Routing in SWMM5
Flow routing within a conduit link in SWMM is governed by the conservation of mass and momentum equations for gradually varied, unsteady flow (i.e., the Saint Venant flow equations). The SWMM user has a choice on the level of sophistication used to solve these equations:
|·||Steady Flow Routing|
|·||Kinematic Wave Routing|
|·||Dynamic Wave Routing|
Each of these routing methods employs the Manning equation to relate flow rate to flow depth and bed (or friction) slope. For user-designated Force Main conduits, either the Hazen-Williams or Darcy-Weisbach equation can be used when pressurized flow occurs.
|Starting a Simulation|
To stop a run before its normal termination, click the Stop button on the Run Status window or press the <Esc> key. Simulation results up until the time when the run was stopped will be available for viewing. To minimize the SWMM program while a simulation is running, click the Minimize button on the Run Status window.
If the analysis runs successfully the icon will appear in the Run Status section of the Status Bar at the bottom of SWMM’s main window. Any error or warning messages will appear in a Status Report window. If you modify the project after a successful run has been made, the status flag changes to indicating that the current computed results no longer apply to the modified project.
|Setting Simulation Options|
SWMM has a number of options that control how the simulation of a stormwater drainage system is carried out. To set these options:
|1.||Select the Options category from the Project Browser.|
|2.||Select one of the following categories of options to edit:|
|·||Time Step Options|
|·||Dynamic Wave Routing Options|
|·||Interface File Options|
|3.||Click the button to invoke the appropriate editor for the chosen option category (the Simulation Options dialog is used for the first 5 categories while the Reporting Options dialog and the Events editor are used for the last two, respectively).|
|Simulation Options Dialog|
The Simulation Options dialog is used to set various options that control how a SWMM simulation is made. The dialog consists of the following tabbed pages:
|·||Time Step Options|
|·||Dynamic Wave Routing Options|
|·||Interface File Options|
After selecting the desired options, click the OK button to save your choices or the Cancel button to abandon them.
|Simulation Options – Time Steps|
The Time Steps page of the Simulation Options dialog 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.
Reporting Time Step
Enter the time interval for reporting of computed results.
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.
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.
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:
|1.||The percent difference between total system inflow and total system outflow is below the System Flow Tolerance,|
|2.||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.
|Simulation Options – Interface Files|
The Interface Files page of 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 lists the currently selected files, while the buttons are used as follows:
|Add||adds a new interface file specification to the list.|
|Edit||edits the properties of the currently selected interface file.|
|Delete||deletes the currently selected interface from the project (but not from your hard drive).|
When the Add or Edit buttons are clicked, an Interface File Selection dialog appears where you can specify the type of interface file, whether it should be used or saved, and its name.
|Simulation Options – General|
The General page of the Simulation Options dialog sets values for the following options:
Select which process models (Rainfall/Runoff, Rainfall Dependent I/I, Snow Melt, Groundwater, Flow Routing, and Water Quality) should be included in the analysis.
This option controls how infiltration of rainfall into the upper soil zone of subcatchments is modeled. The choices are:
Changing this option will require re-entering values for the infiltration parameters in each subcatchment, unless the change is between the two Horton options or the two Green-Ampt options.
This option determines which method is used to route flows through the conveyance system. The choices are:
See the Flow Routing topic for more details.
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).
|Simulation Options – Dynamic Wave|
The Dynamic Wave page of 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.
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.
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.
Clicking the Apply Defaults label will set all the Dynamic Wave options to their default values.
|Simulation Options – Dates|
The Dates page of the Simulation Options dialog determines the starting and ending dates/times of a simulation.
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.
|If rainfall or climate data are read from external files, then the simulation dates should be set to coincide with the dates recorded in these files.|
|Dynamic Wave Routing|
Dynamic Wave routing solves the complete one-dimensional Saint Venant flow equations and therefore produces the most theoretically accurate results. These equations consist of the continuity and momentum equations for conduits and a volume continuity equation at nodes.
With this form of routing it is possible to represent pressurized flow when a closed conduit becomes full, such that flows can exceed the full normal flow value. Flooding occurs when the water depth at a node exceeds the maximum available depth, and the excess flow is either lost from the system or can pond atop the node and re-enter the drainage system.
Dynamic wave routing can account for channel storage, backwater, entrance/exit losses, flow reversal, and pressurized flow. Because it couples together the solution for both water levels at nodes and flow in conduits it can be applied to any general network layout, even those containing multiple downstream diversions and loops. It is the method of choice for systems subjected to significant backwater effects due to downstream flow restrictions and with flow regulation via weirs and orifices. This generality comes at a price of having to use much smaller time steps, on the order of thirty seconds or less (SWMM can automatically reduce the user-defined maximum time step as needed to maintain numerical stability).
|Kinematic Wave Routing|
This routing method solves the continuity equation along with a simplified form of the momentum equation in each conduit. The latter assumes that the slope of the water surface equal the slope of the conduit.
The maximum flow that can be conveyed through a conduit is the full normal flow value. Any flow in excess of this entering the inlet node is either lost from the system or can pond atop the inlet node and be re-introduced into the conduit as capacity becomes available.
Kinematic wave routing allows flow and area to vary both spatially and temporally within a conduit. This can result in attenuated and delayed outflow hydrographs as inflow is routed through the channel. However this form of routing cannot account for backwater effects, entrance/exit losses, flow reversal, or pressurized flow, and is also restricted to dendritic network layouts. It can usually maintain numerical stability with moderately large time steps, on the order of 1 to 5 minutes. If the aforementioned effects are not expected to be significant then this alternative can be an accurate and efficient routing method, especially for long-term simulations.
|Steady Flow Routing|
Steady Flow routing represents the simplest type of routing possible (actually no routing) by assuming that within each computational time step flow is uniform and steady. Thus it simply translates inflow hydrographs at the upstream end of the conduit to the downstream end, with no delay or change in shape. The normal flow equation is used to relate flow rate to flow area (or depth). rout
This type of routing cannot account for channel storage, backwater effects, entrance/exit losses, flow reversal or pressurized flow. It can only be used with dendritic conveyance networks, where each node has only a single outflow link (unless the node is a divider in which case two outflow links are required). This form of routing is insensitive to the time step employed and is really only appropriate for preliminary analysis using long-term continuous simulations.
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