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

On-grade
test data
Directly
calculate
efficiency
as Q captured
divided by
Q Total
Determine
efficiency from
regression
equations for
type 13, 16,
and R inlets
Determine
efficiency using
current UDFCD
methods for type
13, 16, and R inlets
Determine
efficiency using
improved UDFCD
methods for type
13, 16, and R
inlet
Compare
results for
efficiency with
test data and
recommend
improvements
Figure 5-1: Analysis flow chart
5.1 Efficiency from UDFCD Methods
In this section, efficiency is determined for the Type 13, 16, and R inlets using the
currently-accepted calculation methods presented previously in Section 2.2. For the Type 13 and
16 combination inlets, efficiency was determined using Equation 2-1 through Equation 2-4, and
Equation 2-7 through Equation 2-10 as a direct calculation. Guidance in the USDCM to ignore
the curb component of these combination inlets was followed by applying those equations. It
was necessary to match the Type 13 and 16 inlets to comparable inlets from the UDFCD
methods given in Section 2.2. From Table 2-4, the Type 13 inlet grate was found to be most
similar to the Bar P-1-7/8-4 (also known as a P50x100 in HEC 22) by visual inspection, and the
Type 16 grate was most similar to the vane grate. Applicable coefficients from Table 2-4 were
used in Equation 2-8 for calculation of splash-over velocity. The local gutter depression (a),
shown in Figure 2-2, was determined as a function of cross slope and gutter width. For a cross
slope of 1% the value of “a” was 0.13 ft, and for a cross slope of 2% the value of “a” was 0.11 ft.
Additional parameters were determined directly from the collected test data. Efficiency was then
calculated from Equation 2-10, and compared to the observed efficiency in Figure 5-2 and Figure
5-3. Deviation of UDFCD methods from the observed test data becomes greater with increasing
flow depth and increasing inlet length for Type 13 and 16 inlets. Differences in efficiency are
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