Bank Lengths and Manning’s N

[Anonymous]

In HEC RAS 5.0 my Cross Section Data has blank Downstream Reach Lengths for LOB and ROB, but the Channel is populated. Also all my Manning’s n Values are blank. In ArcGIS, I have a channel with banks and a Land Cover shapefile indicating Manning’s n-values. I used the RAS Geometry All Cross-Section tool, which populates the XSCutLines Attribute table for the LeftBank, RightBank, and ChLength, but the LLength and RLength are all “0”. All of my XS extend over the banks and are from the left to the right looking downstream. Additionally I get an error when extracting N Values “Operation fail on geometries of features: Landuse feature OID 2, XS feature OID 1, Err.Number-2147220971.”

Question: 1) How to populate the LLength and RLength for the Banks? I believe this will fix my error “Left and Right overbank length is less than or equal to zero, HEC-RAS requires a positive reach length” when running my model.

Question 2) How to populate the Manning’s n data? I have the LandCover with the info in RAS Mapper, and my Land Cover to Manning’s n (2D Flow Areas Only) table looks good.

Thanks,

[AJ]

I realize this is a late response but figured I would respond in case anyone else has these questions.

Response to question #1: Missing reach length data will occur when your XSCutLines do not intersect your Flow Paths. The cross sections must intersect at least one Flow Path on the ROB and LOB.

Response to question #2: If you already created the LandUse layer in GeoRAS, you also need to use the “Create LU Manning Table” and “Extract N Values” tools (found under RAS Geometry –> Manning’s n Values) prior to creating the export file. As far as I know, the land cover layer created in RAS Mapper only applies to the 2D area. If anyone knows of a way to get cross section manning’s n data from the layer created in RAS Mapper, please post.

[Author]

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