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When computing a model with one or more ponds routes set to Interconnected Ponds (ICPM), the below user notification displays and/or the flow/HGL results are unstable / Oscillate rapidly.
"Mass balance for routing volumes vary by more than 0.5% "
In some challenging situations, instability may occur in the interconnected pond routing (ICPM) calculations. Below are some possible steps to resolve the issue.
1) Make sure that the ponds connected to the ICPM outlet structures are not flooding. This can cause instability in the calculations and make it difficult for the program to achieve mass balance.
2) You can try adjusting the calculation options. For instance, try a different value for Output Increment and ICPM time step. A smaller value will take longer to compute, but the results should be more stable since the calculation is able to capture the smaller changes in headwater and tailwater.
3) Try to set the tailwater type all of the pond outlet links to ICPM. This includes all of the links that connect to outfalls. If the outfall was originally set a free outfall what you will need to do is set the outfall to the boundary condition type of "Time-Elevation Curve" and enter a time elevation curve such that the elevations still create the free outfall effect. This means that the elevation will be lower than the lowest elevation of the upstream outlet structure. Example: The lowest elevation of the upstream outlet structure is 5.00 ft. In this case you could set up your outlet structure with a time elevation curve as follows:
Time Elevation
0.0 hours 0.01 ft.12 hours 0.01 ft.24 hours 0.01 ft.
4) You can adjust the ICPM settings in the outlet structure itself. Go to Components > Composite Outlet Structures and click on the outlet structures that are mentioned in the user notifications. If you click on the name of that outlet structure you have the option to change the increment at which the headwater and tailwater are calculated. You can try to make this smaller than what it is at. A value of .5 ft. or .25 ft. usually works well for outlet structures that have very fast changes in flow vs. elevation that need to be captured on a smaller scale, but this value will be different depending on your outlet structures.
User notification "Downstream TW out of range. Looking for TW value that is less than lowest table value."