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

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

# Problem

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

# Solution

This case occurs whenever critical depth flow at the face of the culvert is the controlling flow condition. Critical depth control can be categorized as a type of inlet control.

Example Message:
CRIT.DEPTH CONTROL.. Vh= .140 ft Dcr = .360 ft H.JUMP IN PIPE

In this example, for the given pond water surface elevation, the culvert structure is operating under critical depth control. That is, critical depth at the face of the culvert was found to be the controlling flow regime (IE, not inlet control).

Vh refers to a velocity head of 0.140 ft. Dcr refers to a critical depth of 0.360 ft at the upstream inside faceof the culvert. H.JUMP IN PIPE means that the flow regime is switching from supercritical to subcritical flow somewhere inside the culvert.

The HW Depth on the upstream face of the culvert is calculated as:

HW Depth = Dcr + ((1+Ke) * Vh)

Where:

Ke = Entrance coefficient; Dcr & Vh are variables described above

If Ke=0.5 for this example: HW Depth = 0.360 + (1+0.5)* 0.140 = 0.57 ft

If upstream invert = 98.50 ft, then the water surface elevation on the upstream face of the culvert = 98.50ft + 0.57 ft = 99.07 ft.