You can create a catchment that uses a polyline as its boundary and a polyline as its flow path.
The catchment's flow path can be selected at the same time you create the catchment. The flow path slopes can be derived from the selected polyline flow path, or from a selected surface.
- Create a polyline around the area you want to define as a catchment. Note: Optionally you can use the lineworkshrinkwrap command to wrap multiple small catchments into one larger polyline for analysis.
- Optionally create a polyline to represent the flow path. The polyline can be 2D or 3D. Note: One method of generating the polyline to use as the flow path is to use the Water Drop command, with 3D Polyline selected as the Path Object Type.
It is not required to select a flow path when you create the catchment. You can define a flow path later with the Create Flow Path From Surface or Create Flow Path From Object command. Or you can opt to not add flow paths and calculate the Time of Concentration using another method.
- Click Analyze tab
Ground Data panel Catchments drop-down 
Create Catchment From Object. - Select the polyline to use as the catchment boundary.
- Optionally select the polyline to use as the flow path, nearest to the uphill end, or press ESC to skip this step. Note: The uphill end of the polyline defines the start point (or hydraulically most distant point) of the flow path. The downhill end of the polyline defines the discharge point.
- Use the Create Catchment From Object dialog box to define the catchment properties and click OK.