3D Terrain model modification

I need modifications made to a polygon model of a creek (the polygon model will be supplied). I need to show the contractor where work needs to be performed on two creek crossings to help estimate costs, and (see enclosed image). Both crossings (the white gravelly areas at the back left and right of the pond) need texture maps applied where fleximat ([login to view URL]) will be installed. The right creek crossing needs to be lowered and converted into a spillway. The left creek crossing needs fleximat, and also a concrete culvert head wall and soft shoulder reinforcement, although that is something I can do and is lower priority. ZBrush seems like the most efficient tool for the job.