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.

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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