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

5.6 Summary
The current state-of-the-art in determining inlet efficiency was illustrated in this chapter
by application of methods provided in the USDCM to the Type 13, 16, and R inlets. Agreement
with observed test data was generally very poor with efficiency over-predicted by an average of
20% for the Type 13 and 16 inlets and under-predicted by an average of 7% for the Type R curb
inlet. Methods given in the USDCM were improved by developing splash-over velocity
coefficients specifically for the Type 13 and 16 combination inlets. While splash-over velocity
was not specifically sought in the testing, it was determined analytically from the collected test
data for the combination inlets. This was done by utilizing the accepted calculation procedures
given in the USDCM to back-calculate the splash-over velocity for each test. A third-order
polynomial regression was then fitted to the calculated splash-over velocity data to provide
updated coefficients. The splash-over velocity coefficients are reflective of the combinationinlet
performance, not the grate-only inlet performance, and provide a considerably improved fit
to the observed efficiency data with efficiency errors averaging 10%. USDCM calculation
procedures for the Type R curb inlet were improved by re-developing the regression coefficient
and exponents for the original equation. The form of the original equation was preserved, and
the overall fit to the observed efficiency data was improved considerably with efficiency errors
averaging 3.8%.
Development of independent empirical equations by dimensional analysis provided an
alternative approach to the currently used UDFCD methods. Physically-meaningful parameters
were combined to produce a single, dimensionally consistent, equation for each inlet. These
equations were found to predict efficiency values that differed by an average of 5% from the
observed test data for each of the Type 13, 16, and R inlets. A comparison, by depth and inlet
type, for all methods is presented in Table 5-3. In this table, each method is compared to the
observed test data for maximum and average efficiency error. The original UDFCD methods
were most accurate at the lowest test depth of 0.333 ft for the Type 13 and 16 inlets. For the
Type R inlet they were most accurate at larger depths. Improved UDFCD methods show
significant improvement at larger depths. Empirical equations were most accurate at 0.5- and 1-
ft depths. Recommendations for calculation method use are given in the conclusion chapter of
this report. A tabular, test-by-test efficiency calculation comparison is presented in Appendix H.
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