Catchment snowmelt is a Global Database which can be optionally referenced from each of the subcatchments. The parameters defined here are adopted for each subcatchment that references this Global Database.

When snowmelt is simulated, a fourth sub-area is added to each subcatchment. Its main purpose is to permit part of the impervious area to be continuously snow covered (eg. due to windrowing or dumping) and part to be "normally bare" (eg. streets and sidewalks that are plowed or otherwise cleared). During single event simulation this sub-area retains 100% snow cover until it has all melted. During continuous simulation an areal depletion curve is used.

Subcatchment surfaces are classified into 4 sub-areas as in the following table:

TypePerviousness

Depression
Storage 

Snow Cover and Extent
Single Event*Continuous*
1ImperviousYesBare

Normally bare, but may have snow cover over 100% of type 1 plus type 3 area.

2PerviousYes

Constant fraction, SNCP, of area is snow covered.

Snow covered subject to areal depletion curve.

3ImperviousNoBareSame as type 1.
4ImperviousYes100% covered.Snow covered subject to areal depletion curve.

*Single event or continuous event snowmelt is set in the Snowmelt section of the Runoff Mode Job Control dialog.

During periods of no rainfall, snowmelt is computed by a degree-day or temperature index equation.

SMELT = DHM x (TA - TBASE)

where:

SMELT = snowmelt rate, in./hr [mm/hr] water equivalent.

DHM = melt coefficient, in./hrF [mm/hrC] water equivalent.

TA = air temperature, F [C].

TBASE = snowmelt base temperature, F [C].

There is no melt when TA <= TBASE.

Initial Snow Depth (I1 - WSNOW1)

Initial snow depth of the pervious and impervious areas respectively that is normally snow covered, in. [mm] water equivalent. This is likely to be the only source of snow for a single event simulation although snowfall may also be added as negative precipitation. During continuous simulation, the effect of initial conditions will die out, given a simulation of a few months.

Initial Free Water (I1 - FW1)

Initial free water on snow covered pervious and impervious areas respectively, in. [mm]. No liquid runoff will leave the snow pack until its free water holding capacity (due to its porosity) has been exceeded. The initial free water (FW) should maintain the relation:

FW <= FWFRAC * WSNOW

where

FWFRAC = fraction of snow depth, from Job Control

WSNOW = initial snow depth

Base Temperature (I1 - TBASE1)

Temperature (F [C]) at which snow will begin to melt on a snow covered pervious and impervious areas respectively.

Single Event Snowmelt Data

This button leads to additional snow-melt parameters for the single event mode. This button is made active by selecting the Single Event Data option from the Catchment Snowmelt dialog under Global Data.

Fraction 100% Covered - Fraction of pervious and impervious areas remaining 100% snow covered.

Melt Coefficient - Melt coefficient for snow covered pervious and impervious areas, in./hr°F [mm/hr°C] water equivalents. The melt coefficient is constant.

Continuous Simulation Snowmelt Data

This button leads to additional snow-melt parameters for the continuous mode. This button is made active by selecting the 'Continuous' option from the 'Snow Melt' Dialog under Job Control.

Global data for continuous snowmelt simulation consists of information concerning plowing (or similar clearing), normally snow covered area and normally bare impervious surfaces.

Each subcatchment that references the current Snowmelt Global Database adopts these parameters.

 

Snow Covered Area

Mandatory data relating to the subcatchment Snow Covered Area. This part of a catchment is continuously snow covered, due to windrowing or dumping for example. During continuous simulation, the whole pervious area is subject to the areal depletion curve.

June 21 Melt Coeff. (Impervious Area) (I1 - DHMAX1)

Maximum annual melt coefficient for snow covered pervious and impervious areas, in./hr°F [mm/hr°C] water equivalents (Minimum if in the Southern Hemisphere). The melt coefficient varies sinusoidally to reflect seasonal changes.

