TUFLOW SWMM FAQ Draft

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TUFLOW-SWMM (engine and linkage)

Does TUFLOW-SWMM require a separate licence?

TUFLOW-SWMM doesn't need a separate licence module. It is included in the standard TUFLOW engine licence.

Do inlets in TUFLOW-SWMM use EPA SWMM's inlet capture at links or TUFLOW's inlet capture at nodes?

Inlets are calculated using the EPA SWMM inlet methods, the approach flows and velocities are calculated in the 2D and this information is passed to the EPA SWMM at each timestep.

QGIS Plugin SWMM Tools

When converting a SWMM inp file to SWMM GPKG, is everything supported?

Everything from the SWMM inp file gets converted except for pollutants and pollutant related information.

Can we use the TUFLOW plugin to work with SWMM data without using TUFLOW?

Yes, that is fine. Let us know if you find the plugin useful or give it stars in the plugin repository.

With the ability to work with the files in GIS, is there ever a reason to use the EPA SWMM GUI?

Yes. Rather than think about using the plugin tools or the EPA SWMM GUI, think about how to use the plugin tools and the EPA SWMM GUI. Some features such as Low-Impact-Developments are easier to edit in the EPA SWMM GUI than in GIS tables. Because the systems are interchangeable using SWMM inp files, you can leverage the benefits of both systems.

How would you get data from a non-SWMM GIS format into the SWMM GPKG format?

The plugin can help with converting this data by creating the template for the input layers. You would create the SWMM GPKG file using the plugin tools. You would need to do some data translations to convert the attribute data into the correct fields. Reach out to support@tuflow.com if you need advice on how best to do this.

How can I review SWMM results?

If run as a TUFLOW-SWMM model, the SWMM results can be viewed in the QGIS TUFLOW Viewer in the same way as ESTRY results. Working with the results in the Viewer plot window is no different to ESTRY. SWMM does include the option to symbolize nodes and conduits using model results which is not yet supported when reviewing ESTRY outputs.

Can I work with SWMM GeoPackages in ArcGIS Pro?

ArcGIS Pro supports GeoPackages so this data can be viewed and edited in ArcGIS Pro. The simplest option would be to do conversions in QGIS and then work in ArcGIS Pro but would require closing the files in QGIS before working with them in ArcGIS Pro. If you have python experience, it should be possible to run the conversion tools using the ArcGIS Pro python environment but this has not been tested. Reach out to support@tuflow.com for assistance if you would like to go this route.

Can I look at TUFLOW-SWMM model results in ArcGIS Pro?

Yes. The SWMM 1D time-series results are written to GeoPackage files that can be read by ArcGIS Pro. It is possible to display time-series graphs. You can symbolize the nodes and links using the results which can be animated through the simulation. Especially when incorporating time, it sometimes works better to save the GeoPackage layers to an ESRI File Geodatabase for post-processing. For the 2D results, NetCDF files either the High-Resolution (HR) or standard results can be brought into ArcGIS Pro and linked to the time slider for reviewing results or creating animations. Sometimes converting this data to the ESRI CRF (Cloud Raster Format) works better especially when generating animations.

Comparisons to other Models and Model Conversions

ESTRY

Do the results generated by the SWMM engine differ much from the ESTRY engine?

Different engines will never produce identical results, however, SWMM and ESTRY results should be generally similar. It will depend a little on the features in the model, for example, SWMM does not have an approach specifically for manhole losses and does not support dynamic losses for inlet and exit losses like ESTRY. We had a model we converted from TUFLOW-ESTRY to TUFLOW-SWMM and were getting quite different results. The ESTRY input files (and therefore the converted SWMM files) used full inlet and outlet losses (0.5 and 1.0). However, because ESTRY adjusts these dynamically, the applied losses were much lower. Once we used the average applied losses in SWMM, the results became very similar.

How do I choose whether to use SWMM or ESTRY?

SWMM and ESTRY are similar 1D models solving the same equations using a similar solution scheme. Either model is capable of providing excellent results as part of a 1D/2D model. Both models can be setup in GIS and can load boundary conditions from the BC database. Both support operable structures though the syntax for instructions varies. Overall, the differences are mostly minor. The biggest deciding factors are probably familiarity of the engine with the modelers and client preferences.

Some of the differences include:

ESTRY

• Dynamic losses for entry, exit, and manhole losses (see previous question)
• Support for bridges as a separate channel type
• 2D output for open channel sections using water level lines
• Flexible virtual pit inlets and outlets (some of the same functionalities can be obtained in SWMM using dummy conduits but with less automation and flexibility)

SWMM

• Supports for subcatchment based hydrology feeding directly to 1D nodes in addition to rain on grid
• Has built-in support for Low Impact Developments (LIDS) which requires the use of subcatchment based hydrology
• Supports on-grade inlets (pits) to determine flows from the 2D based on approach flows in addition to water level (ESTRY supports water level only but on grade can be approximated)
• Has additional defined channel geometries (these can be used in ESTRY with manually defined Height vs Width tables)

How would I convert an ESTRY model to SWMM?

There is a tool in the plugin for converting ESTRY layers to SWMM GeoPackage layers.

PCSWMM

PCSWMM uses SWMM by itself to run 1D/2D urban drainage models. How is this different from what TUFLOW-SWMM is doing?

Our understanding is it represents the overland flow that it is running the SWMM engine using a lattice of 1D cells in multiple directions. If you look at the 1D SWMM equations and the 2D equations used by TUFLOW, you can see that the 2D equations include additional parameters for things like turbulence and some components in the x direction affect the y direction (and the reverse). Because of this, running a 1D model in multiple directions is not the same as running a full 2D model. Depending on the situation the results may be similar. SWMM is also not optimized to be a 2D solver. We were given a SWMM based 1D and overland flow model that took hours to run. We then converted the model to TUFLOW-SWMM with similar resolution and it ran in under 10 minutes.

XPSWMM

Why would I convert existing models from XPSWMM to TUFLOW-SWMM and how hard is it?

TUFLOW-SWMM is a natural choice for migrating existing XPSWMM models. It uses the same 2D engine with SGS and Quadtree and a newer version of the same 1D engine, so generally results should be nearly identical when converted. There may need to be some setup changes that might lead to minor differences in results. There are tools to convert existing models that do most of the work during a conversion, so it is quite easy. Uncommonly used features may require manual editing. Reach out to support@tuflow.com for assistance.

Can XPSWMM models be converted to TUFLOW-SWMM without an XPSWMM licence?

Depending on how the model was archived, this is possible. There are two parts to converting an XPSWMM model. The hydrology and 1D portions and the 2D model data. The hydrology and 1D model components can be written using the XPSWMM Viewer (2024+) without a license. The 2D model data is written by XPSWMM when running the model and is required for a full conversion. If these were not kept when the model was run, the model will need to be rerun from a licensed version of XPSWMM before conversion. Because XPSWMM writes these files using absolute paths, the file paths in the file will need to be modified if converting on a different machine or file path than was used when running the model. Reach out to support@tuflow.com for help resolving these issues.