Non-Convergence During Routing

Typical Problem 2: Non-Convergence During Routing

Possible Causes and Solutions
1. Sources of information

o Non-convergence message
When and where does the problem first appear?

o Re-run for a detailed printout at apparent trouble spots--Froude number, velocity, top width, etc. Check for these often going to critical depth. Froude number near 1, velocity suddenly changing, top-width suddenly changing.

o Look for inconsistencies in initial conditions.

o Review graphics if available--run up just prior to time-step with convergence problem and look at stage hydrographs.

o Try taking out possible trouble-makers one by one (i.e., bridges, downstream boundary, etc).

2. Look for sudden changes with respect to time or space where the problem first appears

o Vertical change in flow width at a cross-section

o Substitute storage for active flow

o Use flood plain and convergence

o Longitudinal change in flow width between cross-sections

o Substitute storage

o Decrease distance between interpolated cross-sections

o Sudden change in discharge (e.g., dam failure)

o Adjust time step

o If acceptable, increase failure time (you may have to perform a substitute study to see what effect this has on results)

o Sudden change in rating curve at a boundary (e.g., bridge overtopping)

o Storage Adjustments

3. Other Problem Sources

o Flow depth becomes too small

o Prevented by "safety-net" feature

o Supercritical flow (or just barely critical flow)

o Use internal boundary at location of critical depth (as with dam and rating curve)

o Use sub/super critical flow options

o Adjust Manning’s n to get rid of supercritical flow for Froude number near 1 (i.e., critical depth)

o Break model into separate models.

o Downstream Boundary, (esp. loop rating curve)

o Inconsistent cross-section geometries