§ 4.3.2. Inlets On Grade With Gutter Depression
A.
Curb Opening Inlets on Grade (Type G-1).
Source: Hydraulic Engineering Circular No. 22, Third Edition, FHWA, Sept. 2009
Curb-opening inlets are effective in the drainage of highway pavements where flow depth at the curb is sufficient for the inlet to perform efficiently. Curb openings are less susceptible to clogging and offer little interference to traffic operation.
Curb opening heights vary in dimension; however, a typical maximum height is approximately 4 to 6 in. The length of the curb-opening inlet required for total interception of gutter flow on a pavement section with a uniform cross slope is expressed by Equation 4-7:
L T = K u Q 0.42 S L 0.3 [1/(n Sx)] 0.6 (Eq. 4-7)
where:
K u = 0.6 in
L T = Curb opening length required to intercept 100% of the gutter flow (ft)
S L = Longitudinal slope
Q = Gutter flow (ft3/s)
The efficiency of curb-opening inlets shorter than the length required for total interception is expressed by Equation 4-8.
E = 1 - [1 - (L/LT)] 1.8 (Eq. 4-8)
where:
L = Curb-opening length (ft)
Fig. 4-14 is a nomograph for the solution of Equation 4-7, and Fig. 4-15 provides a solution of Equation 4-8.
The length of inlet required for total interception by depressed curb-opening inlets or curb-openings in depressed gutter sections can be found by the use of an equivalent cross slope, Se, in Equation 4-7 in place of Sx. Se can be computed using Equation 4-9.
S e = S x + S′ w E o (Eq. 4-9)
where:
S′ w = Cross slope of the gutter measured from the cross slope of the pavement,
S x (ft/ft)
S′ w = a/[12 W], for W in ft or = S w - S x
a = Gutter depression (in)
E o = Ratio of flow in the depressed section to total gutter flow determined by the gutter configuration upstream of the inlet.
Figure 4-16 shows the depressed curb inlet for Equation 4-9.
As seen from Fig. 4-14, the length of curb opening required for total interception can be significantly reduced by increasing the cross slope or the equivalent cross slope. The equivalent cross slope can be increased by use of a continuously depressed gutter section or a locally depressed gutter section.
Using the equivalent cross slope, Se, Equation 4-7 becomes:
LT = K T Q 0.42 S L 0.3 [1/(n S e )] 0.6 (Eq. 4-10)
where:
K T = 0.6
Equation 4-8 is applicable with either straight cross slopes or composite cross slopes.
Figs. 4-14 and 4-15 are applicable to depressed curb-opening inlets using Se rather than Sx.
Equation 4-9 uses the ratio, Eo, in the computation of the equivalent cross slope, Se.
Example 4-3
Given: A curb-opening inlet with the following characteristics:
S L = 0.01 ft/ft
S x = 0.02 ft/ft
Q = 1.77 ft3/s
n = 0.016
Find:
(1) Q i for a 9.84 ft curb-opening.
(2) Q i for a depressed 9.84 ft curb opening inlet with a continuously depressed curb section.
a = 1 in
W = 2 ft
Solution:
Step 1. Determine the length of curb opening required for total interception of gutter flow using Equation 4-7 or Fig. 4-14.
L T = K u Q 0.42 SL 0.3 (1/(n Sx)) 0.6
L T = 0.6(1.77) 0.42 (0.01) 0.3 (1/[(0.016)(0.02)]) 0.6
L T = 23.94 ft
Step 2. Compute the curb-opening efficiency using Equation 4-8 or Fig 4-15.
L/L T = 9.84/23.94 = 0.41
E = 1-(1-L/LT) 1.8
E = 1-(1-0.41) 1.8
E = 0.61
Step 3. Compute the interception capacity.
Q i = E Q
= (0.61)(1.77)
Q i = 1.08 ft /s
Example 4-4
The following information is given:
S L = 0.01 ft/ft
S x = 0.02 ft/ft
T = 8.2 ft
Q = (2.26 ft /s)
n = 0.016
W = 2.0 ft
a = 2.0 in
E O = 0.70
Find: The minimum length of a locally depressed curb-opening inlet required to intercept 100% of the gutter flow.
Solution:
Step 1. Compute the composite cross slope for the gutter section using Equation 4-9.
S e = S x + S′ w E o
S e = 0.02 + (2/12/0.6)0.60
S e = 0.07
Step 2. Compute the length of curb opening inlet required from Equation 4-10.
L T = K T Q 0.42 S L 0.3 (1/n S e ) 0.6
L T = (0.60)(2.26) 0.42 (0.01) 0.3 [1/(0.016)(0.07)] 0.6
L T = 12.5 ft
B.
Grate Inlets on Grade (Type G-2).
The depression of the gutter at a grate inlet decreases the flow past the outside of the grate. The effect is the same as that caused by the depression of a curb inlet.
The bar configuration of grate inlets greatly affects the hydraulic efficiency of the inlet. To determine the hydraulic capacity of a grate inlet on grade, consult with the grate manufacturer regarding the grate design, clear openings, laboratory test results for capacity, recommended coefficients, etc.
Grate inlets have a tendency to trap debris such as leaves and paper which results in increased ponding in the street. A reduction factor of 35% to allow for clogging shall be applied. Since clogging problems require additional maintenance to keep the inlets free of debris and functioning as designed, the use of grate inlets is discouraged and will only be allowed with written approval from the Director of the Watershed Protection Department.
C.
Combination Inlets on Grade (Type G-3).
Combination inlets (curb opening plus grate) have greater hydraulic capacity than curb opening inlets or grate inlets of the same length. Generally speaking, combination inlets are the most efficient of the three (3) types of inlets on grade presented in this manual. The basic difference between a combination inlet and a grate inlet is that the curb opening receives the carry-over flow that passes between the curb and the grate. The reduction factor for clogging of this type of inlet shall be 0% for the curb opening and 35% for the grate inlet. Since clogging problems require additional maintenance to keep the inlets free of debris and functioning as designed, the use of combination inlets on grade is discouraged and will only be allowed with written approval from the Director of the Watershed Protection Department.
D.
Recessed Inlets on Grade (Type G-1R, G-3R).
Capacities for recessed inlets on grade shall be calculated as 0.75 times the capacity for non-recessed inlets. The clogging factors shall remain the same for the various types of inlets.