hec-ras program 2

[bjm1]

How do you define your last x-section so tht the program interprets the right and left bank properly. Every thing I have tried switches my right bank to left unless I put a false x-section 5 feet down stream. any sujestions?

 

[francesca]

I have never encountered that problem. What version of the program are you using? By last cross-section, do you mean the first downstream cross-section or the last upstream cross-section? If you add a cross-section 5' downstream, do you get the same error on the newly-added ("false") cross-section?

I would suggest that you check the stationing along your cross-section. Make sure that the LOB is stationed from 0+00.00.

 

[RWF7437]

The first cross section is always the section farhest downstream. As such, it has the lowest numerical station number. Also, the left and right bank distances downstream are left blank. That's how HEC-RAS "knows" it is the most downstream section.

Also, be certain no two cross sections cross each other. HEC-RAS can't understand this geometry even though it may be phsically possible.

Also HEC-RAS insists that distances along a cross section increase from left to right. HEC-RAS calls these distances "stations" even though any surveyor would call them "offsets". You may use negative distances but you cannot use increasing and decreasing distances.

Good luck

 

[francesca]

Also, be certain no two cross sections cross each other. HEC-RAS can't understand this geometry even though it may be phsically possible.

My understanding is that the cross-sections are supposed to be perpendicular to the direction of water flow. If it is physically possible for cross-sections to cross, then, by this definition, water can flow upstream and/or be in two places at the same time.

If you're defining cross-sections around a tight meander, instead of crossing cross-sections, you should have cross-sections that are almost on top of one another on the inside bank, with downstream reach length equal to zero. I can't think of another situation that might tempt the modeler to have cross-sections that cross, but I'm happy to be drawn into debate on any someone can suggest.

 

[RWF7437]

To Franseca,

Surveyors are perfectly happy to survey, plot, and report cross sections which cross each other. At least all the surveyors I've worked with over 45 years have been able to perform this feat. But, HEC-RAS is a "model" of the surveyor's world and, as such, cannot tolerate such ambiguities.

Thanks for your post.

good luck