Using Extreme Flow Factors (Peaking Factors) in SewerCAD and SewerGEMS

Product(s): SewerCAD, SewerGEMS
Version(s): CONNECT Edition, V8i
Area:  Modeling

Background

Extreme flow factors are generally used for computing peak discharges, and therefore are typically referred to as peaking factors or peaking equations. However, since they can also be used to compute minimum discharges, the term "extreme" flow factor is more accurate and will be used throughout the program and documentation. This article applies to SewerCAD as well as SewerGEMS, when the numerical solver is set to "GVF-Convex".

Extreme flow factors work by applying a multiplier against the base flow for a given manhole, but that multiplier decreases as the total population or total incoming load at a given point (manhole) in the network increases. This is done because, as the total contributing population increases, there is a decreasing chance that all of the population would be contributing a load at the same time (for example flushing their toilets at the same time).

Steps to Accomplish

Loading in SewerCAD (or the GVF-Convex solver in SewerGEMS) is done through the Sanitary Load Control Center. Patterns can be applied are generally used for Extended Period Simulations (EPS). With Steady State runs using Unit Loading, a user will want to look at the peak flow to get a conservative analysis of system. That is where Extreme Flow Factors come in.

To begin, you need to set up the Extreme Flow. To do this, go to Components > Extreme Flows > Extreme Flow Setups (in V8i, the path is Components > Extreme Flow Setups).

To create a new Extreme Flow Setup, click the New button in the upper left. When you do this, a row will be added for each Unit Load associated with the model. You have the option to use a given load by placing a check box in the Use column. The other columns include the Extreme Flow Method and a couple of multiplier options.

There are a couple of options available to the extreme flow method. If you leave the Extreme Flow Method column set to "None" you can enter a constant. This is the multiplier that will be used on the loading derive from the Unit Load in the Sanitary Loading Control Center. However, you also have the ability to use one of several Extreme Flow Methods.

To create these, go to Components > Extreme Flows > Extreme Flows (in V8i, Components > Extreme Flows), or click the ellipsis button in the Extreme Flow Method cell in the Extreme Flow Setups dialog. Click the New button and choose from one of the four methods available.

If you choose the Table methods, you will have to manually enter the base load value or the population and the extreme flow factor associated with it. If you have a range of populations for each factor, enter a row in the table for the high and low point of the range. For example with the Table-Population method:

Note that the keyboard shortcuts work with this table, so if you have the table values in a format like Excel, you can copy and paste the data into the table.

If you choose the Equation methods, you will need to enter the coefficients for the equation. For convenience, the equation used to calculate the Extreme Flow Factor can be found at the bottom of the Extreme Flows dialog.

NOTE: When choosing the method, be sure that it matches with your unit loads. For instance, if your unit loading is based on population, you should use a population-based extreme flow factor method. 

Once you have created the Extreme Flows, you must apply it to the Unit Load. Return to the Extreme Flow Setups dialog. In the Extreme Flow Method, select the table- or equation-based method you wish to use. If there is an extra adjustment you need to apply, you can enter a value for the Adjustment Multiplier. The Adjustment Multiplier is applied to the load that has the Extreme Flow already applied. If you do not need to apply any extra adjustment, simply set the values in this column on 1.0.

Now that the Extreme Flow Setups are completed, you need to apply them to the Calculation Option of the model. Go to Analysis > Calculation Options and double-click the active calculation option to view the properties. Find the properties field Extreme Flow Setup and set this to the Extreme Flow Setup you wish to use.

Now when you compute the model, the extreme flow setup will be applied to the loading of an element.

Please note:

1. Extreme flow setups are only used for steady state simulations. They are not used for EPS runs. This is because a peak flow analysis is only valid in a steady state (snapshot of time).

2. Extreme flow setup can be set for only one type of "extreme flow method" at a time for simulation. If you have multiple methods then you can choose to apply any one method at a time.

3. If you have multiple "extreme flow methods" for a single model to be applied for different population ranges then they need to be used together under a common method.

For Example, if you have the following setup;

Peak factor for Population less than value X  = using a specific equation

Peak factor for Population from X to Y = using Babbitt's equation

Peak factor for Population higher than Y = Constant

For the above setup you might have used "Equation - Population Factor" for the first two cases and "Table - Population Factor" for the third case. However, only one of them can be selected in your Extreme Flow Setup. For this you have to convert the first two cases into "table" format. Since the first and second population ranges use different equations you can calculate the resulting peak factors by inputting the population ranges for the particular equations. Then using these peak factors you can develop a "table" for population vs. peak factors which incorporates all the three conditions.

The resulting table would have specific peak factors against the population ranges. Hence your setup would look something like this;

Population less than value X  = Peak Factor 1 (calculated by solving "specific" equation)

Population from X to Y = Peak Factor 2 (calculated by solving "Babbitt's" equation)

Peak factor for Population higher than Y = Peak Factor 3 (=Constant)

See related forum thread for details.

Troubleshooting

Extreme flow factors are not being applied

If it does not appear that the extreme flow factors are applying when computing your model, make sure your model is set to use the GVF-Convex (SewerCAD) numerical solver and that you have the time analysis type set to Steady State, as extreme flow factors do not apply in EPS (extended period simulation). 

The decrease in peaking factor results in a decrease in flow and decrease in designed pipe size

Although the peaking factor will decrease as the population increases, the total population would still be increasing and therefore the total "base" flow (before peaking factors are applied) will increase. Although it is certainly possible, it would be unusual to see the decrease in peaking factor be large enough to cause a decrease in the total peaked flow. To see this reduction in flow also cause a reduction in designed pipe size would be even more rare. Further, if using the constraint based design feature, one of the highest priority constraints is that the downstream pipe should be at least as large as the upstream pipe (to avoid clogging). See: Order of constraint based design priorities for StormCAD (GVF-Rational) and SewerCAD (GVF-Convex)

If you are encountering this situation and do not wish to see it occur in your model, the decision of what you do is ultimately up to you as the modeler, but one option to consider is to add a more gradual decrease in peaking factor with respect to population. For example, instead of jumping straight from 3.0 to 2.5, add one or more points in between. For example 3.0, 2.75, 2.5. Or, 3.0, 2.9, 2.8, 2.7, 2.6, 2.5.

See Also

Troubleshooting Extreme Flow Results with Count based Unit loads

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