Modeling 3D Ex-Filtration Process of a Soak-Away Rain Garden

Sivarajah Mylevaganam^*, Ting Fong May Chui, Jiangyong Hu

Department of Civil and Environmental Engineering, National University of Singapore, Singapore, Singapore.

DOI: 10.4236/gep.2015.33004                

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.

Keywords

COMSOL Multiphysics, 3D Ex-Filtration, Soak-Away Rain Garden, Average Vertical Ex-Filtration Rate, Horizontal Flow Coefficient

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	Allan, P.D., Robert, G.T. and William, F.H. (2010) Improving Urban Stormwater Quality: Applying Fundamental Principles. Journal of Contemporary Water Research and Education, 146, 3-10. http://dx.doi.org/10.1111/j.1936-704X.2010.00387.x
[2]	Jia, L., David, J.S., Cameron, B. and Yuntao, G. (2014) Review and Research Needs of Bioretention Used for the Treatment of Urban Stormwater. Water, 6, 1069-1099. http://dx.doi.org/10.3390/w6041069
[3]	Hunt, W.F., Jarrett, A.R., Smith, J.T. and Sharkey, L.J. (2006) Evaluating Bioretention Hydrology and Nutrient Removal at Three Field Sites in North Carolina. Journal of Irrigation and Drainage Engineering, 132, 600-608. http://dx.doi.org/10.1061/(ASCE)0733-9437(2006)132:6(600)
[4]	Jones, M.P. and Hunt, W.F. (2009) Bioretention Impact on Runoff Temperature in Trout Sensitive Waters. ASCE Journal of Environmental Engineering, 135, 577-585. http://dx.doi.org/10.1061/(ASCE)EE.1943-7870.0000022
[5]	Li, H., Sharkey, L.J., Hunt, W.F. and Davis, A.P. (2009) Mitigation of Impervious Surface Hydrology Using Bioretention in North Carolina and Maryland. ASCE Journal of Hydrologic Engineering, 14, 407-415. http://dx.doi.org/10.1061/(ASCE)1084-0699(2009)14:4(407)
[6]	Richards, L.A. (1931) Capillary Conduction of Liquids through Porous Mediums. Journal of Applied Physics, 1, 318-333. http://dx.doi.org/10.1063/1.1745010
[7]	COMSOL AB (2012) COMSOL Multiphysics User’s Guide (Version 4.3). Stockholm, Sweden.
[8]	COMSOL AB (2012) COMSOL Multiphysics Reference Guide (Version 4.3). Stockholm, Sweden.
[9]	Li, Q., Ito, K., Wu, Z., Lowry, C.S. and Loheide II, S.P. (2009) COMSOL Multiphysics: A Novel Approach to Ground Water Modeling. Groundwater, 47, 480-487. http://dx.doi.org/10.1111/j.1745-6584.2009.00584.x
[10]	Chow, V.T., Maidment, D.R. and Mays, L.W. (1988) Applied Hydrology. McGraw Hill, New York.