The following section describes the the parameters used by the Hydraulics Mode Job Control Facility. Layer follows. 

Note

  • NEW FOR 2016: Users can now set the Initial Water Surface Elevation (configuration parameter ZREF=X) from the Hdr Job Control dialog. Read the Initial Water Surface Elevation section for details.
  • The Title and Simulation control fields are optional. Time Control data are required.

Title

Description for simulation run (2 lines - 80 characters maximum). This will be printed as the title in the output file. The first line is common for all layers; the second line is specific to the Hydraulics layer.

Simulation Tolerances

The following tolerances may be used to control the simulation:

Flow Tolerance

The iterative solution technique uses an under-relaxation technique to solve the dynamic flow equation and continuity equation. The convergence criterion for this method is related to the maximum number of iterations and relative accuracy, specified in the Job Control dialog.

This is the convergence criterion used during flow iterations. If the difference in both conduit flow and cross-sectional area between successive iterations is less than this value (SURTOL) and the number of iterations is less than the maximum allowable (ITMAX), then convergence is deemed to have occurred. The convergence error for flow is given by:

Error = (Qnew - Qold) / Qnew

where:

Qnew = Current iteration value of flow in the conduit

Qold = Previous iteration value of flow in the conduit

Typical values used in simulations are 0.001 to 0.005. Conduits that appear to have small instabilities in flow or velocity can be fixed by using a smaller flow tolerance.

The flow convergence is tested for all CONDUITS, WEIRS, ORIFICES, PUMPS, and OUTFALLS in the model.

Head Tolerance

This is the convergence criterion used in the continuity equation at junctions. If the difference between successive iterations of junction depth is less than this value (SURJUN) and the number of iterations is less than the maximum allowable (ITMAX), then convergence is deemed to have occurred. The convergence error for junction depth is given by:

Error = (Ynew - Yold) / Ycrown

where

Ynew = Current iteration value of depth at the junction

Yold = Previous iteration value of depth at the junction

Ycrown = Total depth of the junction, ground - invert elevation

Typical values used in simulations are 0.001 to 0.005. 

Minimum Orifice Length

The minimum orifice length in m(ft) for creating an equivalent circular or rectangular conduit for orifices. Typical values are 100 m to 300 m

Default Head Loss Coefficient

The junction head loss coefficient. This multiplier of the velocity head applies both to entrance and exit losses at a junction. The loss is actually modelled in the conduit momentum equation since only a continuity equation is used at the junctions. This coefficient is global and applies to all conduits, unless a value is also entered in the special conduit dialog for that conduit.

Default Contraction Loss

The abrupt cross section shape change from one conduit to the next creates turbulence. The loss in velocity from this change can be modelled by using a contraction/expansion loss coefficient. This coefficient is global and applies to all conduits unless a value is also entered in the special conduit dialog.

Routing Control

By selecting Routing Control users can control the stability of the model results and the speed of execution of the run. This dialog contains the normalised tolerances the program uses to decide whether a conduit or junction has achieved convergence.

 

Performance/Stability Factors

XPSWMM/XPStorm uses an enhanced version of the EPA-SWMM Version 4 Iterative Explicit Solution to solve the gradually varied unsteady flow (St Venant) equations. The convergence criteria for this method is related to the maximum number of iterations and the relative accuracy of model and conduit computations as specified in the simulation tolerances dialog.

Convergence is deemed to be satisfied if, within the maximum number of iterations specified, successive iterations yield results within the tolerance specified by the relative accuracy.

Explicit methods are "conditionally" stable and often require short time steps. Experience has found that the program will be numerically stable when the time step chosen is limited to the time required by a dynamic wave to propagate the length of the shortest conduit.

This is known as the Courant time step and it is printed in a table in the output file. The user should ensure that the time step chosen is less than or equal to the Courant time step listed in the output file.

The Iterative Method uses a variable time step. The time step entered by the user in the Job Control Dialog is set as the maximum allowable time step used during the simulation. The model will select a time step based on the smallest conduit Courant time step at the beginning of each long time step. The model then determines the number of equal length small time steps required to equal the long time steps. This results in a saving in computation time and a higher degree of stability for a given situation.

Performance and Stability are affected by a number of factors, including:

Under Relaxation Parameter (OMEGA)

The under relaxation parameter used during the simulation. It typically has values between 0.60 and 0.85 and should be used in conjunction with the Time-weighting factor (THETA) to minimise hunting and seeking behaviour. The parameter is used in the following way in the program:

Newest = New * omega + (1-omega)*Old

where

Old = Value at previous iteration,

New = New iteration value, and

Newest = Weighted new value after under-relaxation.

