Convergence problems, very slow calculations or "out of range" message when using culvert in series with another outlet component

  Product(s): PondPack, CivilStorm, SewerGEMS
  Area: General

Problem

One or multiple of the following problems occur when attempting to compute a model, or a composite outlet structure, when a composite outlet structure includes a culvert in series with other outlet components. For example a weir and culvert, with the weir's "Downstream ID" field set to the culvert, or any other outlet component in series with a culvert.

  1. Convergence problems - high convergence error seen in the outlet result details, or generally unstable-looking rating curve is seen.
  2. Very Slow Performance - the model calculations get stuck for a long time on the step of computing that specific composite outlet structure (an indication of convergence issues)
  3. "Out of range" error occurs during composite outlet structure calculations

Solution

These issue can happen with culverts in series because they tend to be very sensitive to convergence and may "fight" against each other. Possible solutions:

1) Consider if a culvert is needed - if the capacity of the culvert is always greater than the capacity of the upstream components, then the culvert component may not be necessary. Consider doing a quick outlet calculation with the culvert alone to check. If this is the case, simply delete the culvert component and set the Downstream ID of the upstream component to "Tailwater".

2) Check all input parameters and adjust the Convergence Tolerance if needed - review at the root outlet structure component's configuration, as well as each individual component, to ensure that input parameters are correct. If the culvert barrel is flat, consider adding a small amount of slope. If the convergence issues persist, try setting the culvert component's "Convergence Tolerance" to a large number, to make convergence easier to achieve. For example 0.01 or 0.1 feet.

3) These problems can sometimes occur when trying to model an inlet box structure as a weir (using the Weir component instead of the Riser). If there is a box (or standpipe) structure that is discharging directly into an underground culvert (pipe), then try modeling the structure as an inlet box (or standpipe) with Downstream ID set to a culvert

A special algorithm will check whether the inlet box (or standpipe) is operating under weir or orifice conditions, and whether the downstream culvert is the controlling flow element. The user can specify the weir and orifice coefficients pertinent to each case.

For standpipe and inlet box examples, for PondPack, refer to the "Riser" example file included with the program installation. A profile view of the standpipe can be found in the Help.

See Also

In the Composite Outlet Structure results, what does the message "CRIT.DEPTH CONTROL" mean for a culvert?

In the Composite Outlet Structure results, what does the message "BACKWATER" mean for a culvert?

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