![]() I believe this all this is consistent with your recommendations above, but felt that elaborating on the fact that there are cases where the roughness break is not necessarily the conveyance break may be helpful to new HEC-RAS users. This is similar to the "law of the wall" adjustment common to many flume studies. Placing the bank stations at the "bottom" of the physical bank may overestimate the conveyance capacity of the main channel, as only bed friction (not additional friction caused by rough banks) will be accounted for in the hydraulic calculations for the main channel. ![]() This allows a composite Manning's n to be used for the main channel that accounts for the flow resistance imparted by the banks on the main channel. 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.Well done! One clarifying comment on delineating bank stations based on conveyance: for narrow channels, rough banks may affect the conveyance capacity of the main channel such that the bank stations should still be placed at the top of bank / break in grade. Once HEC/RAS 5.0 is installed, change the system units in Options > Unit System (US customary SI). This example consists on a water channel that overflows. 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. Here we will present the steps and needed considerations to model river/channel flow in HEC-RAS. (20 Pts) Q1 20000 cfs Q2 30000 cfs Assume the Manning's coefficient n-0.24 for the two overbanks and 0.04 for the channel and that the overbank distances between stations are the same as the distances between the stations in. ![]() 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.” Use HEC-RAS to calculate the water surface profiles for two discharges in the channel described in the following tables. All of my XS extend over the banks and are from the left to the right looking downstream. 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”. In ArcGIS, I have a channel with banks and a Land Cover shapefile indicating Manning’s n-values. Also all my Manning’s n Values are blank. used in HEC-RAS is to subdivide flow in the overbank areas using the input cross section n. ![]() In HEC RAS 5.0 my Cross Section Data has blank Downstream Reach Lengths for LOB and ROB, but the Channel is populated. Skyline Solution of a Sparse System of Linear Equations. ![]()
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