Abstract
The use of green roofs to help mitigate storm water contributions to urban flooding has been gaining popularity but is hindered by the limited data on the performance of such roofs with regard to storm water runoff mitigation. The underlying issue stems from the inherent complexity of modeling subsurface multiphase flow. Modeling of this phenomena requires calculating the contributions of substrate microstructure characteristics, the influence of the wetting and non-wetting phases upon each other, and the effect of the microstructure on the wetting phase. Previously we have observed how the microstructure can affect detention, however the quantification of this relationship is still missing. In the present paper we present numerical simulations of wetting phase infiltration of a thin monodisperse packed bed in order to understand and quantify the impact of microstructure geometry on storm water infiltration of a green roof substrate. For a slightly hydrophilic case, (θ=82°), we find that a dominant mechanism underlying this relationship is the microstructure-induced dynamic behavior of the capillary pressure. We determine that at larger packing ratios (ratio of packed bed depth to particle size), the influence of hydraulic head diminishes and behaves conversely for thinner layers, particularly when larger pores are present. Indeed, thin beds composed of large particles can exhibit high flow velocities that in turn affect the capillary pressure within the substrate. We observe that the capillary pressure can shift from negative values denoting capillary suction to positive ones which cause valve-like blocking effects on the flow; dependent upon the flow velocity as determined by the microstructure. In particular, we find that the capillary pressure depends on the value of the pore-scale gravity-induced flow velocity, quantified through a characteristic Capillary number. The provided quantification of this relationship can be invaluable from a design perspective to understand the behavior of capillary pressure of different substrates under a variety of flow rates prior to testing substrate candidates. In addition, a comparison of the behavior of the dynamic component of capillary pressure to other works is undertaken. Flow homogeneity is also found to be linked to the flow velocity, and consequently to the microstructure.
Highlights
With the predicted increase in precipitation frequency and intensity in northern latitudes and the current issues regarding urban storm water management, green roofs have emerged as an attractive technology for urban areas (Bliss et al, 2009)
This work provides an analysis of the impact of the porous micro structure on infiltration dynamics of thin porous media by establishing the relationships between saturation, dynamic capillary pressure, effective capillary number, and characteristic capillary number
This is due to the fact that dynamic effects and gravitational forces at the pore scales, become important
Summary
With the predicted increase in precipitation frequency and intensity in northern latitudes and the current issues regarding urban storm water management, green roofs have emerged as an attractive technology for urban areas (Bliss et al, 2009). The primary driving forces for green roof usage are their ability to reduce peak storm water runoff loads through retention of water in the soil and vegetation as well as detention through mechanical processes within the soil or alternative drainage layer (Johannessen et al, 2017). The term retention refers to the entrapment of liquid within the drainage layer as well as in the vegetation and is removed through evapotranspiration. Detention is defined as the ability of the drainage and vegetation layers to impede liquid infiltration and drainage, lowering the peak flow intensity and lengthening the drainage period. While experiments have been able to provide information regarding the hydrological performance of different layering designs and types of vegetation under both laboratory and in situ conditions, accurately capturing the behavior from the modeling
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