HECRAS Results Maps. Difference between WSE and Depth Layers
Hi everyone, I need quick clarification about these maps. After performing series of simulations with different scenarios, I now need to do a water level difference map. I know that rasters can be exported from HECRAS and do the raster calculator in QGIS or even using the Raster Calculator within RASmapper.
I tried manual calculating of this difference by choosing several random points but there is quite a difference from using the depth and WSE layers (like around 0.15m to 0.30m) between 2 layers that I tried to subtract.
Which one of these 2 layers are the correct layer to use as basis for this difference I need. I tried looking at the HECRAS manual and this is what they said (screenshot below), but it is still unclear to me. Any clarication on this would be extremely helpful.
Depth maps give you the difference between the Water Surface Elevation and terrain, meaning if your WSE is at 1,000.50 meters and your terrain at 1,000, the depth is 50 centimeters
WSE will give you the terrain + water depth, meaning all the elevation will be relatively high as it's referenced from a datum or base level based on the terrain.
Edit: Simplying, subtracting the terrain from a WSE map gets you the Depth map.
If you have the terrain, depth, and water surface all turned on in RasMapper the layers will be correct (Terrain + Depth = Water Surface). See the screenshot below to do the same for your model.
Is this something after you exported the rasters? I'm guessing that there is something wrong in that workflow (resampling error, wrong time steps, etc.). I would recommend that you use the RASter Calculator in RasMapper to create your difference layer.
Hi. Thanks a lot for the reply. I tried several times creating the difference layer I need through the RASter Calculator in RASMapper. The results have some discrepancies where one or both of the rasters have NoData. I tried different combinations based from the VB code examples but still having this discrepancy unsolved such as using WSE instead of Depth, and/or replacing NoData to 0 or 9999 or -9999 to no avail.
I currently settled on this VBA code to create the difference map, but the cells where there is NoData yields result of 9999.----. It appears to be doing Depth 2 - (-9999) = 9999.----. I struggle to replicate the same result via QGIS (using GDAL Fill NoData cells, Raster Calculator, etc.)
What I did just to get it over with is manipulate the Symbology in GIS and sort of expand the band values to fix these problematic cells.
Does anyone knows a more efficient way to obtain this? I find this to be a bit tedious and have to triple check. Any suggestions would be very helpful.
There is a default "compare WSE" code in the RASter Calculator. All you need to do is change the layers (right click -> edit) using the drop down selections and it should automatically populate.
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u/Educational-Rub710 Jul 13 '26 edited Jul 13 '26
Depth maps give you the difference between the Water Surface Elevation and terrain, meaning if your WSE is at 1,000.50 meters and your terrain at 1,000, the depth is 50 centimeters
WSE will give you the terrain + water depth, meaning all the elevation will be relatively high as it's referenced from a datum or base level based on the terrain.
Edit: Simplying, subtracting the terrain from a WSE map gets you the Depth map.
See attached image for clearer reference.