Time Weighting Parameter (THETA)

The implicit time weighting for all conduits. Typically, this parameter should have a value between 0.55 and 1.0. This parameter is used to decrease the oscillations in “hunting and seeking”. 

Conduit Roughness Factor (FMAX)

A global adjustment to conduit roughness. Used as an aid in calibration of your model. The conduit roughness is multiplied by this factor (i.e. a value of 1.20 increases the roughness by 20 percent). The default value = 1.0, or no change.

Flow Adjustment Factor (QREF)

A global adjustment factor for all flows from the interface file and user-defined time series. This allows the user to globally alter the inflows to the model by a constant factor.

Initial Condition Smoothing (ISMTH)

Instead of using a hot start file to make the initial conduit conditions smoother it is now possible to use a new parameter, ISMTH, to dampen oscillations arising from the choice of initial conditions. This parameter works as follows:

ISMTH = 0 All conduits use their normal roughness.

ISMTH > 0 All conduits have their roughness increased by a factor of ten for ISMTH time steps.

The increase in roughness has a dampening effect on the potential buildup of initial transients. Typical values of ISMTH would be 10 to 100 time steps. 

Minimum Courant Time Step Factor (FMIN)

As part of the solution the model calculates the minimum courant time step at each big time step based on the previous conduit depth and velocity. The time step used in the model is based on the calculated courant time step multiplied by the courant time factor. The default time step factor is 1.0, and typical values are 0.5 to 3.0.  

Max. Time Step Iterations

The maximum number of iterations allowed during a time step. Typically, the maximum number should be 50 to 100. If convergence is not achieved by this number of iterations then the model moves on to the next time step and accepts the last value computed.

Report Non-Convergence

By selecting this option the model will display, during run-time, those junctions that fail to reach convergence by the time the maximum number of iterations it reached. |The junction name and the junction heads of the current and previous iteration will be outputted to the screen, and if no junctions are reported then all junctions have reached convergence

Hydrograph Methods

Dynamic Wave

This option implements all terms in the St Venant equations as discussed in the theory section of this manual and automatically adjusts the time step as mentioned above. This option is the default method and is recommended where there is any chance of a backwater effect in the system or where tidal boundary conditions are being modelled.

Kinematic Wave

This option implements a sub-set of the dynamic wave option above and is discussed in the theory section of this manual. The kinematic wave method does not allow for backwater effects or modelling of tidal boundary conditions.

Conduit Equivalencing (Modify Conduits)

An equivalent pipe is the computational substitution of an actual element of the drainage system by an imaginary conduit which is hydraulically identical to the element it replaces (except for volume). Usually, an equivalent pipe is used when it is suspected that a numerical instability will be caused by the element of the drainage system being replaced in the computation. 

Weirs and short conduits are known at times to require short time steps and thus may occasionally be replaced by an equivalent pipe. Orifices are automatically converted to equivalent pipes by the model.

The equivalent pipe substitution used by the model involves the following:

First the flow equation for the element in question is set equal to the flow equation for an "equivalent pipe." In effect, this says that the head losses in the element and its equivalent pipe are the same. The length of the equivalent pipe is computed using the numerical stability equation. Then, after making any additional assumptions which may be required about the equivalent pipe's dimensions, a Manning's 'n' is computed based on the equal head loss requirement. In the case of orifices, this conversion occurs automatically, but in those cases where short pipes or weirs are found to cause instabilities, the user must make the necessary decision and select the appropriate equivalent pipe option.

The desired equivalent pipe length is either no shorter than 1/4 or 1/5 of the length of the longest pipe in the system, or large enough to give a delta tc within the range indicated above. Through experience, the user will be able to determine the pipe length changes required to achieve the desired result and an acceptable time-step for the simulation.

By choosing a value of 1.0 for the equivalent pipe option the model will automatically adjust the pipe or channel lengths using an equivalent longer length to achieve a delta tc in balance with the user-selected time-step (delta t). All pipes in which (delta t)/(delta tc) exceeds 1.0 will be adjusted, with the new pipe/channel lengths and roughness printed.

When a value greater than 1.0 is used an equivalent pipe or channel length will be created based on this value (in seconds). For example, specifying a value of 15 will create an equivalent pipe based on a time step of 15 seconds. A before and after analysis of the full flow-system volume is printed by the model for values greater than 1. This enables the user to estimate the effect of the increase in system volume from using equivalent pipes or channels.

The user can modify the pipe length and roughness below or above limits specified in this dialog box. The model will automatically create an equivalent longer or shorter pipe for the affected conduits.

