Composite Gutter Flow and Spread Calculator
A composite gutter is a curbed street gutter whose first width W from the curb face is depressed by a depth a. That depression gives the depressed width a steeper cross slope, Sw, than the pavement cross slope Sx, so more of the water runs against the curb and the gutter carries more flow for the same spread T, the width of flow measured from the curb face.
The flow outside the depressed width, Qs, comes from the Izzard form of Manning's equation with the constant Ku = 0.56, working in feet, inches and cubic feet per second. A ratio Z links that outer flow to the share Eo carried in the depressed width, and the total gutter flow follows as Q = Qs (Z − 1 + S)/S, where S = Sw/Sx.
Finding the flow for a known spread is a direct calculation. Finding the spread that carries a known flow needs iteration, because T sits inside both Qs and Z; this calculator solves it by bisection, which works because Q rises steadily with T. Enter lengths in feet or metres, the depression in inches or millimetres, and flow in cfs, m³/s or L/s; results come back in the units you choose.
Inputs
Results
| Spread T | —ft |
|---|---|
| Spread ratio T/W | —– |
| Total gutter flow Q | 2.300cfs |
| Depressed cross slope Sw | 0.1033ft/ft |
| Spread outside depression Ts | 6.200ft |
| Flow outside depression Qs | 0.6645cfs |
| Share of flow in depressed width Eo | 0.7111– |
Formulas used
- Inputs are converted to ft, in and cfs; results are converted back to the chosen units.
- Sw = a/(12 W) + Sx, with a in inches and W in feet
- S = Sw/Sx · x = T/W · Ts = T − W
- Qs = (Ku/n) Sx1.67 SL0.5 Ts2.67, Ku = 0.56
- Z = (1 + S/(x − 1))2.67
- Eo = (Z − 1)/(Z − 1 + S)
- Q = Qs (Z − 1 + S)/S
- Spread from flow: bisection on T between 1.000001 W and 200 ft, 200 halvings.
How to use the calculator
- Choose the mode: flow from a given spread, or spread from a given flow.
- Pick the length unit (ft or m) beside W and enter the depressed section width W measured from the curb face. The same unit applies to the spread T.
- Pick the depth unit (in or mm) and enter the depression depth a; enter 0 for a gutter with no depression.
- Enter the pavement cross slope Sx and the longitudinal grade SL in ft/ft, then the Manning n of the gutter.
- Enter the spread T from the curb face, or, in spread-from-flow mode, the flow Q with its unit (cfs, m³/s or L/s).
- Read the results, which update as you type; press Calculate to recompute. If a field shows an inline error, correct that value and the results return.
Frequently asked questions
What is a composite gutter?
A composite gutter is a curbed street gutter with a depressed section of width W next to the curb face. The depression depth a makes the cross slope of that section, Sw = a/(12W) + Sx, steeper than the pavement cross slope Sx, so the depressed width carries a large share of the flow.
Why does the spread for a given flow need iteration?
The spread T appears in both the flow outside the depression and the ratio Z, so the total flow equation cannot be rearranged for T. Because Q rises with T, the calculator brackets T between W × 1.000001 and 200 ft and halves the interval 200 times.
What does Eo represent?
Eo is the share of the total gutter flow carried in the depressed width, computed as Eo = (Z − 1)/(Z − 1 + S), where S = Sw/Sx. The flow outside the depression, Qs, is the remaining share, so Q = Qs/(1 − Eo).
Why is a small flow rejected in spread-from-flow mode?
If the flow is below the flow computed at a spread of W × 1.000001, the water surface would end inside the depressed width. This method assumes the spread extends beyond W, so that case is not covered and the calculator shows an inline error instead of a result.
Disclaimer: results are an estimate only and are not a substitute for a qualified drainage engineer. The method assumes one straight depressed section next to the curb, a uniform pavement cross slope, steady uniform flow and no clogging; it does not cover curved pavement sections or inlet interception.