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;f':Design infiltration rate (length per time, usually mm/hr) after application of safety correction factor (SCF)
;h :height Height (units of length)
;i :Intensity of the design storm event (in units of depth/time, usually mm/hr)
;L :Length, e.g. of a reach, channel or swale (usually in m)
;m :run Run of side slopes in channel cross sections (length, but usually expressed as a ratio in relation to rise. e.g. side slopes = 1:3, so m = 3)
;n :Porosity, applies to aggregate and other void-forming materials, filter media and native soils (unit is vol/vol, so dimensionless)
;Q<sub>p</sub> :Peak flow in units of volume per time (e.g. m<sup>3</sup>/s)
;R :Ratio of catchment impervious area (A<sub>ci</sub>) to the practice footprint area (A<sub>p</sub>). ''syn.'' I/P ratio. For [[green roofs]] (and often [[permeable paving ]] systems) that do not receive drainage from external impervious areas this will be 1:10. In For [[bioretention]] , [[infiltration trenches]] and [[infiltration chambers]] receiving pavement runoff, it may is recommended to be kept between 15 or to 20 to 1.
;R<sub>a</sub> :Existing rise of swale gradient (length, usually m)

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