Manning Pipe Capacity (Full-Bore Gravity Flow)
The discharge a circular gravity pipe carries flowing full at a given slope - the direction manning-slope leaves out (it solves for slope, this solves for flow): V = (1.486/n) R^(2/3) sqrt(S) with R = D/4, Q = V (pi/4) D^2. An 8 in concrete pipe (n 0.013) at 1% slope carries 1.21 cfs (542 gpm) at 3.46 ft/s; because Q scales with sqrt(S), doubling the slope raises capacity only 1.41x. Circular pipe flowing full, normal depth; the partial-flow depth is separate. A design aid; the engineer of record and the sewer code govern.
Formula and source
English Manning V = (1.486 / n) R^(2/3) S^(1/2); full-bore circular pipe R = D/4, A = (pi/4) D^2, Q = V A; Q_gpm = 448.831 Q_cfs.
Manning equation; n-values from USGS WSP-2339 (public domain).
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