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

Flow entering and exiting the model was measured as part of the data-collection process.
Flow entered the model headbox through pipes as pressurized flow. Measurement-instrument
selection for inflow was based on the anticipated flow required for each test, and the associated
pump and pipelines used. Two instruments were used: 1) a differential pressure meter (annubar)
manufactured by the Rosemount division of the Emerson Process Management Company, and 2)
an electro-magnetic flow meter (mag meter) manufactured by the Endress and Hauser Company.
Table 3-7 summarizes flow-measurement characteristics of each instrument.
Table 3-7: Discharge measurement-instrument ranges
Instrument Type Flow Range
(cfs)
Pipeline
(in.)
Pump
(hp)
Accuracy
(%)
mag meter 0.13 - 10 18 40 0.5
annubar 6.5 - 15 24 75 2.5
Outflow from the model flume section was either conveyed through the inlets or
bypassed off the road section. In either case, the flow passed through an opening in the tailbox
of the flume and into channels below. Flow exiting the channels was measured by either a
rectangular weir for bypassed flow or V-notch sharp-crested weir for inlet captured flow. Both
weirs were constructed in accordance with published specifications (Bos, 1989; USBR, 2001).
Calibration was performed for each weir prior to testing of the model. Rating equations in the
form of Equation 3-1 were developed by regression analysis of depth-flow data over the
expected operating range of each weir. Coefficients and exponents used in these equations are
given in Table 3-8. For slope configurations greater than 0.5% longitudinal, the tailwater depth
was noted to rise significantly in the tailbox of the model. When this occurred, the weirs were
raised and recalibrated:
b
Q = aH
Equation 3-1
where:
Q = discharge (cfs);
a = coefficient of discharge;
H = head above the weir crest (ft); and
b = depth exponent.
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