Notation

From LID SWM Planning and Design Guide
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Ac
Area of the catchment, whole contributing drainage area (usually in m2).
Ap
Area of the practice e.g. footprint area of a bioretention cell (usually in m2)
d
Depth e.g. of an infiltration practice (units of length, usually mm or m)
f
Infiltration rate (usually mm/hr)
h
height (units of length)
i
Thornthwaite monthly heat index (-)
I
Thornthwaite annual heat index (-)
L
Length, e.g. of a reach, channel or swale (usually in m)
m
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
Manning's coefficient (unitless) Note: for clarity porosity is being expressed as 'void ratio' and sharing the notation VR
N
Number of days in the month
x
Perimeter of infiltration practice or swale x-section (m)
f'
Design infiltration rate (length per time, usually mm/hr) after application of safety correction factor (SCF)
Qp
Peak flow in units of volume per time (e.g. m3/s)
R
Ratio of catchment (Ac) to the practice footprint area (Ap). syn. I/P ratio. For green roofs (and often permeable paving systems) this will be 1:1. In bioretention it may be 15 or 20 to 1.
Ra
Existing rise of swale gradient (length, usually m)
Rb
Rise of compensation gradient for an enhanced swale (length, usually m)
RVCT
Runoff volume control target (as outlined by MECP, units are mm)
S0
Longitudinal slope (m/m, i.e. dimensionless)
ST
Storage (m3)
Sx
Cross slope (m/m, i.e. dimensionless)
SCF
Safety correction factor (dimensionless)
t
time (usually in hrs)
T
Temperature (°C)
V
Volume (usually in m3)
n
Porosity, applies to aggregate and other void-forming materials, filter media and native soils (unit is vol/vol, so dimensionless)
y
Depth of water (L, usually m or sometimes mm)