a. Private storm drainage collection systems shall be designed in conformance with main line standards specified herein when plumbing code grade requirements of Oregon Plumbing Specialty Code (OPSC) cannot be met. The private storm drainage collection systems shall conform to the detention requirements contained herein as applicable.

b. These provisions of the PWDS do not, nor are they intended to supersede the OPSC, but are intended to allow the design engineer flexibility in the design of private storm drainage systems where the OPC minimum slope requirements cannot be satisfied.