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W Beck

Aligning DEM-extracted and field survey cross sections

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I am attempting to align (merge) my DEM-extracted cross sections with field survey cross sections (step 6.4 in "Managing HEC-RAS Cross Sections" tutorial). 

Can you provide any general rules of thumb when attempting alignment? I have attached a series of screenshots to illustrate my questions. 

Shot A: I have added the field survey cross section (dark line) - I have offset it so you can see it better. Notice that on my field survey, the west top bank is lower than the east top bank. For the DEM extracted cross section (light line), the east and west banks are generally on the same level. Should I line up east bank (DEM) to east bank (field survey), as in shot B? Or, should I line up west bank (DEM) to west bank (field survey), as in shot C? If I line up west-west (e.g., lowest bank-lowest bank), and then align using "insert all", the resulting cross section is odd looking (shot D). Will this odd shape affect the HEC-RAS model? 

So, my field-survey cross sections do not perfectly match the DEM extracted cross sections, making for an uneven overlay. Is it more important to line up the lower or higher top banks prior to merging? Any advice on merging cross sections would be great. Thank you.

A.PNG

B.PNG

C.PNG

D.PNG

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I don't have any general rules of thumb regarding which bank to use to align your cross section in your model.  I can tell you how to fix the problem you're having with the strange shape when you insert the survey cross section based on the lower bank.  One way to fix this is to add an additional point to your survey cross section so your cross section is long enough to be inserted into your extracted cross section without any gaps.  I don't know if this makes sense.  You should probably always use the "Insert All" option when merging instead of the "Blend All" option when merging.

Hope this helps, I'll think more about which bank station to use and if anything comes to me I'll reply to this thread.

Chris

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I had another few thoughts on this issue you're having with the cross sections.  I will insert some of your original questions:

On 4/26/2017 at 11:40 AM, W Beck said:

Can you provide any general rules of thumb when attempting alignment?

I don't have any general rules of thumb, though I think you can generally assume that your surveyed cross sections are more correct than your extracted cross sections.

On 4/26/2017 at 11:40 AM, W Beck said:

Should I line up east bank (DEM) to east bank (field survey), as in shot B? Or, should I line up west bank (DEM) to west bank (field survey), as in shot C?

Since your surveyed cross sections are more correct than your extracted cross sections, I think you can assume that you can line up your cross sections using the east bank, as in screen shot B.  This is assuming that your surveyed cross sections extend to the east bank as extracted from the DEM.  It could be that your surveyed cross section extends only to a certain point (not to the bank point) on the extracted cross section.  In this case, you would line up the east bank of your survey cross section at that same point on your extracted cross section.  If you have good elevation values on your surveyed cross section, you could approximate the location of this point along your extracted cross section.

On 4/26/2017 at 11:40 AM, W Beck said:

If I line up west-west (e.g., lowest bank-lowest bank), and then align using "insert all", the resulting cross section is odd looking (shot D). Will this odd shape affect the HEC-RAS model? 

Yes, I think the odd shape will impact your RAS model.  You want your cross sections to be as similar as possible from one cross section to the next downstream cross section in your model.  This is especially true for running an unsteady RAS model, where abrupt changes in cross section geometry can cause model instabilities.

You could try filtering/removing points along your extracted cross section using the filter tool in the cross section attributes dialog or just by removing them manually before merging cross sections.  It looks like you have lots of points in your channel cross section.  This can be a good thing, but can be problematic if you're trying to merge cross sections where the geometry does not match.

I hope this helps,

Chris

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Chris, 

Thank you for the advice. I ended up removing points from the extracted cross section prior to merging - this worked nicely. 

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