how can I use the reynolds stress model for 3D open channel flow in cartesien coordinates and also how can I define separetly the roughness high for each wall. Many thanks.

I have never used the RSTM, you might try and find some library cases which use it as an example. If this is a choice, are you sure you want to use this rather then a KE type model.

To put different roughgness on different walls, you will need to define your own patches, such as

PATCH (FLOOR ,LWALL ,1,0,0,0,0,0,1,1) COVAL (FLOOR ,V1 , GRND5 , 0.000000E+00) COVAL (FLOOR ,KE , GRND5 , GRND5 ) COVAL (FLOOR ,EP , GRND5 , GRND5 )

then to assign a roughness to it put;

SPEDAT(SET,ROUGHNESS,FLOOR,R,2.20000E-02)