How to use ChannelMAC
ChannelMAC gives a preliminary capacity for a trapezoidal open channel from
Manning's equation, in US customary units. Set the channel geometry, pick the unknown to solve
for, and read the discharge (or normal depth), velocity, and section shape as you type.
1The equation it solves
ChannelMAC evaluates uniform (normal-depth) flow with the US customary form of Manning's equation,
using true trapezoidal-section geometry:
Q = (1.486 / n) · A · R2/3 · S1/2
A = (b + z·d) · d
P = b + 2·d·√(1 + z2)
R = A / P
T = b + 2·z·d
V = Q / A
- Q
- Discharge (flow), cfs
- n
- Manning's roughness coefficient, dimensionless — always an input
- b
- Bottom width, ft — may be 0 for a triangular section
- z
- Side slope, z horizontal per 1 vertical (z H : 1 V) — 0 = vertical walls
- S
- Channel slope, ft/ft (entered as a percent)
- d
- Depth of flow, ft
- A
- Flow area, ft²
- P
- Wetted perimeter, ft
- R
- Hydraulic radius (A ÷ P), ft
- T
- Water-surface top width, ft
- V
- Mean velocity, ft/s
- 1.486
- US customary Manning constant (KN)
2Channel & flow parameters
The first panel is a ledger of parameters. Enter the four geometry values —
bottom width b, side slope z, channel slope S
(as a percent), and Manning's n. A bottom width of 0 gives a triangular section;
a side slope of 0 gives vertical walls (a rectangular section).
Every field is color-coded so you always know what you are looking at:
- yellow — editable. Type or change the value here.
- green — computed. The value ChannelMAC solved for; it is read-only.
- A red outline means a value is missing or out of range — the message under the panel says what to fix.
3Solve for flow or depth
The last two rows — Flow Q and Depth d — each carry a
solve toggle. Pick the one you want ChannelMAC to compute; that row is
highlighted and its field turns green, while the other becomes a yellow field you fill in.
- Solve for flow. You enter the depth; ChannelMAC returns the discharge Q directly.
- Solve for depth. You enter the discharge; ChannelMAC back-calculates the normal depth d.
Everything recomputes as you type — there is no calculate button.
4Read the results
- The dark result tile shows the variable you solved for — discharge Q (cfs)
or normal depth d (ft) — with a line beneath it recapping the channel configuration.
- Two cells below report top width T and mean velocity V, each to
one decimal. The velocity cell turns amber if it drops below about 2 ft/s (sedimentation) or red
above about 12 ft/s (erosion / scour) — a prompt to check the lining, not a pass/fail.
- Open Calculation audit for the full worked set: slope, geometry, area, wetted
perimeter, hydraulic radius, Manning n, top width, discharge, and velocity, alongside the
governing equations.
- The cross-section panel draws the channel to scale with the water body, the
water-surface line and top width, the side-slope ratio, and 1.0 ft of freeboard
above the water. The vertical scale is exaggerated so shallow, wide sections stay readable.
5Scope & limits
Results assume steady, uniform flow at normal depth in a straight prismatic channel, and are
reported in English (US customary) units only. ChannelMAC does not check freeboard adequacy, critical
depth, sub-/super-critical transitions, scour, or channel-lining stability — the 1 ft
freeboard in the drawing is a drawing aid, not a design check.
Intended for use by or under the supervision of a registered professional engineer. The engineer is
responsible for confirming that results are correct and appropriate for the problem being solved.
