HEC-RAS field data collection 1

[dlnva350]

Does anyone know of a good set of instructions to provide surveyors collecting HEC-RAS field data? Our surveyors have not collected field data for HEC-RAS model input before. I'm sure I could let them know what's needed but perhaps someone out there has already gone through this. I'm not being lazy mind you, I just have limited HEC-RAS experience so I want to ensure I'm considering everything.
Thanks,
DLN

 

[francesca]

I have always found that channel x-sections (bank-to-bank) are of limited value and a good aerial topo works best for the overbanks. You can easily blow a project budget on field survey data that is of limited value, particularly as it's hard to make sure while in the field that you're surveying perpendicular to the flow lines.

 

[dlnva350]

So, typically you use flown topo francesca? Then I assume that you're building a surface and picking points perpendicular to the channel to get the station and elevation data for HEC-RAS? A co-worker has told me that this doesn't work well because the flown topo often misses the flowline. He is an environmental scientist but has some HEC-RAS experience (I'm not ruling out the possibility that he is wrong). The county GIS topo is at 5' intervals and doesn't include elevations for the flowline so that is of no use.
By the way, thanks for the quick reply.
-DLN

 

[cvg]

you probably need minimum 2 foot contour intervals and 1 foot is better. If you do have topo, have the surveyors get cross sections to verify the accuracy of your topo. Also, have them pick up all the structures.

 

[francesca]

A flown topo won't help you with what's under the water surface in the channel, and if it's densely wooded and a narrow channel and flown in the summer you may miss some bank detail... however, in a flooded stream, the majority of the flow is on the overbanks so you will need a detailed survey of your overbanks, which on a large area is prohibitively expensive to be done by a field survey crew.

As cvg mentions, you do need the structures to be picked up, and depending on budget and your tolerance for error, a detailed survey of the bank/channel, though again, especially with possible debris in the channel, the fraction of water flowing in the channel during a flood is small and "holes" in the channel, though contributing some turbulence, are ineffective flow areas, and not necessarily worth the time and effort of survey.

I suggest that you use the 5' topo to select your cross-section locations so that your field survey crew has an idea where to go; cross-sections should not be straight lines, but rather perpendicular to the flow direction. What do you do when HEC-RAS gives you an error indicating that more x-sections are needed? Do you send the crew out again or interpolate x-sections? If the latter, wouldn't a 2' or 1' flown topo be more accurate?