Interpolating cross sections for meandering river

[Alexander Betsholtz]

Hi,

I have run into issues trying to interpolate additional cross sections for my river. I have a DEM based on LIDAR and so need to interpolate the river bathymethry from existing cross sections. The river I look at is meandering a lot, and so the XS interpolation tool doesn’t seem to work well as the interpolated cross sections don’t follow the river curvature.

I’m looking for a tool that can help me interpolate cross sections that follow the river curvature in the same way as the bathymethry can be interpolated for the river only (http://hecrasmodel.blogspot.se/2014/12/including-channel-bathymetry-into-your.html).

When I used the latter option to interpolate the bathymethry, flat banks are created in the generated around curves in the DEM(even though these banks are outside the river banks). Theoretically, this DEM could be exported back to GIS where additional cross sections could be added after the flat banks are corrected, but I would expect there might be an easier approach?

Thanks in advance,
Alexander

[Anonymous]

Hi,

When you interpolate between your cross sections have you tried the Generate for Display Option, in the Cut Line GIS coordinates box?

http://hecrasmodel.blogspot.com/2014/05/exploring-hec-ras-xs-interpolation.html

[Anonymous]

Alexander,

I am currently facing the same problem in an highly-meandering river. I’ve tried interpolation of cross-sections within the HEC-RAS geometry editor and checking “generate for display as perpendicular segments to reach invert” but that has gotten me worse results … with strange jagged edges jutting beyond the banks. I am now about to try a workaround in QGIS. I plan to take the GeoTiff with the detailed channel and lesser-detailed overbank data, and “snip” out everything beyond the banks using QGIS, then resave the file, and then create the channel terrain model. But first I must determine how to convert the banklines into a polygon layer that I’ll use as the “snip to” layer. Let’s update each other until the problem is solved.

Bill

[Anonymous]

Hi Toby,

Yes I’ve tried that option, but then the cross sections don’t get georeferenced.

/Alex

[Anonymous]

Hi Bill,

Seems like we are on the same track. I used the “create terrain GeoTIFF from XS’s ( channel only)” In this terrain, some weird flat banks were generated around meanders. I opened this terrain in ArcGIS, created a polygon within the bank lines (using “feature to polygon”). Then used “extract by mask” to the get rid of the flat banks outside the banklines. Finally, I used the raster calculator to add the DEM together with the river bathymetry. The idea is that I can now add cross sections manually in geo-ras, and that these cross sections will be interpolated based on the original ones. Still haven’t looked into the interpolation details used by HEC-RAS when the layer XS interpolated surfaces is created, and still have to see if the interpolated cross sections make sense..

Alex

[Anonymous]

Alexander,

I finally succeeded in merging the detailed channel bathymetry with the detailed overbank data from the DEM. I did this using QGIS. First step was to make a copy of the banklines shapefile within QGIS, then adding lines (i.e. features) in order to close the open upstream and downstream ends of the reach. Then I used QGIS to convert the shapefile with lines into a polygon shapefile. Last step was snipping the terrain (GeoTiff) with the detailed channel bathymetry data so that it didn’t extend past the banklines polygon. Brought it back into RASMapper where the remainder of the merging steps worked correctly.

In summary, the RASMapper terrain merger works correctly in general, but perhaps not so great in highly meandering reaches (with cross-sections not closely spaced?), even when you use the interpolate cross-sections feature. At least that’s what I found after numerous attempts with different interpolated x-sec spacing and channel raster cell sizes.

Bill

[Anonymous]

Hmmm … here’s an update. I just repeated the process of trying to Create Terrain GeoTIFF from XS’s (channel only) for the river that included two highly-meandering reaches and widely-spaced cross-sections that missed most of the meanders. In a previous attempt at this process the created terrain GeoTIFF showed flat overbanks in the meandering reaches! This time I did NOT use XS interpolation in the meandering reach, also I used a smaller output cell size (1 versus 3 previously) and HEC-RAS worked correctly with the created channel terrain GeoTIFF limited to the extent of the banks throughout the entire river reach.

[Anonymous]

Ok. I believe they had fixed the “flat overbanks” bug in 5.0.3. but this version crashes for me when I tick the “interpolated cross section” box in 5.0.3.

If you didn’t use the XS interpolation method, how did you do it?

/Alex

[Anonymous]

I did no interpolation whatsoever.

There was one other difference between previous unsuccessful runs (i.e. with flat overbanks) and my new successful run. I used a different mix of DEMs, this one with no “No Data” cells. It is possible that the sprinkling of a small amount of “No Data” cells, visible when zoomed in, had something to do with the flat overbank delineation.

In any case, my problem is solved. Good luck with resolving yours!

[Author]

Posts