Set Simulation Options
99989796GeoSWMM has a number of analysis options that control how the simulation of a Stormwater drainage system is carried out. These options are provided in the Options in the SWMM data tree. There are six pages provided under the Options setting (Figure 9.1):
- General
- Dates
- Time Steps
- Dynamic Wave
- Interface Files
- Reporting9998979695
Features of these options are described in following.
General Options
The General page in the Simulation Options setting contains following items.
Process Models
This section allows the user to select which of SWMM’s process models will be applied to the current project. For example, a model that contained Aquifer and Groundwater elements could be run first with the groundwater computations turned on and then again with them turned off to see what effect this process had on the site’s hydrology. Note that a specific process model may be disabled if there are no relevant items present in the SWMM project. For example, if there are no Aquifers defined for the project, then the Groundwater option will appear in unchecked and disabled state.
Infiltration Model
This option controls how infiltration of rainfall into the upper soil zone of subcatchments is modeled. The choices are:
- Horton
- Green-Ampt
- Curve Number
Changing this option will require re- entering values for the infiltration parameters in each subcatchment.
Routing Model
This option determines which method is used to route flows through the conveyance system. The choices are:
- Steady Flow
- Kinematic Wave
- Dynamic Wave
Miscellaneous
In addition to the above simulation options, GeoSWMM enables the user to set some special considerations in processing the model simulation. These are optional choice for a user and they have been provided under the Miscellaneous items. The choices are:
Allow Ponding: allows excess water to collect atop nodes and to be re-introduced into the system as conditions permit. In order for ponding to actually occur at a particular node, a non-zero value must be set to its Ponded Area attribute.
Report Control Actions: lists all discrete control actions taken by the Control Rules associated with a project (continuous modulated control actions are not listed). This option should only be used for short-term simulation.
Report Input Summary: enables listing of summary of the project's input data.
Skip Steady Periods: checking this option will make the simulation use the most recently computed conveyance system flows during a steady state period instead of computing a new flow routing solution. A time step is considered to be in steady state if the change in external inflows at each node is below 0.5 cfs and the relative difference between total system inflow and outflow is below 5%.
Minimum Conduit Slope: allows to set the minimum value allowed for a conduit's slope (%). If blank or zero (the default) then no minimum is imposed (although SWMM uses a lower limit on elevation drop of 0.001 ft (0.00035 m) when computing a conduit slope).
Date Options
The Dates page in the Simulation Options dialog contains the following items.
Start Analysis On: allows setting of the date (month/day/year) and time of day when the simulation begins.
Start Reporting On: allows setting of the date and time of day when reporting of simulation results is to begin. This must be on or after the simulation starting date and time.
End Analysis On: allows to set the date and time when the simulation is to end.
Start Sweeping On: allows to set the day of the year (month/day) when street sweeping operations begin. The default setting is January 1.
End Sweeping On: allows to set the day of the year (month/day) when street sweeping operations end. The default setting is December 31.
Antecedent Dry Days: allows the user to set the number of days with no rainfall prior to the start of the simulation. This value is used to compute an initial buildup of pollutant load on the surface of the subcatchments.
Time Step Options
The Time Steps page establishes the length of the time steps used for runoff computation, routing computation and results reporting. Time steps are specified in days and hours: minutes: seconds except for flow routing which is entered as decimal seconds. The input parameters are:
Reporting Time Step: allows the user to set the time interval for reporting of computed results.
Runoff - Wet Weather Time Step: allows to set the time step length used to compute runoff from subcatchments during periods of rainfall or when ponded water still remains on the surface.
Runoff - Dry Weather Time Step: allows to set the time step length used for runoff computations (consisting essentially of pollutant buildup) during periods when there is no rainfall and no ponded water. This must be greater or equal to the Wet Weather time step.
Routing Time Step: allows the user to set the time step length in seconds used for routing flows and water quality constituents through the conveyance system. Note that Dynamic Wave routing requires a much smaller time step than the other methods of flow routing.
Dynamic Wave Options
The Dynamic Wave page in the Simulation Options dialog sets several parameters that control how the dynamic wave flow routing computations are made. These parameters have no effect for the other flow routing methods.
The Inertial Terms indicates how the inertial terms in the St. Venant momentum equation will be handled. The choices are:
- KEEP, which maintains these terms at their full value under all conditions.
- DAMPEN, which reduces the terms as flow comes closer to being critical and ignores them when flow is supercritical.
- IGNORE, which drops the terms altogether from the momentum equation, producing what is essentially a Diffusion Wave solution.
The Define Supercritical Flow By option selects the basis used to determine when supercritical flow occurs in a conduit. The choices are:
- Water surface slope only (i.e., water surface slope > conduit slope)
- Froude number only (i.e., Froude number > 1.0)
- Both water surface slope and Froude number
The first two choices were used in earlier versions of SWMM while the third choice, which checks for either condition, is now the recommended one.
The option for Force Main Equation selects which equation will be used to compute friction losses during pressurized flow for conduits that have been assigned to a Circular Force Main cross-section. The choices are:
- Hazen-Williams equation
- Darcy-Weisbach equation
The Variable Time Step option checks the box if an internally computed variable time step should be used at each routing time period and selects an adjustment (or safety) factor to apply to this time step. The variable time step is computed so as to satisfy the Courant condition within each conduit. A typical adjustment factor would be 75% to provide some margin of conservatism. The computed variable time step will not be less than 0.5 seconds nor be greater than the fixed time step specified on the Time Steps page of the dialog. If the latter was set lower than 0.5 seconds then the variable time step option is ignored.
The Time Step for Conduit Lengthening option (time step in seconds) is used to artificially lengthen conduits so that they meet the Courant stability criterion under full-flow conditions (i.e., the travel time of a wave will not be smaller than the specified conduit lengthening time step). As this value is decreased, fewer conduits will require lengthening. A value of zero means that no conduits will be lengthened. The ratio of the artificial length to the original length for each conduit is listed in the Flow Classification table that appears in the simulation’s Status Report.
The Minimum Surface Area option specifies a minimum area that is used at nodes when computing changes in water depth. If 0 is entered, then the default value of 12.566 sqft (1.167 sqm) is used. This is the area of a 4-ft diameter manhole. The value entered should be in square feet for US units or square meters for SI units.
Interface File Options
The Files page (Figure 9.2) in the Simulation Options dialog is used to specify which interface files will be used or saved during the simulation. The page contains a list box with three buttons underneath it. The list box shows the currently selected files. The command buttons are:
Add, which allows the user to add a new interface file specification to the list.
Edit, which allows the user to change the properties of the currently selected interface file.
Delete, which allows the user to remove the currently selected interface from the project (but not from the user's hard drive).
When the Add or Edit buttons are clicked, an Interface File Selector dialog (Figure 9.3) appears where the user can specify the type of interface file, whether it should be used or saved, and its name. The entries on this dialog are as follows:
The File Type allows the user to specify the type of interface file.
The Use or Save button selects whether the named interface file will be used to supply input to a simulation run or whether simulation results will be saved to it.
The File Name enters the name of the interface file or the user may click the Browse button to select from a standard file selection dialog box.