HEC-RAS Cross Sections 2

[dinol]

I have a very flat flood plain with a farmers ditch as the main channel. The channel travels north then bends 90 degrees West. What do you do when the sections overlap? What do you input for ROB and LOB reaches? Any help would be greatly appreciated. Thanks in advance.

 

[RWF7437]

In HEC-RAS cross sections may not overlap. HEC-RAS cannot understand overlapping sections. When the surveyed sections overlap, in fact, you must edit them by narrowing them or by other means. Section lines may bend so that is often the easiest way to get around this problem.

good luck

 

[francesca]

Cross section lines must always be perpendicular to the flow direction. Consider them slices of flow; in that sense, they bend around corners as the inner bank slows down and the outer bank speeds up.

 

[dinol]

Thank you for the information so far. So if my understanding is correct, I can shorten the cross sections at the bend(e.g. -100' and +100') while the cross sections leading up to the bend and past the bend can be the necessary -1000' and +1000'?

I don't fully understand bending the cross sections? Is there a tutorial floating around that I can look at regarding that? How do you let the program know that there is a bend in the cross section? Thanks in advance.

 

[RWF7437]

You understand correctly. Do get the Manuals for the program ( 3 Volumes ). You can download them at this web address:

http://www.hec.usace.army.mil/software/hec-ras/

 

[danryan]

One of the most important tips I learned doing cross sections is to always have the cross section perpendicular through the contours. This way you are going to identify the cross section geometry from the perspective of the water flowing through it.

I have seen people just cut cross sections straight across a channel regardless of whether one side was sloped differently than the other which distorts the length of the cross section and thereby the area.