How to Decide on a Time Step in InfoSWMM

Note: How to Decide on a Time Step in InfoSWMM

 Step 1: Your first guess was 300 seconds which had a very large continuity and unstable links. If you look at the your average time step a good time step would be 10 seconds based on your average time step of 2.6 seconds.

Step 2: Run your model with a time step of 10 seconds and check the flows and the continuity error. In this case by using a maximum time step near the average time step you got rid of the continuity error and the unstable link flows.

Export from WeatherUnderground using the CSV File Export Option to InfoSWMM

Note:  Export from WeatherUnderground using the CSV File Export Option to InfoSWMM

 Weather Underground is a site that provides excellent local weather information in the form of graphs, tables and csv files. You can use the data very easily in InfoSWMM by copying from Excel to a time series in InfoSWMM. Here is the rainfall for a storm event in Tampa, Florida in September 11, 2010

Step 1: Export from WeatherUnderground using the CSV File Export Option

Step 2: The data imported from the csv file to Excel and after the text to columns tool is used looks like this in Excel. The data is now ready to be imported into InfoSWMM after the time column is adjusted to fall on even 5 minute intervals. In Excel you can use the formula @ROUND((B2)/"0:05:00",0)*"0:05:00" to round all of the time values to 5 minutes. If you do not do this step then you will have problems in InfoSWMM due to the rainfall interval not being equal to the defined raingage interval.

Step 3: You will need to format the new rounded time as a time format for import into a InfoSWMM time  series.  The time series is created in the operation tab of the attribute browser.

Step 4: Open up and make a new time series in InfoSWMM.

Step 5: Copy and then paste the date, rounded time column and rainfall column into the InfoSWMM time series columns.

 Step 6:  Make a raingage in the DB Table in InfoSWMM that will use the time series you just made.  In the case of the Weather Underground data we will use inches, intensity, a rainfall interval of 5 minutes, time series and the name of the time series.  SCF should be 1 for Snow conversion and do not need to include a Filename or Station name as we are not using an external file.

Flow Units In InfoSWMM may be different then the Output Link Flow Units

Note:  Flow Units In InfoSWMM may be different then the Output Link Flow Units

The flows units selected in Run Manager determines the flow units of all incoming units including DWF, Inflow Time Series and other features in pump curves and other curves.  The output unit manager determines what you see when you make a graph of the link flow.  For example, you can have dry weather inflow of CFS and output units of GPM or MGD if you so request and set the correct flags in the interface.

Hysteresis Effect in the Link Flow versus Depth Relationship in SWMM 5

Subject:  Hysteresis Effect in the Link Flow versus Depth Relationship in SWMM 5

You can often get a hysteresis effect for the Flow versus Depth relationship in SWMM 5 due to the five component St. Venant equation used to simulate the flows (http://swmm5.blogspot.com/2010/12/what-are-units-for-five-st-venant-flow.html) .  A hysteresis effect is having two or more flow values for the same depth value in the link. For example, this image shows how the link 8100 has a different flow for the same depth in the rising and falling limb’s of the hydrograph.  This is due to the different values for the upstream and downstream head, hydraulic radius and cross sectional area during the falling and rising hydrograph, respectively.

How is RHO computed for a Link in SWMM 5?

Subject:  How is RHO computed for a Link in SWMM 5?

SWMM 5 uses a sliding metric to calculate the cross sectional area and hydraulic radius used in the simulation for the link dynamic flow.  The area and hydraulic radius used moves from the Upstream End of the Link to the Midpoint of the Link based on the Froude number and a few other considerations (see Figure 1 for the other considerations).

The area and hydraulic radius used as a function of the Froude Number:

1.   Upstream cross sectional area and upstream hydraulic radius is used when the Froude Number > 1

2.   Midpoint sectional area and hydraulic radius is used then the Froude Number is < 0.5

3.   An area and hydraulic radius between the upstream and midpoint sections is used then the Froude Number is between 0.5 and 1

Figure 1:  How to compute RHO based on the Froude Number.

Figure 2:  The computed value of the Froude Number and the value of RHO over time.

Figure 3: Relationship between the upstream area, midpoint area and the actual area used during the simulation.