Cross Section Allignments

[Goverdhan Reddy]

Hi,

I am modeling a small creek.

I see every one saying that the hec ras cross-sections shouldn't cross each other. The creek which I am modeling has an approximate 90 degree curve which makes it mandatory for the cross-sections on either side of the curve to cross each other. What should I do in this situation?

Can any one help me regarding this.

Thanks
Goverdhan Reddy

[Canada]

You could simulate the bend (head loss) by increasing the rugosity factor (Manning) and reduce the spacing of the sections in this particular bend.

Cross sections shouldn't cross each others - maybe it could start from a very small distance on the bank to be sure that they never have negative distance.

[jarvus lunt]

Cross sections should not cross each other. But each cross section does not have to be a straight line. Where the river goes around a bend, the cross sections themselves may have to have curves or jogs in them. Ideally, the cross section should be perpendicular to the flow at every point on the cross section. If every cross section is perpendicular to all the flow (in that cross section) then the cross sections will not cross.

Using fewer cross sections (as per the post above) may also help.

[Goverdhan Reddy]

@ Canada

I will try increasing the Mannings n at the bend,

and you mentioned to start the C/S from a very small distance on the bank, but I was directed by FEMA to extend the C/S untill the BFE and when I do this the right side cross section allignments of the downstream reach (which flows west) are intersecting with that of the upstream reach (which flows south).

@ Jarvus

as I mentioned above I was directed by FEMA to extend the C/S untill the BFE and when I do this the right side cross section allignments of the downstream reach (which flows west) are intersecting with that of the upstream reach (which flows south), I considered C/S allignments to be straight lines, so I think this might be causing the problem. Can you explain me how to consider the cross section allignments in this case.

Thanks for your response

[jarvus lunt]

I'm not an expert on FEMA requirements, but obviously if they are directing you to do something, I'm sure you will have to do it. However, I can not imagine that overlapping cross sections is a good idea. It might help if you posted a screen shot or two showing what is going on with your geometry.

[Goverdhan Reddy]

@ Jarvus

Look at the attached image.

I stationed the cross sections from left to right looking downstream, The cross section allignments (straight lines) are shown with the red lines. You can see how the allignment of C/S-1 is intersecting with the allignment of C/S-2 on the right side.

I think I did a mistake by considering the allignments as straight lines, can you explain me how to consider the C/S allignments at C/S-1 and C/S-2 such that they wont intersect.

Thanks
Goverdhan Reddy

[izazazou]

Question:

I read the replies and I am wondering why its hardly recommended to have cross section that cross each others...
In my model I have a junction and a almost 360 degres curve, so I have no choice if I want to close the cross sections with the water surface elevation...I am wrong???

thanks

[jarvus lunt]

RAS is a one dimensional model. For instance, the program assumes that every point along a cross section has the same energy grade. If two cross sections intersect, the program is going to compute two separate energies at the exact same point. That doesn't make sense. Will it cause a big error, maybe not.

But there is no reason that a cross section has to be a straight line. Ideally, the cross section should connects points that all have the same energy (and the flow is perpendicular to the cross section.

So when going around corners, angle or curve the cross section.