I am in doubt about how to draw the stream centerline in HEC-GeoRAS:
- It should be the the line that pass for the lowest points of each cross section of main channel (invert line of main channel)?
- It should be the the center(middle) of my cross section (independent of my the main flow line) or the center considering only the main channel? In this case, the line don't necessary represent the invert
- It has to pass for any specific point of my cross-section?
I was developing my station points based on the distance from the most left point, so my first point always starts with zero. I already read that this don't influence the calculation, so I could state any point of reference and take the distance from this point to my stations values. Its that right?
In figure below, some cross-sections are not perpendicular of flow path, however this is from a previous work of a private company, and surveying is not a option for me (for various reasons, mostly time and money).
Thank you in advance
Francisco,
The centerline is a reach representation. But the HEC-RAS only uses the flowpath distances for calculation (left, channel and right). In HEC-GeoRAS some people use the centerline as Channel Flowpath.
You must to draw and define three flowpats. These features represent the flow over banks and main channel. Flowpath channel is not necessarily the geometric middle of the section.
Station-elevation data: the station value is a reference value. You can start from zero, but it represents the initial point, drawed from left to right.
If any section is not perpendicular whith flow direction, is necessary that you adjust the station information whit the “Skew Cross section” option. That corrects the flow area, and the cross section stations.
Regards,
Thanks for the fast reply, Heber!
About the section, these points in the image are actual surveyed data. As a said, this is a "old" project the I am working with. That being said, is a good practice to just change the location of the surveyed data so the cross-section be perpendicular to the flow? Because this would change where the points were actually measure.
Best regards,
As this seams to be a popular question, I've decided to add the solution that I have used.
The choice how to draw the stream centerline in HEC-RAS has direct impact on the terrain generated by cross-sections. If you don't use the middle point of the cross-section as the location where the river should pass, the terrain generated won't be equal to the cross-section used. As example, I have added the Figure below where we have the cross-section nº 6251.18. As we can see, the river line won't pass through the middle point, as is dislocated for the "left" of middle point of cross section. Because of this, the terrain generated for the cross-section in red is also dislocated in relation to the actual cross-section used for the terrain. However, if we make the river centerline equal to the middle point of the cross section, the terrain generated for that cross-section will mach the cross-section data.
So based on the cross-section data, if you are still using the GeoRAS in ArcGIS, you can use the middle point line feature to draw your river line based on middle point of each cross-section. In RASMapper, you could used the new edit geometry feature to draw the river line (but I think you would have to "guess" where is the middle point for each cross section).
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