Problem

When computing a model with one or more ponds routes set to Interconnected Ponds (ICPM), a message indicates that it was not able to converge, or 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% "

Solution

In some challenging situations, instability may occur in the interconnected pond routing (ICPM) calculations because of the way that convergence is performed. 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. If a pond is overflowing (and other ponds appear to be stable), you may need to choose a larger pond, adjust your outlet structures, and/or check to make sure that the catchment runoff results are appropriate.

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) Review and adjust the settings in the Composite outlet structure(s). Go to Components > Composite Outlet Structures. If there is more than one outlet structure associated with the composite outlet structure, highlight one of these and click the Compute button. If there is an issue, check the setup of the outlet structure to make sure the data in entered correctly. Keep in mind that the elevation information is often invert elevations. If you have an irregular weir, the elevation in the weir table is relative to the weir elevation.

You can also adjust the advanced settings of the composite outlet structure. If you click on the name of the composite 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.

4) Try to set the tailwater type all of the pond outlet links to ICPM because a mix of Free Outfall and ICPM has been known to cause imbalance issues as described in this article. This includes all of the links that connect to outfalls. If the outfall was originally set as Free Outfall, you will need to 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 hours          0.01 ft.
12 hours         0.01 ft.
24 hours         0.01 ft.

Problems with Volume Balance

When using ICPM, you may notice that the volume "out" for a particular pond may not match the value for volume "in" to a downstream pond, even if the mass balance error is low. This can happen if there are multiple ponds connected to the pond in question with potentially forward and reverse flow. In this case the volume out can be considered a net volume. For example to match the volume "in" value, you may need to add any reverse volume from other ponds and subtract any forward volume going toward other ponds. Essentially things get complex with multiple ponds that have both forward and reverse flow, but to gain more confidence in the results, try graphing the flow and volume to get a better feel for where the volume is distributed.

You can see additional volume routing details in the "Interconnected Pond Routing Summary" sections of Report Builder (near the bottom of the list).

See Also

User notification "Downstream TW out of range. Looking for TW value that is less than lowest table value."

Calculation failure or bad results with a mix of ICPM and Level Pool ponds