Importing Land Use Information

[AJ]

I am having trouble getting my LandUse (manning’s n) data from Geo-RAS into RAS. I used the following steps:
(1) created the LandUse layer with the GeoRAS tool bar and digitized polygons corresponding to the different land use areas.
(2) Checked that all fields in the attributes table have been populated.
(3) I then used the Geo-RAS tool bar to “Create LU-Manning Table” and “Extract n values” and then
(4) exported the RAS data to an SDF file using the Geo-RAS toolbar
(5) Finally, I imported the GIS data into RAS

After going through the import process, I checked the mannings n data for all cross section and it is only a single value for all cross sections (the first value listed in the attributes table) but it should vary throughout the cross section.

I have run this process from the step (3) to (5) a few times but there was no change. I also, minimized the number of entries in the attributes table to 9 just to avoid any issues with having too many entries but this did not help either. I have never had an issue with this before so any suggestions on how to solve this would be great.

[Anonymous]

Check you cell spacing. One n value per cell face. You gotta work with your computational mesh as well as the land use grid size to get what you want. Either way you can use the mannings value override button at the top rightish of the geometry data editor

[AJ]

I think I need to clarify the problem, when going from ArcMap/HEC-GeoRAS to RAS (1D model) Only a single n value for all cross sections is imported even though the attributes table in ArcMap shows 9 different n values. The only value imported is the first value in the attributes table.

[Anonymous]

I dont recommend using georas for a mannings value. There is no need. What you should do is use the shapefile itself. Assing the mannings value inside RAS mapper with Tools>New Land Cover…It will bring in the shapefile and you can now select the field which distinctively shows your landuse categories, then assign it a mannings value in the right column. Then rebuild you mesh and you will get a varying n value

[AJ]

From what I understand, the procedure you described using RAS Mapper only applies to manning’s n values for 2D models. It is my understanding that it will not work for assigning manning’s n values to the cross sections of a 1D model.

[Anonymous]

Sorry i assumed 2D you are correct. The fastest way to assign mannings values into n1, n2 and n3 is to define the banks within hec ras. Then in the geometry data editor select the tables>mannings n values

This allows you to assign mannings values for multiple cross sections based on the L and ROB as well as the channel. Let me know if this helps. As modelers are further moving away from GeoRAS and its multiple bugs i tend to stay away from anything beyond a reach and a cross section creation for import

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