Adding cross section 1

[amanmekele]

My x-section for the entire reach (having bridge at the middle) is the same. Is there disadvantage in accuracy of water surface computation if i provide more x-sections especially near bridge? please can you give me hint or direct a reference?

Thanks

 

[cvg]

you need to start and end your contraction and expansion somewhere near the beginning and end of the bridge. add sections at these locations

 

[amanmekele]

Thank you cvg. but i need to elaborate for me that HECRAS manual recommends to provide "enough" x-section in these reaches and the word enough is not clear for the number per length of contraction and expansion length. so, prviding more sections there can have side effect on computation or we are in the conservative side?
Thank you in advance.

 

[gbam]

amanmekele - take a look at the hydraulic reference manual figure 5.1 & 5.2 for typical xsec locations for a bridge. In general there are four xsecs needed to define the bridge:
1) located far enough downstream so that flow is completely expanded
2) downstream face of bridge (I usally place at toe of slope)
3) upstream face of bridge (Toe of slope)
4) located far enough upstream such that flow width matches the no bridge scenario.

I do on occasion model xsecs between 1 & 2 and 3 & 4; but only in special circumstances changes in manning's, xsec geometry or etc...

Hope this helps.

 

[amanmekele]

Dear gbam,
Really thank you so much for your attention, I put these 4 sections considering some engineering judgements. However, I put several sections in between section 1&2 and 3&4 by interpolation (the reach has uniform shape). Because i need these points and also i put ineffective flow area locations the same by interpolation b/n 1&2 and 3&4 sections for the interpolated sections. the question is has a negative effect on computation (steady flow WSP)putting more x-sections in between the basic reaches. Another thing is gbam said that section 2&3 usually put at toe of slope, what will be if the embankment is vertical(no slope)?
Thank you.

 

[gbam]

Try running it both ways to determine your differences. As long as you can justify placing additional xsecs then you should be fine.

As for the placement of Xsecs 2&3 at the toe of slope, by tow of slope I mean the roadway embankment toe of slope. If you have a wall one can set it a couple of feet us or ds. In my area we typically only have brdge wingwalls for short distances away from the channel and then the sloping roadway embankment takes over.

Every scenario may be slightly different. It is up to the engineer to evaluate the condition and come up with the best method for modelling the condition.

 

[amanmekele]

Thank you gbam, I appreciate your response. yes, I observed that it is better to add sections in b/n.
By the way, considering small scale flow systems(Eg. lab. flume flow), is that possible to simulate using HECRAS with the actual scale better than large scale taking a big scale ratio?

 

[gbam]

Off the top of my head, I am not sure. One thought is to double check the HECRAS tolerances. Then determine of the small scale Flume condition falls within the allowable tolerances. Worst thing that can happen is that you model it; then calibrate the model to match your measured results and find that your assumptions are way off. If I remember correctly flumes were used to develop the St Veneau (spelling) equations that are used in hydraulic modeling. Man this is really stretching my memory; its been a while since I was in school. But I vaguely remember something like that.

 

[amanmekele]

I appreciate more, Of course the allowable tolerance for water surface calculation is minimum 0.1mm which is acceptable to my profile computation with 1.2m wide and 0.28m dip flume flow, we can take as small (field) irrigation canal. and I read one literature that compares the flume flow with in different 1D modelling softwares and the author mensioned HECRAS did well with Energy method, but there is no full information about his procedur. That is why it made me in doubt. any way you helped me a lot, thank you so much.
If you do not mind can i ask another? I saw that the bridge/culvert can skew up to 45 degree only, is that possible to use more than 45 degree mannualy using "cosine value" in excel, and paste it? the mechanism is shown in manual,and it is clear. why HECRAS limit to 45? (I need to check up to 70 degree)

 

[gbam]

Skewed bridges are always difficult. For one, xsecs are perpendicular to the flow. This creates different water surface elevations across the bridge face, although it can be minor for small skews. If the system is not under pressure then the bridge can be described in xsec and modelled without the bridge routine. In practice, I review alot of models that neglect the skewness of the structure and lengthen the bridge to match xsec locations. This is not correct but one has to determine whether the design condition/constraints are met with that approach. I will model the bridge with xsecs first to see if the lowchord elevation will be submerged creating pressure flow. If not my model is complete.

Do you imagine that the brdge will surcharge? HECRAS will allow caps on xsecs. I have only used this a couple of time but it worked well in my scenario. Hope that helps, I know I did not answer your question directly but it could be a maybe yes or maybe no.