SWMM5, InfoSWMM

Subject:  What are the Units for the five St. Venant Flow Terms in SWMM 5 and InfoSWMM?

The new flow (Q) calculated at during each iteration of time step as

(1)        Q for the new iteration = (Q at the Old Time Step – DQ2 + DQ3 + DQ4 ) /   ( 1.0 + DQ1 + DQ5)

In which DQ2, DQ3 and DQ4 all have units of flow (note internally SWMM 5 has units of CFS and the flows are converted to the user units in the output file, graphs and tables of SWMM 5).  

The equations and units for DQ2, DQ3 and DQ4 are:

(2)        Units of DQ2 = DT * GRAVITY * aWtd * ( H2 – H1) / Length = second * feet/second^2 * feet^2  * feet / feet = feet^3/second = CFS

(3)        Units of DQ3 = 2 * Velocity * ( aMid – aOld) * Sigma = feet/second * feet^2 = feet^3/second = CFS

(4)        Units of DQ4 = DT * Velocity * Velocity * ( aMid – aOld) * Sigma / Length = second * feet/second * feet/second * feet^2 / feet = feet^3/second = CFS

The equations and units for DQ1 and DQ5 are:

(5)        Units of DQ1 = DT * GRAVITY * (n/PHI)^2  * Velocity / Hydraulic Radius^1.333 = second * feet/second^2 * second^2 * feet^1/3 * feet/second / feet^1.33 = Dimensionless

(6)        Units of DQ5 = K * Q / Area / 2 / Length * DT = feet^3/second * 1/feet^2 * 1/feet * second = Dimensionless

The five components calculated at the each time step and at each iteration during a time step and together predict the new Link Flow (Q) in SWMM 5.  The value of the different components can be seen over time in Figure 1 and as a component percentage in Figure 2 and 3.

Figure 1:  The Five St. Venant Components over time.

Figure 2:  The relative magnitude of the St Venant terms over time for the same for the same link as in Figure 1.

Figure 3:  The relative magnitude of the St Venant terms over time for the same for the same link as in Figure 1 shown in an area chart normalized to 100 percent.  Normally the DQ1 and DQ2 terms balance each other except for backwater conditions or reverse flow in which the terms DQ3 and DQ4 can dominate.

Subject:  Risk Assessment Manager in InfoSWMM

 If you can get the Risk Assessment Manager than you can do a flooding extent based on the Maximum Head or just the Flooded Nodes.

Subject:  Making your inactive elements active in different alternative scenarios.

 Step 1:  Open up the Facility Manager and turn off Apply to Active Facility Only, click on Map Selection and then finally the +Add button

Step 2:  Select those elements you want to add to the Facililty (they are yellow in this case)

Step 3:  Save and then Close the Facility Manager Dialog

Step 4:  The Objects should be active now.

Step 5:  Run the Model and check if they are being used in the RPT file.

Step 6:  You can also run the Compare Scenario Command to see the different alternative models use a different set of outfalls.

Subject:  The relationship between the rainfall, total losses from the previous area, evaporation and infiltration only rate.

The total loss from a subcatchment pervious area is the sum of the evaporation + infiltration loss.  Typically the evaporation rate is much less than the infiltration rate. SWMM 5 now has too options – evaporation during only dry periods or evaporation during both wet and dry periods.

Figure 1:  An example network that shows the relationship between the rainfall, total losses from the previous area, evaporation and infiltration only rate. 

Figure 2:  The same model with the Evaporation during only Dry Periods turned on

Subject:  How to Determine if your model is Unstable in SWMM 5 or InfoSWMM

SWMM 5 and InfoSWMM has a good output feature in the RPT file that tells you the list of links with the highest flow instability during the simulation.  If you look at the link flow with the highest instability value and it looks okay to you then it usually means the rest of your model output is stable.  The index is the number of flow turns for the link during the simulation.  A flow turn occurs when

We call DQ the difference between the New and Old flow,

The value of DQ is greater than 0.001 cfs (we do not want to count small perturbations),

The sign difference between the new DQ and the Old DQ is negative.   In other words we want to count those oscillations in which the DQ value was negative and is now positive or was positive and is now negative. We don’t count then when the flow is monotonically increasing or decreasing in the link.

For example, the Link U-104 below has a large number of Flow Turns but a plot of the link flow shows the Flow Turns to mainly unimportant.