2.13 ROADWAY OVERTOPPING
Roadway overtopping will begin as the headwater rises to the elevation of the lowest point of the
roadway. This type of flow is similar to flow over a broad crested weir. The length of the weir can
be taken as the horizontal length across the roadway. The flow across the roadway is calculated
from the broad crested weir equation
Q o = K u k t Cr L s (HWr )1.5
(2.177)
where:
Overtopping discharge in m3/s (ft3/s)
=
Qo
=
Overtopping discharge coefficient
Cr
HWr
=
Flow depth above the roadway in m (ft)
=
Submergence factor
kt
=
Length of the roadway crest along the roadway in m (ft)
Ls
=
1.0 (English)
Ku
=
Ku
9.81 / 32.2 or 0.552 (SI)
The charts in Figure 2.41 indicate how to evaluate the correction factors kt and Cr.
If the elevation of the roadway crest varies, for instance where the crest is defined by a roadway
sag vertical curve, the vertical curve can be approximated as a series of horizontal segments.
The flow over each is calculated separately and the total flow across the roadway is the sum of
the incremental flows for each segment (Figure 2.42).
The total flow across the roadway then equals the sum of the roadway overflow plus the culvert
flow. A trial and error procedure is necessary to separate the amount of water passing through
the culvert, if any, from the amount overtopping the roadway. Performance curves must then
include both culvert flow and road overflow.
2.14 SOLVED PROBLEMS OPEN CHANNEL FLOW (SI)
2.14.1 PROBLEM 1 Evaluation of Correction Factors α and β
Calculate the correction factors α and β for a cross-section given the discharge measurement
during the peak flood event for the year. From Table 2.4, the following values are obtained:
Q
=
152.41 cms
138.20 m2
A
=
W
=
49.71 m
180.46 m4/sec2
Vi∆Qi
=
Vi 2∆Qi
220.88 m5/sec3
=
2.68
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