Notation

A_c

Area of the catchment, whole contributing drainage area (usually in m²).

A_i

Area of impervious cover within the catchment (usually in m²).

A_p

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: the symbol n is also used for porosity so it is important to be sure which term is implied.

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)

Q_p

Peak flow in units of volume per time (e.g. m³/s)

R

Ratio of catchment (A_c) to the practice footprint area (A_p). 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.

R_a

Existing rise of swale gradient (length, usually m)

R_b

Rise of compensation gradient for an enhanced swale (length, usually m)

RVC_T

Runoff volume control target (as outlined by MECP, units are mm)

S₀

Longitudinal slope (m/m, i.e. dimensionless)

S_T

Storage (m³)

S_x

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)

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

The 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.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

A 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.

The 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)

The rate at which stormwater percolates into the subsoil measured in inches per hour.

The ratio of the impervious catchment (drainage) area to the pervious (footprint) area of the receiving BMP.

An 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.

Vegetated open channel, with check dams; designed to convey, treat and attenuate stormwater runoff.

The portion of water precipitated onto a catchment area, which then 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.

The 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)

A 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.

The 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.