When a culvert or storm drainage pipe is being sized on the basis of discharge capacity requirements, knowledge of the hydraulic roughness of the pipe is required. Hydraulic roughness is typically influenced by both the roughness of the pipe material and the inside pipe wall profile (e.g., smooth, corrugated, etc.). When subjected to external loads associated with field installations (e.g., backfill, compaction, etc.), the inside wall profile of thermal-plastic pipe products, like high-density polyethylene (HDPE) M294 drainage pipe, often changes due to circumferential strain. As lined HDPE pipe strains circumferentially, the structural external ribs reduce in diameter, which causes the inside liner to develop a longitudinal corrugated wall profile. Variations in the hydraulic roughness of M294 Type S drainage pipe with 24-in. (0.61-m) diameter that were based on external circumferential strain rates ranging from approximately 0 to 2.4% were identified in this study, conducted at the Utah Water Research Laboratory, Utah State University. The circumferentially strained pipe wall geometries of four in-service M294 Type S drainage pipes with 24-in. (0.61-m) diameter were measured and compared with the laboratory pipe conditions. The results of this study were also compared with a previously published predictive method for determining Manning's n values on the basis of corrugation depth and pipe diameter.