The model will create equivalent pipes based on the specified minimum and maximum lengths. This can be limited to modifying Manning's n only and/or the full extent of pipe hydraulic characteristics.

The following are the available available: 

  1. Preprocesses 'n' Only. When this option is set the model modifies the Manning's n value for all pipes that are below the minimum or above a maximum threshold set in adjacent fields in this dialog box.
  2. Minimum Pipe Length for Modification. Minimum pipe length below which the model will make equivalent pipe assumptions by modifying Manning's n and/or pipe characteristics to optimise stability for particular time steps. 
  3. Maximum Pipe Length for Modification (NEQUAL). Maximum pipe length above which the model will make equivalent pipe assumptions by modifying Manning's n and/or pipe characteristics to optimise performance for particular time steps.

Equivalent to Time Step Factor (NEQUAL)

When this option is set the model modifies the pipe characteristics for pipes that have a Courant time step below the value set in the adjacent field in this dialog box. The characteristics are optimised for particular time step ratios to improve performance.

Junction Defaults

This option defines the default values relating to junctions, such as Depth and Ponding area volume parameters.

Default Node Surface Area (AMEN)

This is the default surface area for all manholes in ft2 (or m2). It is used for surcharge calculations in the model. The manhole default diameter is 4ft (or 1.22m).

Minimum Junction/Conduit Depth

Enter the smallest allowable junction and conduit depth (feet or metres).

Ponding Area

This is used to define the mechanism used for junction flood storage. The default mechanism is for surcharged water that breaks the ground surface to be lost from the network. However, using this option will store the flooded water at this node until the conduit can handle the excess amount of water.

The coefficient used in the exponential equation for the ponded area of the flooded junction has a typical value of 1,000 to 5,000.

The exponent used in the exponential equation for the ponded area of the flooded junction has a typical value of 1.0 to 5.0. Note: A value of zero will default to 1, so use a small value such as 0.00001 to be effectively 0 and turn the storage area to essentially constant.

Output Control

Starting Time Step for Print Cycle (NSTART)

This is the cycle number at which the intermediate printout facility will begin its output. Intermediate printouts will then be continued at intervals specified below. The starting printout is a hold-over from previous versions of EXTRAN and was used for an intermediate review of the model in a search for the source(s) of problems.

Intermediate Print Cycle Interval (INTER)

This value is specified as a number of cycles, and is the interval between intermediate print cycles during the simulation. The intermediate printouts start at the cycle specified in the start print field and are repeated at the specified intervals. 

This value controls the interval at which heads, velocities, and flows are printed during the simulation (intermediate printout), beginning at the starting time step specified above (NSTART). Surcharge information during the simulation is also printed at these intervals.

Intermediate printout is produced for all junctions and conduits, whereas the summary printouts are only produced for those specified by the user. The intermediate printout is very useful in case an error occurs before the program reaches its desired simulation length, however it tends to produce bulky output.

If intermediate printout is to be avoided entirely, set this value to a number greater than total number of cycles in the simulation period. Nodal water depth, elevation, conduit flow, and velocity are all printed in the intermediate printout.

The output looks better if the starting cycle for intermediate printouts and this value are selected so that the first and subsequent output occurs at an even number of minutes or half-minutes.

Summary Print Cycle Interval (JNTER)

This value is specified as a number of cycles, and refers to the interval between time-history summary print cycles at the end of the simulation. The number of cycles printed is equal to the total number of cycles in the simulation period divided by this value.

Summary printouts are only produced for those nodes and conduits specified by the user whereas the intermediate printout is produced for all junctions and conduits.

Echo Natural Section Data

If this flag is enabled any HEC-2 style conduits that are present in your network will have their Station and Elevation data echoed in the output file.

Gauged Pollutants

This flag and the associated dialog allows the definition of the list of pollutants to be used in the Gauged Pollutant List. If this flag is OFF, no gauge pollutants are shown. If this flag is ON, a list of pollutants is selected from the list of gauged pollutants shown in the Pollutants Global Database.

The pollutants to be used are chosen from those defined in the Pollutants Global Database. Any number of pollutants may be defined. To add a pollutant to the "Use Pollutants" list, highlight the pollutant required in the Pollutant List (left-hand list), then select the "Add" button. To remove a pollutant from the list select the pollutant you wish to delete from the "Use Pollutants" list (right-hand list), then select the "Delete" button.

The pollutant load and concentrations may be generated in the Hydraulics, Runoff or Sanitary layers, or read from an interface file.

Hot Restart (REDO)

A hot restart facility allows a file to be read and/or created to establish initial conditions for a run. This may avoid re-running of, for example, dry-weather flow conditions prior to the start of a storm runoff simulation.

