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

test data for typical design depths of 0.5 to 1 ft. A 5% reduction in average efficiency error was
noted, and a 10% reduction in maximum efficiency error was noted for all test depths over the
improved UDFCD methods. UDFCD methods for these inlets were shown to rely heavily on
theoretical parameters that can not be physically determined by a user; parameters are instead
determined from complex empirical relationships. A comparison between splash-over velocity
curves developed for the Type 13 and 16 combination inlets and those for the most similar grate
inlets from the USDCM revealed significant differences. The equations provided in the USDCM
give an unrealistically high splash-over velocity (on the order of 30 ft/s) for a 10-ft Type 13 or 16
combination inlet, which is in sharp contrast to the 4 ft/s determined from the test data. Original
and improved UDFCD methods were most accurate at the lowest flow depth of 0.333 ft. Beyond
that depth, the accuracy was very poor. This is likely due to the limitations of the FHWA model
used to collect data for development of the equations. For the Type R curb inlet, the improved
UDFCD methods were slightly better able to predict the observed efficiency data than the
empirical equation for all test depths. A 1.2% improvement in average efficiency error was
noted over the empirical equation, and a 15% reduction in maximum efficiency error was noted
for all test depths. Typical design depths are 0.5 ft and greater for selection and placement of
street inlets (UDFCD, 2008). With this in mind, recommendations for which calculation method
to use are given as follows: for the Type 13 and 16 combination inlets the empirical equations
are recommended, for the Type R curb inlet the improved UDFCD methods are recommended.
For illustration purposes, the observed test data on efficiency are plotted with the empirical
efficiency and the efficiency determined from the improved UDFCD methods in Figure 6-1
through Figure 6-3.
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