December 21 Melt Coeff. (Impervious) (I2 - DHMIN1)

Minimum annual melt coefficient for snow covered pervious and impervious areas, in./hr°F [mm/hr°C] water equivalents (Maximum if in the Southern Hemisphere). The melt coefficient varies sinusoidally to reflect seasonal changes.

Min. Depth for 100% Cover (Impervious) (I2 - SI1)

Snow depth above which there is 100 percent snow cover on snow covered pervious and impervious areas, in. [mm] water equivalent.

Areal Depletion Fraction (I1 - SNN1)

Fraction of impervious area subject to areal depletion. All of the pervious area is subject to areal depletion.

Plowing

Mandatory data related to Snow Plowing.

SWMM allows snow that falls on the normally bare impervious areas to be redistributed to other parts of a subcatchment or out of the system. This is intended to simulate plowing and snow removal practices in urban areas.

The snow plowing data required is the depth of snow at which plowing will be instigated and the distribution of the plowed snow. The sum of the four distribution fractions must total exactly 1.0.

No pollutants are transferred with the snow. The transfers are assumed to have no effect on pollutant washoff and generation. In addition, all the parameters of this process remain constant throughout the simulation and can only represent averages over a snow season.

The redistribution simulation does not account for snow management processes using chemicals, eg. roadway salting. This is handled using the melt equations.

Snow Covered Imp. Area (I2 - SFRAC1)

Fraction of plowed snow being transferred to the snow covered impervious area. Fraction in the range 0.0 - 1.0.

Snow Covered Perv. Area (I2 - SFRAC2)

Fraction of plowed snow being transferred to the snow covered pervious area. Fraction in the range 0.0 - 1.0.

Out of Catchment (I2 - SFRAC4)

Fraction of plowed snow being transferred out of the subcatchment. Fraction in the range 0.0 - 1.0.

Immediate Melt (I2 - SFRAC5)

Fraction of plowed snow melting ie. not being transferred to any of the regions. Fraction in the range 0.0 - 1.0.

Plowing Snow Depth (I2 - WEPLOW)

Depth of snow (in. [mm] water equivalent) before snow plowing will be instigated. Once plowing is initiated it is assumed that snow will be plowed to zero depth and distributed according to the fractions above. Some guidelines for the value of this snow depth are provided in the following table (Richardson et al, 1974).

Road Classification

Snow depth to Start
Plowing on Pavement
(inches) 

Max Snow Depth
(inches) 
 Low-Speed Multilane Urban Expressway0.5-11

High Speed 4-lane Divided Highways
(ADT > 10,000) 

1 2
Primary Highways (ADT 500-5000)12.5
Secondary Roads ADT < 50023

Normally Bare Impervious Area

Mandatory data relating to the Normally Bare Impervious Area.

 

"Normally Bare" implies surfaces such as roads and footpaths that receive snowfall but are subject to early snow removal.

 Initial Snow Depth (I2 - WSNOW3)

Initial depth of snow on normally bare impervious area, in. [mm] water equivalent. During continuous simulation, the effect of initial conditions will die out, given a simulation of a few months.

Initial Free Water (I1 - FW1)

Initial free water on snow covered pervious and impervious areas respectively, in. [mm]. No liquid runoff will leave the snow pack until its free water holding capacity (due to its porosity) has been exceeded. The initial free water (FW) should maintain the relation:

FW <= FWFRAC * WSNOW

where

FWFRAC = fraction of snow depth, from Job Control

WSNOW = initial snow depth

Snow Melt Base Temp (I2 - TBASE3)

Temperature (°F [°C]) at which snow will begin to melt on a normally bare impervious area.

Initial Snow Depth (I2 - WSNOW3)

Initial depth of snow on normally bare impervious area, in. [mm] water equivalent. During continuous simulation, the effect of initial conditions will die out, given a simulation of a few months.

Initial Free Water (I2 - FW3)

Initial free water on normally bare impervious area, in. [mm]. No liquid runoff will leave the snow pack until its free water holding capacity (due to its porosity) has been exceeded.