Hec-Ras steady flow-Water elevation decreases as flow increases

[Adrian]

Hello! Im a self-learned Hec-Ras user who uses Hec-Ras to evaluate different river restoration actons. Im using a steady flow model to create a rating curve which i will use later on when performing my unstable flow model.

Background: Im working with a modifed river which suffers from minimal flow regimes, dam laggs and has 60+ drop structures to maintain the water level. In this reach, we have a 2 km long slow-flowing area with a constant water level which is maintained by a big drop structure that dams the upstream part of the river 67 cm.

At first i tried to use the inline structure option to model the drop structure, but as i got the same results no matter what settings I used, i decided to just incorporate the drop structure in the raster i used when working in Hec-Geo-Ras.

I have used the steady flow model and the normal depth downstream boundary condition (0.2), and also set a change in the internal energy from the cross-section just above the drop structure (0.35). When using these values, the water elevation obtained from the hec-ras simulation above the drop structure matches with my observed water elevation, and the water elevation below the drop structure also gives good results.

However, when i use more than one profile in the steady flow data, i run in to problems.
I have 5 profiles, where the flow ranges from 6 m3/s (profile 1) to 15 m3/s (profile 5). Water elevation for profile 1 (6 m3/s) is 318.82 meters above sealevel, but in profile 2-5 (8 m3/s – 15 m3/s), the water elevaton decreases down to 318.42 (profile 2) – 318.50 profile 5) meters above sealevel.

The water elevation decreases as the flow increases.

I have tried the model without changing the internal energy from the cross-section above the drop structure, and then the water elevation increases as the flow increases. However, i need that internal energy change to get the correct initial water elevation above the drop structure as well as to get the correct decrease in water elevation downstream the drop structure.

So the problem seems to be that the internal energy change is only valid for profile 1, but gets lost when the flow changes in profile 2-5 thus the water elevation decreases. I have struggled with this for two weeks now so any kind of help would be much appreciated.

Sincerely
Adrian

[Anonymous]

Does the Energy Grade increase for the higher flows?

EG = v^2/2g + WSE

WSE = EG – v^2/2g

If the velocity increases with higher flows, it is sometimes possible for the WSE to drop even though the energy grade is increasing.

[Adrian]

No, as the reach water level is maintained by the drop structure in the end of the reach, the water elevations is about 70 cm higher than it should be during natural conditions. The water surface is horizontal along the whole reach until the drop structure, so every increase in the flow should raise the water elevation.

The problem only appears when I alter the internal energy at the cross-sections above the drop structure. If I use the same energy grade along the whole reach, the water elevation increases with the flow but the calculated water elevations won’t be correct. I need that change in energy to get water elevation that matches with what i have observed, hence the problem 🙂

Thank you for your answer anyhow, If you have any more suggestions, let me know!

[cameron]

can you supply the model for us to look at? What did you set for the inline weir coefficient and other weir parameters? What error/warning messages are you getting for the inline structure?

[Adrian]

Can you tell me how to upload it? I don’t know how to do 😀

As i mentioned, i haven’t used the inline structure option to model the drop structure, instead i have built in elevaton of the drop structure in to the raster i use. This because i couldn’t get the inline structure option to produce reliable results. The results were the same no matter what options i used. I however do have another hec-ras project were the drop is represented as a inline structure, with the same problems as i have mentioned above. I’d actually prefer if i could get help with the project with the inline structure present, as that seems more appropiate to use then the one described above, where the drop is mosaiced in to the raster file used for the project.

Just tell me how to upload the project, and then i’ll upload it together with a description on what i have done and why.

Thank you for your help!

Adrian

[Adrian]

I just read Chris note on how to zip and upload files. Here is a link which i hope will work:

https://drive.google.com/open?id=1-u5ibJgEchTTbpBOGYRlEzF3-1MVJQUO

So here is a little explanation: This reach is dammed by the drop structure represented as a inline structure below cross section 126.6861. The drop structure has forced the water elevation to inrease about 67 cm, and also keeps the water surface above the structure horizontal and at constant elevation 318.96 meters above sea level.

I tried different manning values and downstream boundary conditions in order to get hec-ras to produce a result where the water surface above the structure is 318.96. This was achieved by using the critical depth as downstream boundary condition and set internal changes at cross-section 126.6861 to have a known WS of 318.96.

(Please notice that this is another Hec-Ras model, not the one I described in my first post. It is however a model representing the same reach and with the same problem that the water elevation decreases when adding profiles with higher flows. I uploaded this because here the drop is represented as a inline structure and not mosaiced into the raster as was the case with the first model)

Ok so we now that the water elevation above the drop structure is almost constant at 318.96 meters above sealevel because of the minimum flow regime, which is 6 m3/s. We also know that the flow coming out from the drop structure is almost always 5 m3/s, with a pilot flow of 4 m3/s. So even if the incoming upstream flow increases, the amount of water coming out from the drop structure never exceeds 5 m3/s, unless the water elevation increases over the height of the drop structure. And we also know that the water elevation at the most downstream cross-section (34.55011) is ~67 cm lower than the water elevation above the drop structure.

What i want to do: I want to use multiple profiles with increasing flows to produce a rating curve that is valid above the drop structure. What water surface will i have at 7,8,9,10 m/s etc. The problem is that when i run the model with several profiles of flows higher than the initial 6 m3/s, the water elevation decreases. Profile 1 has flow= 6 m3/s and water elevation=318.96. Profile 2 has flow=7 m3/s and water elevation=318.48. Profile 5 has flow=10 m3/s and water elevation=318.49. I want the water elevation to higher than the initial 318.96.

Sorry for the wall of text, but i figure i rather write too much than too little.

Thanks in advance for your help, i really appreciate it!

[Author]

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