# Notation

Ac
Area of the catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale., whole contributing drainage areaThe total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds. (usually in m2).
Ap
Area of the practice e.g. footprint area of a bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cell (usually in m2)
Depth e.g. of an infiltration practice (units of length, usually mm or m)
Infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. (usually mm/hr)
height (units of length)
Thornthwaite monthly heat index (-)
Thornthwaite annual heat index (-)
Length, e.g. of a reach, channel or swaleA shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades. (usually in 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)
Manning's coefficient (unitless) Note: the symbol n is also used for porosityThe porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e) so it is important to be sure which term is implied.
Number of days in the month
Perimeter of infiltration practice or swaleA shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades. x-section (m)
f'
Design infiltration rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. (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)
Ratio of catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. (Ac) to the practice footprint area (Ap). syn. I/P ratioThe ratio of the impervious catchment (drainage) area to the pervious (footprint) area of the receiving BMP.. For green roofs (and often permeable pavingAn alternative practice to traditional impervious pavement, prevents the generation of runoff by allowing precipitation falling on the surface to infiltrate through the surface course into an underlying stone reservoir and, where suitable conditions exist, into the native soil. systems) this will be 1:1. In bioretention it may be 15 or 20 to 1.
Ra
Existing rise of swaleA shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades. gradient (length, usually m)
Rb
Rise of compensation gradient for an enhanced swaleVegetated open channel, with check dams; designed to convey, treat and attenuate stormwater runoff. (length, usually m)
RVCT
RunoffThat potion of the water precipitated onto a catchment area, which flows as surface discharge from the catchment area past a specified point.Water from rain, snow melt, or irrigation that flows over the land surface. 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)
time (usually in hrs)
Temperature (°C)
Volume (usually in m3)
PorosityThe porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e), applies to aggregateA broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations. and other void-forming materials, filter mediaThe engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles. and native soils (unit is vol/vol, so dimensionless)
Depth of water (L, usually m or sometimes mm)