HY8:Vena Contracta

What is it?

When water is forced through a orifice opening, like a sluice gate, the water continues to decrease in depth as the streamline curves turn to follow the direction of travel. This contraction of depth is called the Vena Contracta.

When and where does it occur in culvert hydraulics?

The Vena Contracta occurs at the inlet of a culvert whenever the inlet control depth is greater than the outlet control depth. These conditions are created when the tailwater is low and the culvert is short.

How does HY-8 handle those computations?

HY-8 neglects the Vena Contracta except when the culvert slope is horizontal or adverse under inlet control.

HY-8 will use the following equation to determine the length of the Vena Contracta:

Where:

• L = Vena Contracta Length
• D = Rise of Culvert

HY-8 uses the following equation to determine the final depth of the Vena Contracta:

Where:

• d_vc = Vena Contracta Final Depth
• c = Vena Contracta Coefficient
• y_inlet = Headwater Depth or Rise of the Culvert, whichever is smaller