1/2. A new length conduit with a lower Manning's N 1/3. The Flow and Velocity are the Same
2/1. How does Conduit Lengthening Work in SWMM5?
2/2. Check the sum of the new Lengths in the Flow Table
2/3 Check the Sum of the New Length - it will give you an understanding of the New Volume in the Model. You do not want to have a large increase in volume, plus longer lengths reduce the slope of the links which may cause problems.
3/1 Check the Sum of the New Length - A surrogate for the New Volume 3/2 You do not want a large increase in Volume 3/3 SWMM5 / InfoSWMM