the natural hydrology of the catchment and are more beneficial to the environment [1]-[5]
As can be observed from the graphs, for a given surface area of the soak-away rain garden, saturated hydraulic conductivity of the in-situ soil, and depth to groundwater table, the horizontal flow coefficient decreases as the saturated hydraulic conductivity of the filter media increases
This paper presents a 3D model developed using COMSOL Multiphysics to understand the 3D ex-filtration process, which plays major roles in reducing stormwater volume, of a soak-away rain garden, shallow, landscaped depressions commonly located in parking lots or within small pockets in residential areas
Summary
To mitigate the adverse impact of urbanization around the world, several best management practices, in other words, green infrastructures, have been used in a way that protect the natural hydrology of the catchment and are more beneficial to the environment [1]-[5]. Though in the past few decades the mathematical modeling work and the science of computer simulation have enhanced the understanding of hydrological processes, there is still a lack of modeling studies that focus on exploring the 3D ex-filtration process of a soak-away rain garden in a sufficient level of details [2]. To understand and get the preliminary view on the 3D ex-filtration process of a soak-away rain garden in a sufficient level of details, a comprehensive mathematical modeling work is required, which is independent of field data. It is the objective of this study to explore the 3D ex-filtration process of a soak-away rain garden, the average vertical ex-filtration rate and the horizontal flow coefficient, by developing a 3D mathematical model based on Richard’s equation [6]-[8] using COMSOL Multiphysics [7] [8]
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