# In the Composite Outlet Structure results, what does the message "BACKWATER" mean for a culvert?

 Product(s): PondPack, CivilStorm, SewerGEMS Version(s): V8i, CONNECT Edition Area: Modeling

# Problem

In the Composite Outlet Structure results, what does the message "BACKWATER" mean for a culvert?

# Solution

This hydraulic case occurs when the backwater (water surface) profile through the culvert barrel results in the highest HGL HW elevation.

Example Message:
BACKWATER CONTROL.. Vh= .110 ft Hwdi = .398 ft Lbw = 50.0 ft

In this example, for the given pond water surface elevation, the culvert structure is operating under backwater control. That is, a backwater profile was analyzed through the barrel and was found to be the controlling flow regime (IE, not inlet control).

Vh refers to a velocity head of 0.110 ft. hwDi refers to the hw Depth of .398 ft at the upstream inside face of the culvert. Lbw = 50.0 ft refers to the length of the section under backwater conditions. If Lbw is less than the culvert length, it indicates that the barrel has full flow conditions for the remaining (downstream) section of the culvert.

The HW Depth on the upstream face of the culvert is calculated as:
HW Depth = hwDi + ((1+Ke) * Vh)
Where: Ke = Entrance coefficient; hwDi & Vh are variables described above

If Ke=0.5 for this example: HW Depth = .398 + (1+.5) * .110 = 0.563 ft
If upstream invert = 98.50 ft, then the water surface elevation on the upstream face of the culvert = 98.50 ft + 0.563 ft = 99.06 ft.

# See Also

In the Composite Outlet Structure results, what does the message "CRIT.DEPTH CONTROL" mean for a culvert?

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