Screen circular culvert barrel size and full-flow capacity with Manning's equation, then continue to the separate FHWA HDS-5 outlet-control workflow; this tool explains inlet control but does not calculate inlet-control headwater.
For educational purposes only. Not a substitute for professional engineering judgment.
Manning's equation is empirical and assumes uniform, steady flow conditions
Not valid for pressurized (surcharged) pipe flow - use pressure flow equations instead
Accuracy decreases for very smooth pipes (n < 0.010) or very rough pipes (n > 0.035)
Does not account for entrance/exit losses or other minor losses
Partial flow calculations assume free surface (atmospheric pressure at water surface)
Maximum Q/Qfull occurs at approximately y/D = 0.94, not at full flow
Air entrainment effects near the pipe crown are not considered
References formatted in APA 7th Edition style.