Sketchup - FOLLOW ME TOOL

 Create the seater profile using Circle and Arc command

Offset to create depth to the seater

Extrude the seater to 1’6” for seater height.

Create 3" sunken landscape deck 

Select the Tool from – Tools - Follow me

Create a profile perpendicular to the path

Select the profile

Follow the path the profile want to flow.

Assign material