Snow Pack objects contain parameters that characterise the buildup, removal, and melting of snow over three types of sub-areas within a subcatchment:
The Plowable snow pack area consists of a user-defined fraction of the total impervious area. It is meant to represent areas such as streets and parking lots where plowing and snow removal can be done.
The Impervious snow pack area covers the remaining impervious area of a subcatchment.
Each of these three areas is characterised by the following parameters:
Minimum and maximum snow melt coefficients.
Minimum air temperature for snow melt to occur.
Snow depth above which 100% areal coverage occurs.
Initial snow depth.
In addition, a set of snow removal parameters can be assigned to the Plowable area. These parameters consist of the depth at which snow removal begins and the fractions of snow moved onto various other areas.
Subcatchments are assigned a snow pack object through their Snow Pack property. A single snow pack object can be applied to any number of subcatchments. Assigning a snow pack to a subcatchment simply establishes the melt parameters and initial snow conditions for that subcatchment. Internally, SWMM creates a "physical" snow pack for each subcatchment, which tracks snow accumulation and melting for that particular subcatchment based on its snow pack parameters, its amount of pervious and impervious area, and the precipitation history it sees.
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Option
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Description |
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Snow Pack Name |
User-assigned snow pack name. |
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Snow Pack Selector |
Select the snow pack to edit. |
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Add |
Click to add a new snow pack to the project. |
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Delete |
Click to delete a snow pack from the project. |
For each of the three categories of subcatchment surface type, you can specify the following:
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Option
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Description |
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Minimum Melt Coefficient |
The degree-day snow melt coefficient that occurs on December 21. Units are either mm/hr-deg C or in/hr-deg F. |
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Maximum Melt Coefficient |
The degree-day snow melt coefficient that occurs on June 21. Units are either mm/hr-deg C or in/hr-deg F. For a short term simulation of less than a week or so, it is acceptable to use a single value for both the minimum and maximum melt coefficients. The minimum and maximum snow melt coefficients are used to estimate a melt coefficient that varies by day of the year. The latter is used in the following degree-day equation to compute the melt rate for any particular day:
Melt Rate = (Melt Coefficient) * (Air Temperature - Base Temperature) |
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Base Temperature |
The temperature at which snow begins to melt (degrees C or F). |
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Fraction Free Water Capacity |
The volume of a snow pack's pore space which must fill with melted snow before liquid runoff from the pack begins. This is expressed as a fraction of snow pack depth. |
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Initial Snow Depth |
Depth of snow at the start of the simulation (water equivalent depth in millimetres or inches). |
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Initial Free Water |
Depth of melted water held within the pack at the start of the simulation (mm or inches). This number should be at or below the product of the initial snow depth and the fraction free water capacity. |
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Depth at 100% Cover |
The depth of snow beyond which the entire area remains completely covered and is not subject to any areal depletion effect (mm or inches). |
You can specify what portion of the impervious area is not subject to areal depletion.
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Option
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Description |
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Fraction of Impervious Area That is Plowable |
The fraction of impervious area that is plowable and therefore is not subject to areal depletion. |
See Also Edit Snow Packs