Curb Opening Hydraulics Calculator

The curb opening calculation is a program that will calculate the capacity of a curb opening on grade conditions, and will determine the length to capture a percentage of flow. This calculation is not to be used in conjunction with the modified Manning calculations for inlet grates.


Enter 4 of the 5 variables and the remaining variable will be calculated.

The normal variables typically entered are Longitudinal (SL) and Transverse (ST) slopes, Manning roughness coefficient n: and either but not both the curb opening length (LT) or the flow (Q).

If an efficiency number from 0 to 1 is entered it will determine the length of a curb opening that will capture that fraction of flow.

Curb Opening Length (LT):
K: 0.6
Flow in ft3/s (Q):
Longitudinal Slope (SL):
n: (usually n=0.016)
Transverse Slope(ST):

The efficiency of curb-opening inlets shorter than the length required for total interception is expressed by the
following equation:

Curb-Opening Efficiency Equation

Efficiency (E):
Curb Opening Length (L):

A modified version of the first equation that deals with gutter depression and curb opening efficiency.

Gutter depression (a):
Width of depression (w):