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> Interceptor > System Design > Grate Inlet Capacity

Grate Inlet Capacity

Interceptor A-67 provides the design flexibility to place the exact amount of open area just where it is required for various drainage conditions. The following design tools should be used to determine the length of channel and grate required under the following drainage scenarios; Curb and Gutter, Sump or Sheet Flow.

Note: Hydraulic testing performed by an independent laboratory has established that the inlet capacity of the A-67 grates will not be the limiting factor. Design is instead controlled by the movement of overland flow to the grates.

Curb and Gutter

This includes typical gutter flow applications adjacent to a curb or barrier. First, establish traditional inlet locations by using normal design criteria (clean out access and pipe directional change). Applying the following design information, use Interceptor A-67 to enhance the inlet capacity and control bypass flows, thereby reducing the number of expensive inlet structures that would normally be used only to restrict spread.

From allowable spread and roadway parameters, first determine maximum gutter flow using:

Q_G = (C_G/n)T^2.67S_T^1.67S_L^0.5
Where:
Q_G = Gutter Flow (cfs)
T = Spread (ft)
S_T = Transverse Slope (ft/ft)
S_L = Longitudinal Slope (ft/ft)
n = Roughness Coefficient of Roadway
C_G = 0.315 metric (0.468 English)

The length of Interceptor A-67 required to intercept 100% of the given gutter flow can be calculated using the following equation derived from tests conducted by FHWA and published in HEC-22, which was developed for slot type drains with widths ≥ 1.75".

L_D = C_CQ^0.42S_L^0.3(1.0/nS_T)^0.6
Where:
C_C = 0.817 metric (0.6 English)

If less than 100% efficiency is required, solve the following equation for L_I:

E = Q_I/Q_G = 1.0 - (1.0 - L_I/L_D)^1.8
Where:
E = Efficiency
Q_I = Flow to be Intercepted (cfs)
L_I = Length of Drain Required (ft)

Sump

This condition occurs where runoff is allowed to pond over the grate as in roadway sag curves or parking lots. Interceptor A-67 can reduce the critical nature of grading to the inlet or dependence on additional inlet structures.

Based on the depth of the water, flow condition will be either weir or orifice. For the A-67 grating, the transition between weir and orifice occurs at approximately 0.6 ft.

Weir Flow: Q = C_WPH^1.5
Solving for length of drain, then:
P = (Q/C_WH^1.5)
L_D = P for flow entering one side of grate
L_D = 1/2 P for flow entering both sides of grate
Where:
L_D = Length of Drain (ft)
Q = Flow (cfs)
P = Perimeter of Grate (ft)
H = Hydraulic Head Above Grate (ft)
C_W = 1.4 metric (2.48 English)

Orifice Flow: Q = 0.8 A(2_gH)^0.5
Solving for length of drain, then:
L_D = (Q/K_O)H^0.5
Where:
L_D = Length of Drain (ft)
Q = Flow (cfs)
A = 0.0828 m²/m • L_D (0.27 ft²/ft • L_D)
g = Gravitation Acceleration 9.81 m/s² (32.2 ft/s²)
H = Hydraulic Head Above Grate (ft)
K_O = 0.29 metric (1.73 English)

Sheet Flow

This runoff occurs perpendicular to the trench and flows with a relatively uniform depth along the entire trench. Interceptor A-67 can help control sheet flows that cause dangerous hydroplaning at commercial or residential entrances or multi-lane super-elevations. Hydraulic testing, performed by an independent laboratory, establishes the intercept capability of the Interceptor A-67 grate at 0.12 cfs/ft, so:

L_D = Q/C_S
Where:
L_D = Length of Drain (ft)
Q = Flow (cfs)
C_S = 0.011 metric (0.12 English)

Nomigraph

The following nomigraph is provided as a quick reference for the majority of applications where ST=0.02 and n=0.015. Locate intersection of spread and ST lines, then drop straight down to find drain length required.