u/Western-Firefighter5

Hi everyone,

Following my previous post about initialization issues, I now managed to run the simulation in HEC-RAS 6.7 Beta 5 (October 2025), but I’m facing instability during the unsteady flow computation on a river network model used for Water Surface Elevation (WSE) analysis.

Here’s the context of my study:

1D unsteady flow simulation
River network model (multiple interconnected branches)
LiDAR-derived terrain (1D cross section based geometry)
Focus on WSE behaviour across the network

The geometry preprocessing completes successfully and the simulation starts, but it becomes unstable very quickly after initialization.

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Important detail:

The solver does not fail immediately, but diverges within the first hours of simulation.

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Error message:

**** ERROR: Solution Solver Failed ****

Minimum error exceeds allowable tolerance
Time of failure: 01JAN2020 07:00:00

**** Warning: Extrapolated above Cross Section Table ****
Rio Apartado (RA1), Rio Leon (RL1, RL2)

Overall Volume Accounting Error: 7431 (1000 m³)
Percentage error: 282.5%

Simulation went unstable at: 01JAN2020 07:15:00

What I already checked / observed:

Geometry preprocessing runs without errors
Model starts correctly but becomes unstable very early
Mass balance error grows very quickly (~280%)
Instability appears on multiple branches of the network
Extrapolation warnings appear during the run

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At this point I suspect the issue may be related to:

instability in flow routing across the network
discontinuities between cross sections or reaches
sensitivity of WSE propagation in early simulation time
vegetation (trees along riverbanks) affecting LiDAR-derived terrain and introducing local elevation errors
or boundary condition effects in a branched system

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Has anyone experienced similar early divergence and solver failure in 1D unsteady network models in HEC-RAS 6.7 Beta?

Any advice on what typically causes this kind of rapid instability would be really appreciated.

Thanks in advance.

Hi everyone, this is my first time posting on Reddit, so sorry if my message sounds a bit strange. I’m trying to run a long unsteady simulation (2020 full year) on a river network model in HEC-RAS 6.7 Beta 5, but the simulation crashes immediately before any actual computation starts. Current error message:

“+STOP! Error in the time window.
HDF_ERROR trying to close HDF output file:
HDF5-DIAG: Error detected in HDF5 (1.8.11)
H5Tclose(): not a datatype”

Here’s the context of my study :
- Unsteady flow simulation
- River network model (1D with terrain / mapper)
- Initially using daily inflow hydrographs for the whole year 2020
- Computation interval initially 15 min
- Output intervals tested at 1 day and 1 hour

I already checked/tested:
- Corrected geometry issues (I had “edge line self intersections” before, now gone)
- Rebuilt mesh / preprocessing
- Created a completely new plan
- Tested with constant inflows instead of imported hydrographs
- Tested short simulations (a few days)
- Deleted temporary HDF files
- Checked river station ordering and cross sections
- Simulation still crashes instantly with the exact same “time window” error

Important detail:
The solver never really starts. In the computation summary:
“Unsteady Flow Computations <1”

So it looks like the crash happens during initialization rather than during hydraulics.

At this point I’m wondering if:
- this is a time window alignment issue,
- a corrupted plan/project state,
- or possibly a HEC-RAS 6.7 Beta HDF bug.

Has anyone seen this exact error before or has ideas on what to check next?

Thanks!