Curb Opening Inlet Interception Calculator
Enter the gutter flow and the inlet geometry. The length needed for total interception, the efficiency of your opening, and the intercepted and bypassed flows update as you type.
- Length for total interception, LT
- — ft
- Length ratio, L/LT
- —
- Interception efficiency, E
- —
- Intercepted flow, Qi = E × Q
- — cfs
- Bypass flow, Q − Qi
- — cfs
The inlet is at or beyond the length for total interception: E = 1 and the bypass flow is 0.000 cfs.
Caution: with LT of 10 ft or more, HEC-22 notes that Eq. 7.10 underestimates the length required for 100 percent interception, so a longer inlet may bypass more flow than computed.
Check case: Q = 1.77 cfs, SL = 0.01, Sx = 0.02, n = 0.016 and L = 9.84 ft give LT = 23.94 ft, L/LT = 0.411 and E = 0.614, so only 1.09 cfs of the 1.77 cfs is intercepted (HEC-22, FHWA-HIF-24-006, Feb 2024, Eq. 7.10 p. 96, Eq. 7.13 p. 97).
Equations used: HEC-22 Eq. 7.10, LT = Ku · Q0.42 · SL0.3 · (1/(n · Sx))0.6 with Ku = 0.6, and Eq. 7.13, E = 1 − (1 − L/LT)1.8 for L below LT, E = 1 otherwise. Inlet on grade, uniform cross slope, no gutter depression.
Source: FHWA-HIF-24-006, Hydraulic Engineering Circular No. 22, Urban Drainage Design, Fourth Edition, February 2024, Chapter 7, Example 7.2 part A, pp. 97–98.
For preliminary estimation only; not a substitute for review by a qualified/licensed engineer.
Sizing a curb-opening inlet on grade
A curb-opening inlet on a continuous grade rarely catches every drop of gutter flow. Water that is not intercepted bypasses the opening and travels downstream to the next inlet, so the question for a drainage engineer is how much of the design flow a given opening length actually captures, and how much is left to carry on.
HEC-22 answers it in two steps. Eq. 7.10 with Ku = 0.6 gives LT, the length needed to intercept all of the gutter flow. Eq. 7.13 then gives the efficiency E of a shorter opening. In the check case, Q = 1.77 cfs, SL = 0.01, Sx = 0.02, n = 0.016 and L = 9.84 ft give LT = 23.94 ft, L/LT = 0.411 and E = 0.614, so only 1.09 cfs of the 1.77 cfs is intercepted (HEC-22, FHWA-HIF-24-006, Feb 2024, Eq. 7.10 p. 96, Eq. 7.13 p. 97).
Q is the gutter flow you already have, for example from the Rational Method; this page does not compute it. It assumes an inlet on grade with a uniform cross slope and no gutter depression, and it is meant for highway and stormwater drainage engineers making a preliminary check.
How to use it
- Enter the gutter flow Q in cfs from your own gutter or Rational Method calculation.
- Enter the longitudinal slope SL and the pavement cross slope Sx as ft/ft (for example 0.01 and 0.02).
- Enter the Manning roughness n for the gutter surface (for example 0.016).
- Enter the curb opening length L in feet you are considering.
- Read LT, L/LT, E, and the intercepted and bypassed flows from the results panel, which updates as you type.
FAQ
How do I calculate the curb opening length needed to intercept all the gutter flow?
HEC-22 Eq. 7.10 gives it: LT = Ku · Q^0.42 · SL^0.3 · (1/(n·Sx))^0.6, with Ku = 0.6, Q in cfs, SL and Sx in ft/ft and L in feet.
What is the interception efficiency of a shorter curb-opening inlet?
HEC-22 Eq. 7.13 gives E = 1 − (1 − L/LT)^1.8 for an opening shorter than LT, and E = 1 once L reaches LT; the intercepted flow is Qi = E × Q and the rest bypasses.
Why does my inlet bypass more flow than the equation predicts?
HEC-22 notes that Eq. 7.10 underestimates the length required for 100 percent interception, especially for inlets 10 ft or longer, so a longer inlet may bypass more flow than the calculated efficiency suggests.
Does this calculator determine the gutter flow Q for me?
No — Q is the gutter flow you already have from another calculation such as the Rational Method; the tool assumes an inlet on grade with a uniform pavement cross slope and no gutter depression.