HEC-RAS unstable WSE_ EG over height

[DEL]

Hello.. I wanna ask about instability WSE, especially of EG of unsteady flow in HECRAS. The results models are success with some warning. I had change Htab parameters in number of points and increments. I had increase and decrease. But it’s not works for my model. Any one can help me please…

[Scott Miller]

The model is 1D only, right? What are the low end flows? How long does it run in model time? How well does the initial flow match the first flows to enter the reach?

The steep downstream end may be contributing to instability. If the model runs for long enough to output a few minutes of data, take a look at what the profile does during that time.

[DEL]

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References: <[email protected]> <[email protected]>
In-Reply-To: <[email protected]>
From: Fadel Khalifah Ibrahim
Date: Tue, 16 Oct 2018 06:53:11 +0700
Message-ID:
Subject: Re: HEC-RAS unstable WSE_ EG over height
To: [email protected]
Content-Type: multipart/alternative; boundary=”00000000000095ec6b05784d2452″

–00000000000095ec6b05784d2452
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Thank you very much Scott. That’s right the model is 1D. Actually i would
like to make 2D model. but first, i started from 1D model and then continue
to 2D. I set unsteady model and running for 3 days.
the initial flow that I use seems very influential. as an example. if I use
the value 5. The model will immediately error. If I use 40, it will error
too, but if I use 36 (recently), the flow will tend to be stable.

As information a few meters before the downstream, there is a downstream
that is quite steep, but sometimes the results of the model will be fine.
when I change the hydrograph, the flow model will then tend to be error.
Overall, the geometry of the river has no significant contraction and
expansion and the too steep cross section. I confused what should i do now.

Best regards.

*FADEL KHALIFAH IBRAHIM, S.T.*
ENVIRONMENTAL AND CIVIL ENGINEER

On Tue, Oct 16, 2018 at 6:11 AM Scott Miller [via HEC-RAS Help] < [email protected]> wrote:

> The model is 1D only, right? What are the low end flows? How long does it
> run in model time? How well does the initial flow match the first flows t=
o
> enter the reach?
>
> The steep downstream end may be contributing to instability. If the model
> runs for long enough to output a few minutes of data, take a look at what
> the profile does during that time.
>
> ——————————
> If you reply to this email, your message will be added to the discussion
> below:
>
> http://hec-ras-help.1091112.n5.nabble.com/HEC-RAS-unstable-WSE-EG-over-he=
ight-tp6516p6522.html
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–00000000000095ec6b05784d2452
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Content-Transfer-Encoding: quoted-printable

Thank you very much Scot=
t. That's right the model is 1D. Actually i would like to make 2D model=
. but first, i started from 1D model and then continue to 2D. I set unstead=
y model and running for 3 days.=C2=A0

the initial flow that I use =
seems very influential. as an example. if I use the value 5. The model will=
immediately error. If I use 40, it will error too, but if I use 36 (recent=
ly), the flow will tend to be stable.

As informati=
on a few meters before the downstream, there is a downstream that is quite =
steep, but sometimes the results of the model will be fine. when I change t=
he hydrograph, the flow model will then tend to be error. Overall, the geom=
etry of the river has no significant contraction and expansion and the too =
steep cross section. I confused what should i do now.=C2=A0

<= /div>

Best regards.

=
FADEL KHALIFAH IBRAHIM, S.T.

ENVIRONMENTAL AND CIVIL ENGINEER

=

=

On Tue, Oct 16, 2018 at 6:11 AM=
Scott Miller [via HEC-RAS Help] <[email protected]> wrote:

=

The model is 1D only, right? What are the low end flows? How long does it =
run in model time? How well does the initial flow match the first flows to =
enter the reach?

The steep downstream end may be contributing to instability. If the=
model runs for long enough to output a few minutes of data, take a look at=
what the profile does during that time.

=09
=09
=09

---

If you reply to this email, your message =
will be added to the discussion below:

http://hec-ras-help.109=
1112.n5.nabble.com/HEC-RAS-unstable-WSE-EG-over-height-tp6516p6522.html

=09
To unsubscribe from HEC-RAS unstable WSE_ EG over height, click here.
NAML

–00000000000095ec6b05784d2452–

[Scott Miller]

Does the model go unstable every run, regardless what the initial flow is? Set the output intervals to 1 minute. Then run the unsteady model and look at the animation of the profile. This can reveal where instabilities start.

(Click on the animation/”play” button, then right click the profile plot for “full extent”… the view ought to zoom in to the channel profile. Click on animation again to get the animation tool bar back.)

The initial flow should almost always match the initial flow in the hydrograph. It may be useful to set a minimum flow in the boundary conditions. Make it a small fraction of typical flows, but not so small the channel goes dry anywhere.

Low flows can be problematic where they arrive at a steep reach. A cross-section may dewater in a time step. It may help to put more cross sections in where the bed slope is steep or the channel changes rapidly. Make sure the distance between cross sections and the time step make sense for velocities in the channel, and that the velocities are reasonable.

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