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

Q
⎛ L L Qw
⎞
= f ⎜ , ⎟
Equation 2-21
⎝ FT h Q ⎠
w
,
where:
Q = total flow;
Q w = captured flow;
L = inlet length;
F = Froude number;
T = top width of gutter flow; and
h = depth of flow in the gutter.
For this inlet, the Froude number appears in the first parameter group, and ratios of
lengths and flows are used. The flow ratio used is typically called the inlet efficiency or capture
efficiency.
2.7 Summary
Currently-accepted design procedures, which represent the state-of-the-art for inlet design
from the UDFCD, were explained for each inlet used in this study. USDCM methods (which
originated in HEC 22) are based upon theoretical parameters which must be determined from
empirical relationships. The FHWA model, which provided data for development of HEC 22
methods, was described. In addition, Manning’s equation and the Froude number were each
defined as unique velocity-depth relationships. The process of dimensional analysis was
explained as a commonly-used method for developing significant parameter groups that can be
used in equation development. A survey of parameter groups identified as significant in
determining inlet efficiency was conducted. Empirical equations have been used for determining
the capacity of curb and grate inlets for composite gutter sections (in which the gutter cross slope
does not equal the street cross slope). Most of the available research has been on gutters with
uniform cross slopes. For gutters of uniform cross slope, Manning’s equation for a triangular
cross section is frequently used for determining flow. Relationships exist for determining either
curb or grate inlet capacity. Few relationships exist for combination inlets; they are typically
treated as only a grate inlet. This is due to the observation that, when the grate is not depressed
below the gutter flow line, little or no gain in performance results from the grate. A need exists
for design equations, based on physically relevant and easy to determine parameters, which
address use of combination inlets with the grate depressed below the gutter flow line.
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