Option — StormNET Help

Simulation Options editor is used to set various options that control how a SWMM simulation is made. The editor consists of the following tabbed pages, click on links below to see details for each option.

General Option
Date Option
Time Step Option
Dynamic Wave Routing Options

The General tab sets values for the following options:

Field	Description
Process Models	Select which process models (Rainfall/Runoff, Rainfall Dependent I/I, Snow Melt, Groundwater, Flow Routing, and Water Quality) should be included in the analysis.
Infiltration Model	This option selects the default method used to model infiltration of rainfall into the upper soil zone of subcatchments. The choices are: Horton Modified Horton Green-Ampt Modified Green-Ampt Curve Number All new subcatchments added to a project will default to using the selected method. For existing subcatchments, their infiltration method will only change if they had been using the previous default option. That would require re-entering values for the infiltration parameters in each such subcatchment, unless the change was between the two Horton options or the two Green-Ampt options. A prompt is issued asking if SWMM should automatically assign a default set of parameter values to all subcatchments that switch between two incompatible types of infiltration methods.
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
Allow Ponding	Checking this option will allow excess water to collect atop nodes and be re-introduced into the system as conditions permit. In order for ponding to actually occur at a particular node, a non-zero value for its Ponded Area attribute must be used.
Minimum Conduit Slope	The minimum value allowed for a conduit's slope (%). If 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).

The Dates tab determines the starting and ending dates/times of a simulation.

Field	Description
Start Analysis On	Enter the date (month/day/year) and time of day when the simulation begins.
Start Reporting On	Enter the date and time of day when reporting of simulation results is to begin. Using a date prior to the start date is the same as using the start date.
End Analysis On	Enter the date and time when the simulation is to end.
Start Sweeping On	Enter the day of the year (month/day) when street sweeping operations begin. The default is January 1.
End Sweeping On	Enter the day of the year (month/day) when street sweeping operations end. The default is December 31.
Antecedent Dry Days	Enter 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 subcatchments.

The Time Steps tab of the Simulation Options 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.

Field	Description
Reporting Time Step	Enter the time interval for reporting of computed results.
Runoff - Dry Weather Time Step	Enter the time step length used for runoff computations (consisting essentially of pollutant buildup) during periods when there is no rainfall, no ponded water, and LID controls are dry. This must be greater or equal to the Wet Weather time step.
Runoff - Wet Weather Time Step	Enter the time step length used to compute runoff from subcatchments during periods of rainfall, or when ponded water still remains on the surface, or when LID controls are still infiltrating or evaporating runoff.
Control Rule Time Step	Enter the time step length used for evaluating Control Rules. The default is 0 which means that controls are evaluated at every routing time step.
Routing Time Step	Enter the time step length 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.
Steady Flow Periods This set of options tells SWMM how to identify and treat periods of time when system hydraulics are not changing. The system is considered to be in a steady flow period if: The percent difference between total system inflow and total system outflow is below the System Flow Tolerance, The percent differences between the current lateral inflow and that from the previous time step for all points in the conveyance system are below the Lateral Flow Tolerance. Checking the Skip Steady Flow Periods box will make SWMM keep using the most recently computed conveyance system flows (instead of computing a new flow solution) whenever the above criteria are met. Using this feature can help speed up simulation run times at the expense of reduced accuracy.

The Dynamic Wave tab 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.

Field	Description
Inertial Terms	Indicates how the inertial terms in the St. Venant momentum equation will be handled. KEEP maintains these terms at their full value under all conditions. DAMPEN reduces the terms as flow comes closer to being critical and ignores them when flow is supercritical. IGNORE drops the terms altogether from the momentum equation, producing what is essentially a Diffusion Wave solution.
Define Supercritical Flow By	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.
Force Main Equation	Selects which equation will be used to compute friction losses during pressurized flow for conduits that have been assigned a Circular Force Main cross-section. The choices are either the Hazen-Williams equation or the Darcy-Weisbach equation.
Surcharge Method	Selects which method will be used to handle surcharge conditions. The Extran option uses a variation of the Surcharge Algorithm from previous versions of SWMM to update nodal heads when all connecting links become full. The Slot option uses a Preissmann Slot to add a small amount of virtual top surface width to full flowing pipes so that SWMM's normal procedure for updating nodal heads can continue to be used.
Variable Time Step	Check the box if an internally computed variable time step should be used at each routing time period and select 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 the minimum variable step discussed below nor be greater than the fixed time step specified on the Time Steps page of the dialog.
Minimum Variable Time Step	This is the smallest time step allowed when variable time steps are used. The default value is 0.5 seconds. Smaller steps may be warranted, but they can lead to longer simulations runs without much improvement in solution quality.
Time Step for Conduit Lengthening	This is a time step, in seconds, 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 0 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 Summary Report.
Minimum Nodal Surface Area	This is a minimum surface area used at nodes when computing changes in water depth. If 0 is entered, then the default value of 12.566 sq. ft (1.167 sq. m) 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.
Maximum Trials Per Time Step	This is the maximum number of trials that SWMM uses at each time step to reach convergence when updating hydraulic heads at the conveyance system's nodes. The default value is 8.
Head Convergence Tolerance	When the difference in computed head at each node between successive trials is below this value the flow solution for the current time step is assumed to have converged. The default tolerance is 0.005 ft (0.0015 m).
Number of Threads	This selects the number of parallel computing threads to use on machines equipped with multi-core processors. The default is 1.