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# Weir and Overtopping Flow at Headwalls

When an overtopping weir has been added to a headwall junction, the software automatically checks for flow going over the weir. Overtopping will begin when the headwater rises to the elevation of the weir crest or roadway. 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 recommended to have the Check for Inlet Control option checked ON before computing. This is located on the Compute tab.

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 weir overtopping flow plus the pipe flow must equal the total flow. A trial and error process is necessary to determine the amount of the total flow passing through the pipe(s) and the amount flowing across the weir at the same energy grade line.

Stormwater Studio uses the following broad crested weir equation:

Where:

Q = overtopping flow in cfs (cms)
Cw = broad crested weir coefficient = 3.09 (1.50)
L = crest width in ft (m)
H = Hw – Weir Elevation in ft (m)

As the depth of the overtopping flow increases the weir crest is extended upwards.

IMPORTANT: Weir flow is not eligible to be used as carryover flow to other inlets. Rather it rejoins the pipe flow at the downstream end. From there the total Q will move to the next downstream Line as usual. A Bypass Target Line cannot be specified.