Hydraulic Efficiency of Grate and Curb Inlets - Urban Drainage and ...

Inlet Name
Bar P-1-7/8
Bar P-1-7/8-4
Bar P-1-1/8
Vane Grate
Table 2-3: Grate nomenclature and descriptions
Description
parallel bar grate with bar spacing 1-7/8 in. on center
parallel bar grate with bar spacing 1-7/8 in. on center and 3/8-in. diameter
lateral rods spaced at 4 in. on center
parallel bar grate with 1-1/8 in. on center bar spacing
curved vane grate with 3-1/4 in. longitudinal bar and 4-1/4 in. transverse bar
spacing
45 o Bar 45 o -tilt bar grate with 3-1/4 in. longitudinal bar and 4-in. transverse bar
spacing on center
30 o Bar 30 o -tilt bar grate with 3-1/4 in. longitudinal bar and 4-in. transverse bar
spacing on center
Reticuline
“honeycomb” pattern of lateral bars and longitudinal bearing bars
The ratio of frontal flow captured by the inlet to the total frontal flow (R f ) can be
expressed by Equation 2-7:
R
f
Q
=
wi
= .0 − 0. 09
Qw
( V −V
)
1 Equation 2-7
o
where:
R f = ratio of frontal flow captured to total frontal flow;
Q w = flow rate in the depressed section of the gutter (cfs);
Q wi = frontal flow intercepted by the inlet (cfs);
V = velocity of flow at the inlet (ft/s) determined from Q/A; and
V o = splash-over velocity (ft/s).
The relationship given in Equation 2-7 is only valid for splash-over velocity (V o ) less than
cross-sectional averaged velocity (V), otherwise R f = 1 and all frontal flow is captured by the
grate. Splash-over velocity is defined as the minimum velocity causing some frontal flow to
escape capture by the grate, and may be defined by Equation 2-8:
V
o
= α + βL
− γL
+ ηL
Equation 2-8
e
2
e
3
e
where:
V o = splash-over velocity (ft/s);
L e = effective length of grate (ft); and
α,β,γ,η = constants from Table 2-4.
11