The best way is to create a shapefile in a GIS package such as QGIS which is free software. You create the polygon shapefiles and then import them( you can drag them) into HECRAS. Then you associate the mannings n areas with a geometry.
Hi Con, First of all thanks for your reply. Can I do the same from a shapefile exported from Civil 3d? Can you also briefly explain how to associate mannings n with my shapefile and override my mesh with this new landcovers manning where the overlap is?
All right for those who require this info I am writing the way I solved it. Actually it was pretty simple. Just type “mapexport” and click on polygons to select your closed polylines.
Important part is to fill “DATA” tab with at least one feature so you won’t get “this shp file contains 0 features” error in hec-ras.
The rest is same as other procedures,
-create manning n layer
-associate it with terrain
To create a mannings layer you can import multiple shape files for each landuse type as follows:
– right click on map layers and select “add new Mannings n Layer”
– hit the plus key and browse and select all your mannings shapefiles
– in the bottom left box type in your description for each landuse
– in the bottom right box for each landuse, enter the mannings n
– save your mannings layer
you can also copy a shapefile imported into the “Map Layers” folder to the Mannings n “override region” layer in the geometry folder. this will override any created mannings layer.