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 | |||||
| subcatchments. | |||||||
| 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 | |||||||
| junctions). | |||||||
| 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 | |||||||
| elevation. | |||||||
| 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 | ||||||
| infiltration. | ||||||
| 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, | ||||||
| etc.). | ||||||
| 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
| 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 | |||