6.2 Model Setup

6.2.1 SWMM Model

The SWMM portion of a TUFLOW-SWMM model is read from one or more SWMM project files (inp). These files can be created within the EPA SWMM GUI, the TUFLOW GIS tools described in Section 6.3, or other tools (for example, exported from other software that have adopted SWMM 5 as their 1D solver).

The EPA Website provides documentation on SWMM model theory and setup. This manual does not contain a description of how to build SWMM models or the SWMM file formats since this information is readily available from the resources provided in Section 6.1.2. The TUFLOW-SWMM Tutorial Modules demonstrate how to build TUFLOW-SWMM models using the TUFLOW GIS tools.

6.2.2 TUFLOW-SWMM Model Simulation

When running a TUFLOW-SWMM model, TUFLOW controls the execution of the simulation. SWMM input commands are specified within a SWMM Control File (*.tscf), documented in Appendix I. The SWMM Control File is referenced in the TUFLOW Control File (*.tcf) using the SWMM Control File command, documented in Appendix A.

The SWMM Control File can reference one or more SWMM input inp files using the Read SWMM command. If multiple input inp files are used, TUFLOW merges the files into a single input inp before simulation. During the merger, if there are SWMM objects that are duplicate in terms of location and “Name” the item that was defined lower in the SWMM Control File (*.tscf) takes precedence. This merger approach has been adopted to allow modellers to override existing objects easily for options assessments (such as, to assess the upgrade of a section of pipe network). After the input files are merged, the final SWMM model in inp format is placed in the TUFLOW output folder. The SWMM model runs from the output folder. The SWMM report and output files are also written to the TUFLOW output folder.

6.2.3 Linking SWMM to TUFLOW 2D and ESTRY

For information relating to the linking of SWMM 1D with TUFLOW 2D, and the linking of SWMM 1D to TUFLOW 1D (ESTRY), see Section 10.4.

6.2.4 SWMM Simulation Option Settings

The SWMM model settings are primarily dependent on the settings defined in the Project–Options section of the SWMM inp file. However, some settings are controlled by TUFLOW when running a TUFLOW-SWMM model, including:

  • END_DATE/END_TIME: Set based upon the SWMM START_DATE/START_TIME and the TUFLOW simulation duration.
  • FLOW_UNITS: the flow units are set to match the TUFLOW simulation, either to CFS for US Customary units (if using Units == US Customary) or CMS for metric units.
  • FLOW_ROUTING: Flow routing is set to DYNWAVE.
  • ALLOW_PONDING: Allow ponding is set to YES. Ponding can be prevented by setting a high Junction attribute value for “YSur” (higher than the maximum water elevation) or setting a Junction “Apond” value to 0.0. Note, it is recommended that only Junctions with TUFLOw 1D/2D HX or SX boundaries should have the Apond attribute set to 0.0, which will create flooding in the 2D domain when the Junction is surcharged.
  • END_DATE/END_TIME: These are set based on the TUFLOW end time and the SWMM start time (simulation duration + start date/time = specified end date/time).
  • VARIABLE_STEP: This is turned off (0). TUFLOW will control the timestep based on the HPC (Heavily Parallelised Compute) timestep and the SWMM Iterations command.

6.2.5 Using the TUFLOW BC Database in SWMM

The fixed structure of SWMM restricts it to the simulation of a single event per SWMM inp file. Manual definition of SWMM boundary condition inputs would become an onerous task during the execution of a project requiring simulation of multiple events. TUFLOW-SWMM has overcome this SWMM limitation by enabling the communication of boundary condition timeseries data from the TUFLOW Boundary Condition Database to SWMM. This feature upgrades SWMM to use TUFLOW’s powerful and flexible event management options. Any number of event simulations can now be executed from a single SWMM inp input file. TUFLOW automatically assigns the chosen boundary condition inputs.

The connection between the TUFLOW Boundary Condition Database and the SWMM model is established by using the SWMM Control File (*.tscf) command, Read BC <curve_type>. The “SourceType” in the SWMM inp file also needs to be set to TIMESERIES for time varying data, such as rainfall depth vs time or discharge vs time to be passed seamlessly to SWMM from TUFLOW. The SWMM Timeseries name must also be specified (matching the TUFLOW Boundary Condition Database Name). TUFLOW writes the event specific information to the SWMM inp file it creates, prior to its simulation.TUFLOW-SWMM Tutorial Module 4 is a demonstration of this TUFLOW-SWMM boundary condition model configuration.