HEC-GeoRAS: Floodplain Delineation Inaccuracies

[Anonymous]

I am finding some inaccuracies with the floodplain delineation of one of my HEC-RAS models. I will preface with the fact that I am using data that I myself did not collect, but rather obtained from a local agency. I have had no issues with floodplain delineation with some of their other data, but I have come across a strange issue with one particular model.

I will also preface that, with the other models I was provided, the data also came with a DTM. For this model I had to request a DEM from the agency that was in a different projection than the rest of the data they provided me. I was able to change the projection of that file without any issue.

However, when I ran the floodplain delineation function I find that my flood boundary far exceeds what I know to be the normal area that is inundated during a regular 100-year event. Also, my depth grid that was produced went in a single gradient from North to South (dark to light blue) instead of showing actual cell-by-cell depths (i.e. a defined dark, deep channel). I tried using a DEM from alternative sources (NYS GIS Clearinghouse, USGS, etc.) and had the same result. I suspect then it’s not the DEM that is an issue, but rather something with the model.

Thoughts?

Thanks!

Edit: To be more specific, when I say the boundary area seems too large I mean that the inundated area covers the entire extent of each X-S instead of the true inundated area.

[cameron]

If you cut a cross-section from the DEM, does it match what is in the HEC-RAS model?

[Anonymous]

I was wondering that myself. Is there a specific method to do this?

[Anonymous]

Problem solved. In my GIS export, I needed to check the Channel only instead of the Entire Cross Section under Interpolated Cross Sections.

Hope this helps anyone who ran into a similar error.

Thanks to Cameron for such a quick response in trying to help me out. I did end up figuring out how to get the DEM cross-sections.

EDIT: In addition and in reference to Cameron’s suggestion, I did come across another issue where my cross sections did not match my DEM. I used this tutorial starting on page 17 to remedy this issue:

http://www.hec.usace.army.mil/misc/files/ras/Combined_1D_and_2D_Modeling_with_HEC-RAS.pdf

[Author]

Posts