Hydraulic propertiey parameters

[László Vas]

I’m creating a steady model of an irrigation channel. It contains several bridges. To start the simulation I have to fill the following data: Bridge –> HTab Param. –> Maximum flow (recommended). I usually write here the maximum flow. In case of unsteady model I write there the maximum flow of the time-serie.

After I fill it, the model is running, but I don’t understand why is it neccesary. I tried to search in the User’s manual and in the Hydraulic reference manual about this, but I didn’t find it.

So is it a good method to write the maximum flow to every bridges, or am I doing wrong? And why is it neccesary? And what will be the consequences if I write a lower or higher discharge than the maximum?

Thanks for your help.

[Anonymous]

RAS uses the Bridge > HTab stuff to create the “curves” for each bridge. You view this under Hydraulic Property Tables > Type > Internal Boundaries. The vertical/Y side of the graph is the Headwater. The curves will go as high as the specified maximum Headwater. The horizontal/X side is the amount of flow. If you do not specify the maximum flow, then RAS will compute the maximum flow based on the maximum headwater. This can sometimes be a huge number. For instance, you may have a model that has an actual maximum flow of 100,000, but if you don’t specify a maximum RAS may compute a maximum flow in the millions. This doesn’t automatically make the curves wrong. However, a lot of the curves will cover flows that will never happen. This means there are fewer points in the curves down where the flow actually happens and you can get some loss of accuracy.

So it is generally good to put in a maximum, but it is bad if you put in a maximum lower than what actually happens in the stream. I think version 5.0 will output a red warning message if this happens but it is probably best to guess/put in a maximum a little on the high side.

When you put in a maximum, RAS does not extend the curves past the user entered flow maximum.

[László Vas]

Thank you, now I clearly understand this. By the way, how do you know that? Because I have tried search about this, but I can’t find anything.

[PaoloP]

You can find in Users Manual Chapter 8 pag. 8-22.
It is only for unsteady simulation, for steady simulation you don’t need to fill this table.

Paolo

[Anonymous]

There is a lot of information about how the program works in the User’s manual and the Reference manual.

There are various people that teach courses on using RAS, which is another way to learn to new stuff.

[rsbe]

Hi, I experience a problem which might be related to your discussion.. When post processing my unsteady simulation i get a weird drop in water surface elevation just in front of a bridge. This drop is not present for the no bridge alternative… Somehow I cannot believe that this difference is only due the post processor which has a different solution scheme than the unsteady solver (which is apparently used in the case of no bridge).. Thanks for any hint in advance.

[Author]

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