HEC-GeoRas – Errors during “Floodplain delination Using Rasters”

[Anonymous]

Hi!

I’m here writing because I have a problem with my Hec-RAS flood simulation.

I’ve already had some help asking here: https://www.researchgate.net/post/HEC-GeoRas-Errors_during_Floodplain_delination_Using_Rasters, but we haven’t found a solution!

The last problem that we are not managing to solve is the following:

I don’t understand why my floodplain get really huge. The water covers areas in which its impossible that it will get, since the bank it’s higher than the water level. However, if I define that bank as a “levee” the water won’t go on the area that was covering before. I don’t know why in the first case the water gets to the low area on the other side of the bank. Do you know how can I solve this?

I know it’s rather confusing with no photos, but I don’t know how to upload them here. However, if you want to understand better you could go to the link that I write at the beginning to see photos and conversation and just check to the last 7 messages.

Thank you so much in advance!!

Riccardo Bianchi

[Anonymous]

GeoRAS is an outdated process for floodplain delineation. HEC-RAS 5.0.7 or any RAS software after version 5.0 has the ability to easily delineate your floodplain based on the import of a terrain file into RAS via RASMapper. This also can be exported to a raster for use in GIS

[cameron]

A couple things going on. First, both Mapper and HEC-GeoRAS create “wse tins” and just apply it to the terrain you provide. The wse tins are based on the cross-sections extents and cover the entire cross-section unless you put a levee feature in the cross-section which it then clips the “tin” to that point.

Second, in the 1D HEC-RAS model, it has no idea if areas should be connected or not. All it does is a calculation based on the shape of your cross-section. This is where understanding limitations of models is important. 2D would more detailed so if you set the model up correctly, it would know if areas should be connected or not.

Not sure what the purpose of your study is for, but if you don’t think water can get to a certain location, I would either put a levee feature there or trim the cross-section shorter. If you are running multiple flows (low and high) where the high flow does get out there, you would have to set the levee elevation just above the low flow wse and below the high flow wse.

If it is for FEMA, they won’t allow a levee feature unless it really is a levee and so you would just have to make it ineffective and then clean it up in GIS.

[Author]

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