This calculator follows HEC-RAS Reference Manual in calculating region composite n using Chow 1959, page 136, equation 6-17 (not 6-18).
Rock lining
Use the Manning Trapezoidal Channel Calculator to design rock lining. This calculator is better suited to natural sections.
Errata
Found and fixed 23 August 2026. A segment drawn exactly vertical — two points at the same station, one above the other, which is how a rectangular channel, a box culvert or a retaining wall is drawn — added no wetted perimeter. Results from before that date are too high for any section with a vertical wall: a channel 10 wide and 5 deep was given a wetted perimeter of 10 instead of 20, and flow about 1.6 times the correct value. A sloping bank, however steep, was never affected, and neither was a wall standing above the water. If you used this page on a section with a vertical wall, please run it again.
Velocity check
High velocity implies that there was a large elevation drop that created such high specific energy. That energy can be lost quickly at expansions, bends, or obstructions. Verify that this is reasonable for the site.
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