Inline Structure Instabilities

[Dr. Khan]

Hi All,
Could someone suggest me on the instability issues of inline structures in HEC RAS.
I am running a 1D/2D model which has only one inline structure. The model results show unacceptable high water surface immediate upstream of the structure as shown in the following image. I am not sure what is causing this.
Please note that the structure has two internal cross-sections which are similar to upstream and downstream cross-sections. I have tried many options to resolve this issue but could not solve it.
Any quick response will be highly appreciated!

The following image shows the inline structure as added in the model. The structure is added as Deck/Roadway.

[Anonymous]

So to clarify,

The instability is occurring within the 1D portion of the model upstream of the 1D/2D inline structure? How is your mass balance output? Time step?

Check you max flow, htab parameters for that bridge and compare to the results. If there are 1D parameters the Htab parameters are very important as well as the computation time step?

[Dr. Khan]

Hi Luis,

Clarifications is as follows:

The instability is occurring within the 1D portion of the model upstream of the 1D/2D inline structure?
Yes.

How is your mass balance output?
The model is generating extra flow and volume.

Time step?
It is flexible based on Counrant number. The maximum Courant is set to 5 and lowest to 0.5. Although I have tried a range of these settings from courant number of maximum 1 to 5 and minimum 0.5-1.

Check your max flow, htab parameters for that bridge and compare to the results. If there are 1D parameters the Htab parameters are very important as well as the computation time step?

Htab parameters are set as:
Headwater maximum Elevation = 50 leaving the optional parametrs blank.

[Anonymous]

Ok what is your weir coefficient for the inline structure? If the weir is supposed to be representing a connection that becomes fully submerged it must be in the range of 0.2 to 0.5 ( I like to use 0.2)

Also i do not like the courant time step stuff. Try it with it off when you get the chance. I have a reason behind it but it is too long to type right now

[Dr. Khan]

Thanks Luis. I am using a weir coefficient of 1.8. This is because the bridge deck hardly gets submerged. Although the water level reaches the sofit level during the peak.
As suggested, I will try to run is with fixed time step.

[Author]

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