Determine the length of slotted drain inlet required for interception of all of the water in the curb and gutter calculated by Equation 10-19.

where:
L
r
= length of slotted drain inlet required for total interception of flow (ft. or m)
z = 0.706 for English measurement or 1.04 for metric
Q
a
= total discharge (cfs or m
3
/s)
S = gutter longitudinal slope (ft./ft. or m/m)
E = function of S and S
x
as determined by
Equation 10-20
S
x
= transverse slope (ft./ft. or m/m)
n = Manning’s roughness coefficient.

total discharge < 5.5 cfs (0.156 m
3
/s)
longitudinal gutter slope < 0.09 ft./ft. (0.09 m/m)
roughness coefficient (n) in the curb and gutter: 0.011 < n < 0.017.
The longitudinal slope exponent (E) is determined with Equation 10-20:

Select the desired design slotted drain length (L

a

) based on standard inlet sizes. If L

a

< L

r

the interception capacity may be estimated using Figure 10-15, multiplying the resulting discharge ratios by the total discharge. Alternatively, the carryover for a slotted drain inlet length may be directly computed using Equation 10-21.

where:
Q
co
= carryover discharge (cfs or m
3
/s)
Q = total discharge (cfs or m
3
/s)
L
a
= design length of slotted drain inlet (ft. or m)
L
r
= length of slotted drain inlet required to intercept the total flow (ft. or m).