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

1
Observed
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1 ft depth
0.5 ft depth
0.33 ft depth
Equal
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Predicted
R 2 Average efficiency error (%) Maximum efficiency error (%)
0.861 6.5 30.2
Figure 5-4: Predicted vs. observed efficiency for Type R curb inlet from UDFCD methods
For the most similar inlets to the Type 13 and 16 combination inlets currently available in
the USDCM, the UDFCD methods over-predict efficiency by an average of about 20%. For the
most similar inlet to the Type R curb inlet currently available in the USDCM, the UDFCD
methods generally under-predict efficiency by an average of 7%. These predictions can be
improved by slight modification of the currently-accepted design methods.
5.2 Improvements to UDFCD Efficiency Calculation Methods
One of the research objectives of this study was to extend the UDFCD methods for
determining inlet efficiency to include the Type 13 and 16 inlets, and to improve methods for the
Type R curb inlet. From the plots presented in Section 5.1, it can be seen that efficiency was
generally over-predicted for the combination inlets and under-predicted for the curb inlet. For
grate-type inlets, the only equation given in the USDCM that is grate-specific was presented
previously as Equation 2-8 for calculating splash-over velocity (V o ). Coefficients used in the
third-order polynomial of Equation 2-8 to calculate V o are what need to be developed for the
Type 13 and 16 inlets. Splash-over velocity is a unique value for a given grate type and length.
By inspection of testing photographs and recorded videos, it was concluded that flow velocity in
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