SewerGEMS and CivilStorm can now more easily leverage SWMM's ability to run simulations over very long durations (years vs. days). Here are few notes to keep in mind when attempting to work through the process.

Rainfall

Commonly, LTCS's are used to test sewer models with many years worth of rainfall data. Although, you can manually enter in this data directly into the Storm Data Group Library, CivilStorm and SewerGEMS now have the capability to reference external rainfall files when using the SWMM engine. This functionality eliminates the necessity of entering years worth of rainfall data into the CivilStorm/SewerGEMS Storm Libraries and allows you to directly access rainfall stored in many common formats including:

• DSI-3240
• DSI-3260
• HLY03, HLY21
• FIF21
• And a generic SWMM specific format which works as follows where each line of the file contains the station ID, year, month, day, hour, minute, and non-zero precipitation reading, all separated by one or more spaces

STA01 2004 6 12 00 00 0.12

STA01 2004 6 12 01 00 0.04

STA01 2004 6 22 16 00 0.07

For more information on how SWMM utilizes external rain files see the SWMM user manual section 11.3 here: http://www.epa.gov/ednnrmrl/models/swmm/epaswmm5_user_manual.pdf

When setting up a reference to a rainfall file in the Storm Data dialog, note that not all data is required for all data types. For example when using the SWMM specific format, you will need to set the Rain Data File Type, the Station ID and the interval, for other file types this data may be implicit in the file. Also, if the Rain Data File contains multiple stations you can create multiple references to the same file, by clicking the add button in the Rain File Storm Data section.

Runoff Calculations

The SWMM engine only directly supports the EPA-SWMM runoff method when computing runoff off of catchments. When using other runoff methods, the hydrographs are computed using a separate engine and passed into the SWMM engine as opposed to the SWMM engine computing the hydrographs directly. Therefore, it is highly recommended, if not necessary in most cases to use the EPA-SWMM runoff method when performing a LTCS for these reasons:

1. You will be able to take into account Climate Data, Snow Melt, and Groundwater, all of which play a more active role in runoff computations over longer durations.
2. When computing the hydrographs independently of SWMM you are restricted to 32,000 points per catchment. This may require you to use larger time steps than desired to span larger periods of time.
3. If you have many catchments with large number of runoff points, not using the EPA-SWMM runoff method could overly bloat the SWMM input data.

When running a LTCS with a SewerGEMS/CivilStorm model using the SWMM engine, you can save considerable time during the overall runs by saving the calculated hydrology data using a Runoff Interface file through the SWMM calc options. This procedure is useful, once catchment and rainfall data has been well established in the model and only data associated with the sewer needs adjusting. To perform the procedure.

1. In the SWMM Calculation Options change the Runoff File Mode from None to Save, and select a location to save the Runoff File.
2. Compute the model.
3. Change the Runoff File Mode from Save to Use.

From now on when you compute the model SWMM will use the runoff hydrographs stored in the Runoff File instead of recomputing them. Obviously, if any of your catchment or climate data changes you will need to rebuild the Runoff File.

Restricting Results

Running a LTCS with the SWMM engine has the potential to generate an unwieldy amount of results. In some cases it might be worthwhile to restrict results generated to a few proxy elements then for every element in the model, especially for very large models over long periods of time. You can do this by setting the Catchments, Nodes, and Links Results Type in the SWMM calculation options. You can set the type to either All Results, No Results, or choose a selection set. If the Selection Set is chosen, results will be generated for elements of the appropriate type in the selection set.