Catchment user notification about the difference between the calculated peak flow vs. interpolated peak flow being greater than 1.5%

Applies To 
Product(s): CivilStorm, SewerGEMS, PondPack
Version(s): CONNECT Edition, V8i
Area:  Output and Reporting
Original Author: Mark Pachlhofer, Bentley Technical Support Group

Problem

After I compute my model I get a user notification for a catchment that says, "The difference between calculated peak flow and interpolated peak flow  X % is greater than 1.5%. Computed peak flow = X cfs Interp. peak flow = X cfs. Output increment for this catchment may be too large."

Background

This is a yellow user notification, which is just a warning message, that let's you know you should check the flow out of your catchment and it's related to the SCS Unit Hydrograph Calculation.

By default, the program computes an internal calculation time step based on 0.133 Tc for each subarea (each subarea has it's own unique time step for runoff). This procedure typically results in runoff time steps that differ from the overall network time step specified on the calculation options dialog (Analysis > Calculation Options).

Once the subarea runoff hydrograph is computed using this internal time step, the calculation options dialog time step is used to interopolate and create flow ordinates using the standard network time step so that all routing and hydrograph addition will be using the same time base.

The default threshold for printing this error is when the interpolated peak differs by 1.5% (or greater) compared to the internal peak.

Solution

Adjust the output increment in the calculation options

  1. Go to Analysis > Calculation options and double click on the current calculation option, which is the one with the red checkmark on it. This reveals the properties of this calculation option

  2. Locate the "Output Increment" property and change it to a smaller number. For PondPack, the default is 3 minutes, so you may want to consider 2 minutes or one minute. For SewerGEMS and CivilStorm, consider a smaller value such as 0.01 hours. Also for SewerGEMS and CivilStorm, consider adjusting the "Calculation Timestep" to a smaller value if needed. This is the increment for the calculations, whereas the output increment is how often the calculated results are reported.

For PondPack 10 (circa ~2006) or earlier 

 This error message can usually be avoided by narrowing the timestep on the GO dialog in order to include an ordinate closer to the internally computed time peak.
For an illustration, see the graph below.  As you can see from the picture using a timestep of 1 hour would have taken readings at the two places marked.  The peak outflow occurs between those two places, and therefore there is not an interpolated peak at the true calculated peak.

This default threshold can also be adjusted in version 7.5 via the Options / Global Settings menu item.  For example, if you feel the 1.5% is too strict you can set PondPack to trigger the warning if there is 2.5%.

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