Overtopping is automatically computed for bridge and culvert sections and will begin when the headwater, (Hw) rises to the elevation of the roadway or deck as shown in the profile below. The overtopping will usually occur at the low point of a sag vertical curve on the roadway. The flow will be similar to flow over a broad crested weir.
It is a simple matter to calculate the flow across the roadway for a given upstream water surface elevation using the weir equation. The problem is that the roadway overflow plus the flow through the culvert barrel must equal the total flow. A trial and error process is necessary to determine the amount of the total flow passing through the culvert and the amount flowing across the top chord.
For Weir, Bridge and Culvert sections, Channel Studio uses the following weir equation:
Where:
Q = Overtopping flow
Cw = Broad crested weir coefficient = 2.66 (1.5) – Bridges, Culverts and Weirs
Cw = Sharp crested weir coefficient = 3.33 (1.84) – Weirs only
L = Crest width
H = HGL – High Chord elevation (The approach velocity is neglected)
Adjustment for Submergence
Weir performance can be affected by submergence, i.e., when the tailwater rises above the weir’s crest and reduces the flow.
The equation for the reduction in flow is:
Where:
Qs = Submerged flow
Qr = Unsubmerged flow from standard weir equation
H1 = upstream head above embankment
H2 = downstream head above embankment