Rectangular Channel Normal Depth (Manning)
The Manning normal (uniform-flow) depth of a rectangular channel - the depth the Froude tile leaves out - solved by bisection of Q = (1.486/n) A R^(2/3) sqrt(S) with A = b y, R = A/(b + 2y). A 10 ft channel carrying 200 cfs on a 0.1% slope at n 0.015 runs 3.82 ft deep at 5.24 ft/s, Fr 0.47; because that normal depth exceeds the 2.32 ft critical depth, the reach is a mild slope (subcritical). Prismatic rectangular section, uniform flow; trapezoidal sections and backwater profiles are separate. A design aid; the engineer of record governs.
Formula and source
Q = (1.486/n) A R^(2/3) S^(1/2), rectangular A = b y, R = A/(b + 2y); solve for the normal depth y by bisection; slope class from yc = (q^2/g)^(1/3).
The Manning normal (uniform-flow) depth of a rectangular channel and its slope class against the critical depth, as compiled in Chow's Open-Channel Hydraulics, by name.
Audience
This tile is built for plumbers and the adjacent professions in the Plumbing and Gas group. The interactive calculator runs entirely in your browser. No account, no fee, no advertising, no tracking.
Related tools
Posture
Rough Logic answers the math question the working professional asks on the job. The site is a calm, fast, ad-free, account-free, ever-free reference. It does not interpret code. It does not replace the licensed professional. It does not store your inputs. The Authority Having Jurisdiction governs all installations and inspections.