XS should be straight, shouldn't it?

[Van PD Tri]

Hi to all,

I have question related to delineate the cross-section in HEC-GeoRAS.

In theory, the XS should be straight. However, in some applications, the XS line can be curve as well (like the poster from RiverCAD).

Can anyone explain me where (both along the channel and along a XS) the XS should be straight and where it can be curve?

Best regards,

Tri Van

[jarvus lunt]

"In theory, the XS should be straight"

No, I would say in theory the XS should be perpendicular to the direction of the flow. Flow perpendicular to the cross section is the assumption of one-dimensional programs like RAS.

For an "S" shaped river, sometimes you have to make an arbritrary XS shape just to keep the XSs from crossing each other.

I usually draw the channel part of the XS as a straight line. I adjust the shape of the XS in the overbank to try and keep the flow perpendicular to the XS section line. Ideally, the XS should be whatever shape is required to keep all the flow lines perpendicular, but it depends on how much time I want to spend messing around with the XS and whether it seems hydraulically relevant for the given data set.

[Van PD Tri]

Hi,

Thank you very much for your reply.

I have tried a curved cross section which make the length of that cross section is longer than others. Therefore, the calculated U at the cross section is much smaller than at the others as well.

What do you think about this?

Besr regards,

Tri Van

[jarvus lunt]

I'm not totally sure what you are asking. Is the question: "if the cross section is really curved, the cross section would be longer than a straight cross section, and because the cross section is longer the cross section area would also be larger and this could effect the hydraulics?" Yes, I would agree that is correct. If the length in the overbank is much bigger (because of the length added by the curve) the area will be bigger and the velocity will be lower. Depending on the data set, this might be something to worry about.

[Van PD Tri]

Yes, it is the issue I worry about.

Thank you for sharing idea with me.

Best regards,

Tri Van