AquaSWMM

Migration guide

Opening a Hydraflow Storm Sewers project in AquaSWMM

Hydraflow Storm Sewers is the Rational-method storm-sewer extension for Autodesk Civil 3D. AquaSWMM opens its .stm project files directly. There is no export step and no intermediate format. You get two things from one file: the same kind of Rational-method design, with flows, capacities and a hydraulic grade line, and a full EPA SWMM 5 model of the same network that you can run in dynamic wave.

AquaSWMM is a standalone desktop app for Windows, macOS and Linux. It costs $490 a seat per year, or $1,200 for five seats, and it doesn’t need Civil 3D to run.

1. Open the .stm file

Use File > Import > Hydraflow / Civil 3D Storm Sewers (.stm)…, or simply File > Open, or drop the file on the window (manual §13.1). AquaSWMM reads the text .stm format, including the files Civil 3D’s Storm Sewers writes. The source file is never overwritten: the converted model has no path until you save it as a .inp.

What comes across:

  • The network. Lines and structures with their coordinates, each line’s own upstream and downstream inverts, rims, sizes and shapes (circular, box, elliptical) and the pipe’s Manning’s n.
  • The drainage areas. Each line’s area, runoff coefficient C and inlet time.
  • The design basis. The project’s IDF curves, return period, loss coefficients and starting HGL (tailwater). They become the design basis for Analyze (manual §12.1).
  • Inlets. Grate size and cross slope, for the HEC-22 inlet check.
  • The drawing. The project’s background image becomes the backdrop, placed where the plan had it, and its DXF underlay stays the underlay.

2. Run the Rational-method design: Analyze

Analyze (F6) runs AquaSWMM’s own storm-sewer engine on the converted model: Rational method, Manning, standard-step HGL, HEC-22 inlets and access-hole losses, catalog sizing and a design review. It writes a design report as PDF or HTML (manual chapter 12). The equations are in chapter 15.

If you compare the report with your Hydraflow printout line by line, expect the inputs to match (slopes, ΣC·A, intensity for a given Tc) and the results to be close but not identical. Most of the difference comes from three choices AquaSWMM makes:

  • Velocity for travel time. AquaSWMM times each line at its normal-depth velocity, even where backwater would slow the flow. Travel times can come out shorter and flows on the lower lines slightly higher, which is the conservative side for sizing.
  • Surcharged lines. AquaSWMM applies the full-flow friction slope at the design flow over the whole length of a pressurised line, so it can show a little more head than a method that averages friction along the reach.
  • Structure loss coefficients. AquaSWMM uses one access-hole K for the project, and the importer takes the most common value in your file. Where your project set a different K at some structures, such as a higher one at terminal inlets, the HGL there will differ.

The method is set out in manual chapter 15. Document the differences in your submittal the way you would any change of software.

3. Run the same network in EPA SWMM 5

The import also writes a standard SWMM model (manual §13.1). Junctions and outfalls get their inverts and rims. Conduits keep their lengths, n values and offsets. Each drainage area becomes a subcatchment, and the gage gets a 2-hour design storm built from your IDF curve by the alternating-block method.

That lets you check things a Rational-method run can’t show: timing, storage and backwater from a real hydrograph. AquaSWMM runs EPA’s runswmm.exe unmodified and stamps every run with the engine version and the SHA-256 of the executable. The saved .inp stays EPA’s format, so it also opens in the free EPA SWMM program.

Review the hydrology before you rely on the SWMM run. A .stm file holds a C value, not the impervious fraction, width and slope that SWMM needs. The importer derives them and lists its assumptions (manual §12.2 and §15.7). AquaSWMM’s attribute table can set a whole column at once (manual chapter 6).

4. Send the design back to Civil 3D

Design Reports > Export Design Network (LandXML)… writes the analysed network as a LandXML pipe network, which Civil 3D imports as a pipe network (manual chapter 12). Any sizes you changed during design go back with it.

Help while you convert, at any hour

AquaSWMM includes an AI support assistant that has read the whole manual. Ask it what a design-review finding means, where a setting lives, or how the imported hydrology was derived. It answers in seconds and names the section to check. It is on every plan and the free trial.

Try it on your own project

The 30-day trial is the full app, so you can open your own .stm on the first day. If something in your file doesn’t come across, email support@aquaswmm.com. I’ll send a 90-day evaluation key and look at the import with you.

Moving a Civil 3D pipe network instead? See Civil 3D pipe networks in AquaSWMM.

Autodesk, Civil 3D and Hydraflow are trademarks of Autodesk, Inc. AquaSWMM is independent software from HydroComplete LLC and is not affiliated with or endorsed by Autodesk or the U.S. EPA. AquaSWMM reads the .stm text format for interoperability. The method comparison describes AquaSWMM 0.12.7. Check your own results against your Hydraflow reports.