Unsteady HEC-RAS Initial Conditions 2

[Erin05]

I am developing an unsteady HEC-RAS model which includes a channel segement which outfalls to a detention pond, which is then routed through another channel before outfalling to the receiving channel. Thus far, I have not been able to successfully run a stable model. Since the model goes unstable within the first time steps, I believe my problem lies in the combination of initial conditions.

Does anyone have any basic advice on the best way to establish the initial conditions for unsteady HEC-RAS?

Thank you!

 

[chicopee]

Which HEC-RAS version are you running?

 

[Erin05]

HEC-RAS v3.1.3

Thanks!

 

[Drew08]

Hi Erin05,

Not many people have responded to your question because it is very vague. There dozens of reasons why your initial conditions may be unstable. I'm sorry to say the best tool to use to solve unsteady state problems is experience. Unsteady state modeling is as much art as it is technical. If you go about it with a completely technical mindset, you'll have problems.

Some of the more common problem areas are:

Make sure all initial flows at external and internal boundaries match.

Cross-sections that expand or contract too rapidly, both between sections and vertically within a cross-section, may cause problems. Ineffective flow areas may be needed to ease transitions. A rule of thumb is never allow the active flow area of consecutive sections to change by more than 50%.

HEC-RAS methodology was developed to deal with subcritical flow, which was later adapted to super critical flow. Even though you have super critical flow checked, the model always has difficulty dealing with super critical flow at initiation.

You can also mess around with time steps, theta weighting factor, distance of interpolated cross-sections and several other computational parameters that may make your model work, at least temporarily, in order to help identify problem areas.

If all else fails, you can try creating a "hot-start file", which gets fairly complicated and usually can be avoided with a little finesse.

Try rephrasing your question to be a little more specific, you may get a better response.

 

[chicopee]

I have the same problem eventho,I switched from vers. 4.0 Beta to vers. 4.0. My steady state model under various flows of my stream came out well however I have the same problem as you have in the unsteady state flow.
The 50% change in X-S mentioned by Drew08 is a good point and I'll check that out carefully.

 

[MattZ99]

Another item to consider is to make sure that you have a little bit of water in your detention basins. If you are using storage areas, set the initial elevation to 0.1 foot above the invert. If you are modeling the basins with cross sections, make sure you have a minimum flow in your flow hydrographs. If you are still stuck, paste in what the error says.

 

[Erin05]

Thanks for all of the advice.

After consulting with some other engineers on the project, we've decided that we're trying to make it too complicated. So we have decided to simplify our approach, which should resolve the issue. But I appreciate the help and will return if the need arises again!

 

[Drew08]

I think Erin05 hit on a very valuable lesson. Far to often, we engineers run to quickly to the time consuming complex models before we truly understand what our final goal is. After all, you wouldn't take the space shuttle to the grocery store if the yugo was gassed and in the garage. Often times you can reach the same goal with a simple Manning's, mass balance, or other simple hydraulic formula and leave HEC-RAS on the shelf.

 

[ptmoss]

Are you kidding me? I would absolutely take that Space Shuttle. Even just to the end of my driveway!!

(Does it come with a plow?)

 

[Crystalct1]

Unsteady models are often more trouble than they are worth. Storage and routing effects are more easily handled by a hydrologic program. Look at your output and decide where, when, and what your worst-case is. You can usually model just that one case in a steady state model.

Even environmental permit reviewers do not recommend, expect, or mandate a complicated unsteady model. And most clients would complain about the extra level-of-effort.