The user can create a hot start file from a normal run or from a previous hot start run. Only one hot start file can be created.

Start Cold, Create Hot-Start File

If this option is selected then the model does not use a hot restart file to set initial conditions and creates a hot restart file for the next simulation run.

Start Hot Using Hot-Start File

If this option is selected then the model uses a hot start file to set initial conditions and does not create a new hot restart file for the next simulation run.

Start Hot, Create New Hot-Start File

If this option is selected then the model uses a hot restart file to set initial conditions and then creates a new hot restart file for the next simulation run.

Pollutant List

Defines the list of pollutants to be used in the simulation. Any number of pollutants may be defined, but a maximum of 20 may be used in any particular simulation (This is the typical configuration but if errors are encountered the SWMM.PAR file should be checked for the actual maximum number allowed).

The pollutants to be used are chosen from those defined in the 'Pollutants' Global Database. To add a pollutant to the "Use Pollutants" list, highlight the pollutant required in the Pollutant list (left-hand list), then select the "Add" button. To remove a pollutant from the list select the pollutant you wish to delete from the "Use Pollutants" list (right-hand list), then select the "Delete" button.

Design Constraints

Use this option to allow xp to select pipe sizes based on user defined criteria. 

The Available Pipes option is not allowed if the Limit d/s discharges optimization is enabled in the Basin Optimization dialog.

Available pipes

Use this dialog to define the catalog of pipe diameters to be used during conduit design. Units are (m) for metric and (ft) for US Customary design.

Design For

The following options are available for this section: 

No Design. Pipe diameters are not changed

% of Depth. Pipe diameters are selected based on a maximum % of depth

Minimum Cover. Sets the minimum freeboard and cover. Units can be in meters or feet

Evaporation

Evaporation is used to renew surface depression storage and is also subtracted from rainfall and/or snowmelt at each time step. It has a negligible effect on single event simulation (ie. less than a week long), but is important for continuous simulation.

This parameter is not used to deplete the snow pack. Ie., it does not act as sublimation, nor does it affect regeneration of infiltration capacity. The evaporation input in Hydraulics Mode also acts as an upper bound for evapo-transpiration losses from groundwater and soil moisture. Evaporation also occurs from open channels such as trapezoidal, parabolic or natural channels.

Save All Results for Review

This flag will save all the results for nodes and conduits from analysis to a special file for later graphical post-processing.

Run Hydrology/Hydraulics Simultaneously

Check this box to make XPSWMM/XPStorm calculate the Hydrology and the Hydraulics simultaneously at each time step. Under this option, the Solve Mode (under Configuration > Mode Properties) should have Runoff and Hydraulics modes checked and any Flow Interface File (under Configuration -> Interface Files) will not be used.  Interface files are used to pass the flows from Runoff to Hydraulics nodes when the solve is not simultaneous.  Simultaneous solves are recommended. 

Time Control

The Time Control section of the Hydraulic Job Control contains the following time control input fields.

Simulation Start Time (TZERO)

Start and end of the simulation are defined by entering the year month, day, hour, minute and second. For the Simulation Start Time (TZERO), enter the starting date and time of the simulation.

Simulation End Time (TSL)

For the Simulation End Time (TSL), enter the ending date and time of the simulation.

While it is only necessary to enter the last two digits of the year it is good practice to use four digits to remove any year 2000 ambiguity.

Simulation Time Step (DELT)

The simulation Time Step (DELT) is input as seconds into the Time Control section of the Hydraulic Job Control and discussed further in the Time Step (DELT) section. The simulation results values calculated at the simulation Time Step are reported in the *.out file and in XP-Tables.

Save Results Every (SAVERES)

The Save Results Every time step can be input as either seconds or minutes and is the time step used to produce the hydrographs reported in Review Results, Dynamic Long Section and Dynamic Section Views. The hydrographs produced for Review Results, Dynamic Long Section and Dynamic Section Views only report data points at the time interval selected in Save Results Every. The Save Results Every time step will be larger than the simulation Time Step and will produce hydrographs with fewer points than what is calculated in the simulation. This will cause the reported peak results values in Review Results, Dynamic Long Section and Dynamic Section Views to be less than the actual peak values taken from the entire simulation, which are reported in the *.out file and within XP-Tables.

Initial Water Surface Elevation

The configuration parameter ZREF=X, where X is the initial elevation for all nodes in the model, can be set from the Initial Water Surface Elevation field. To enable this parameter, place a check mark on the box, and enter the desired values in the provided field. 

 This parameter applies globally to all objects in the model. Varying levels are supported in each node dialog.