SWMM Process Models and Modeling Objects, Units from Hydraulics Reference Manual

1.3 SWMM’s Process Models and Modeling Objects, Units

Table 1-2 SWMM’s modeling objects
Category Object Type Description
Hydrology Rain Gage Source of precipitation data to one or more
Subcatchment A land parcel that receives precipitation associated
with a rain gage and generates runoff that flows into
a drainage system node or to another subcatchment.
Aquifer A subsurface area that receives infiltration from the
subcatchment above it and exchanges groundwater
flow with a conveyance system node.
Snow Pack Accumulated snow that covers a subcatchment.
Unit Hydrograph A response function that describes the amount of
sewer inflow/infiltration generated over time per
unit of instantaneous rainfall.
Hydraulics Junction A point in the conveyance system where conduits
connect to one another with negligible storage
volume (e.g., manholes, pipe fittings, or stream
Outfall An end point of the conveyance system where water
is discharged to a receptor (such as a receiving
stream or treatment plant) with known water surface
Divider A point in the conveyance system where the inflow
splits into two outflow conduits according to a
known relationship.
Storage Unit A pond, lake, impoundment, or chamber that
provides water storage.
Conduit A channel or pipe that conveys water from one
conveyance system node to another.
Pump A device that raises the hydraulic head of water.
Regulator A weir, orifice or outlet used to direct and regulate
flow between two nodes of the conveyance system.
Category Object Type Description
Water Quality Pollutant A contaminant that can build up and be washed off
of the land surface or be introduced directly into the
conveyance system.
Land Use A classification used to characterize the functions
that describe pollutant buildup and washoff.
Treatment LID Control A low impact development control, such as a bio-
retention cell, porous pavement, or vegetative swale,
used to reduce surface runoff through enhanced
Treatment Function A user-defined function that describes how pollutant
concentrations are reduced at a conveyance system
node as a function of certain variables, such as
concentration, flow rate, water depth, etc.
Data Object Curve A tabular function that defines the relationship
between two quantities (e.g., flow rate and hydraulic
head for a pump, surface area and depth for a storage
node, etc.).
Time Series A tabular function that describes how a quantity
varies with time (e.g., rainfall, outfall surface
elevation, etc.).
Time Pattern A set of factors that repeats over a period of time
(e.g., diurnal hourly pattern, weekly daily pattern,
Control Rules IF-THEN-ELSE statements that determine when
specific control actions are taken (e.g., turn a pump
on or off when the flow depth at a given node is
above or below a certain value).

Figure 1-3 depicts the processes that SWMM models using the objects described previously and how they are tied to one another. The hydrological processes depicted in this diagram include:

Figure 1-5 Flow chart of SWMM’s simulation procedure
Figure 1-3 Processes modeled by SWMM
Figure 1-3 Processes modeled by SWMM
Table 1-3 State variables used by SWMM
Process Variable Description Initial Value
Runoff d Depth of runoff on a subcatchment surface 0
Infiltration* tp Equivalent time on the Horton curve 0
Fe Cumulative excess infiltration volume 0
Fu Upper zone moisture content 0
T Time until the next rainfall event 0
P Cumulative rainfall for current event 0
S Soil moisture storage capacity remaining User supplied
Groundwater u Unsaturated zone moisture content User supplied
dL Depth of saturated zone User supplied
Snowmelt wsnow Snow pack depth User supplied
fw Snow pack free water depth User supplied
ati Snow pack surface temperature User supplied
cc Snow pack cold content 0
Flow Routing H Hydraulic head of water at a node User supplied
Q Flow rate in a link User supplied
A Flow area in a link Inferred from Q
Water Quality tsweep Time since a subcatchment was last swept User supplied
mB Pollutant buildup on subcatchment surface User supplied
mP Pollutant mass ponded on subcatchment 0
cN Concentration of pollutant at a node User supplied
cL Concentration of pollutant in a link User supplied


*Only a sub-set of these variables is used, depending on the user’s choice of infiltration method.

Table 1-4 Units of expression used by SWMM
Variable or Parameter US Customary Units SI Metric Units
Area (subcatchment) acres hectares
Area (storage surface area) square feet square meters
Depression Storage inches millimeters
Depth feet meters
Elevation feet meters
Evaporation inches/day millimeters/day
Flow Rate cubic feet/sec (cfs) cubic meters/sec (cms)
gallons/min (gpm) liters/sec (lps)
106 gallons/day (mgd) 106 liters/day (mld)
Hydraulic Conductivity inches/hour millimeters/hour
Hydraulic Head feet meters
Infiltration Rate inches/hour millimeters/hour
Length feet meters
Manning’s n seconds/meter1/3 seconds/meter1/3
Pollutant Buildup mass/acre mass/hectare
Pollutant Concentration milligrams/liter (mg/L) milligrams/liter (mg/L)
micrograms/liter ( g/L) micrograms/liter ( g/L)
organism counts/liter organism counts/liter
Rainfall Intensity inches/hour millimeters/hour
Rainfall Volume inches millimeters
Storage Volume cubic feet cubic meters
Temperature degrees Fahrenheit degrees Celsius
Velocity feet/second meters/second
Width feet meters
Wind Speed miles/hour kilometers/hour

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